A mix of pink sand verbena (Abronia villosa) and dark green pre-flowering Sahara mustard rosettes (Brassica tournefortii) growing on the sand plains southeast of Parker, Arizona on January 17, 2019.

This is going to be a long post, but I will try to make it worth your while to read with good information and nice photos of the desert and its wildflower displays, even as I discuss the threat posed by Sahara mustards and other invasive weeds to the desert ecosystem. North American deserts on both sides of the US/Mexican border are known globally for their capacity to burst into glorious bloom in the springtime thanks to a wide variety of colorful annual and perennial plant species that respond to autumnal and winter rainfall. How and why this happens is a complex and interesting topic in its own right, one that I will have to fully address in a separate post at some point. But for this article I want to focus primarily upon the threat to these stunningly beautiful and ecologically valuable mass blooming events posed by invasive weeds. Most specifically the Sahara mustard, Brassica tournefortii.

The plant to the left is a rosette of Sahara mustard, while the plant to the right is a dune evening primrose (Oenothera deltoidea) – they look superficially similar at this stage of development.

Sahara mustard is an accidental introduction to North America. It first appeared in the Coachella and Imperial Valleys of southeastern California in the 1930s, and was possibly brought in as a contaminant of date palm plant materials originating in the Middle East back when the date industry was becoming established, although no one really knows for sure at this point how exactly it got here. Sahara mustard is native to North Africa and southwestern Asia, where it survives as part of the desert ecosystem and is occasionally used as a low-grade seed oil crop. The plant is in the same genus as a number of important food crops derived from Brassica oleracea, including cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, Savoy cabbage, and kohlrabi. It is also in the same genus as oilseed crops like rapeseed and canola, varieties of Brassica napus, as well as turnips, Brassica rapa. 

Sahara mustards on the Parker, AZ sand dunes, with some already starting to initiate flower stalk development in mid-January 2019, well before the nearby dune primroses have even started to bud. This fast growth and early development trait is part of what makes the mustard able to outcompete native wildflowers.

Upon arriving in the United States, the mustard found the warm-winter Sonoran Desert ecosystem in particular to be much to its liking and commenced to spread rapidly via its windborne “tumbleweed” growth habit, which can spread the seeds far and wide. (More on this later.) It has also found purchase in the Mojave Desert and is starting to spread into somewhat colder-winter deserts such as the southern Great Basin, Chihuahuan, and lower parts of the Colorado Plateau Deserts, although it does not appear to be quite as invasive in those zones as it is in the Sonoran and Mojave Deserts.

I yanked the mustard rosettes after I took their photo. Obviously I left the dune primroses alone after removing their competition.

Sahara mustard is a winter annual plant which grows in the cool season and sets seeds before the rising heat of mid-spring leads into blazing summer temperatures. It does not germinate with warm-season summer monsoon rains, at least not if those rains occur before late September at the tail end of the summer monsoon. The most favorable timing for mustard germination in the deserts appears to be the months of October and November, although December and even as late as early January is possible, depending upon the year and the quantity of rainfall. Unfortunately this is also the same autumn rainfall timing profile as the showy and much-beloved annual wildflowers that rightfully draw so much attention. And Sahara mustard is a formidable competitor to these native plants….

Sahara mustards have invaded the Cactus Plain Wilderness Study Area northwest of the town of Bouse, Arizona. The sandy plain is named partially for the predominance of the devil’s club cholla (Grusonia wrightiana) although a number of other cactus species are also found here.

Sahara mustard LOVES sand and grows to its greatest size and highest density upon it, including flat windblown plains, stabilized dune fields, and along dry wash channels. However this invader is by no means restricted to sandy substrates and can be found on gravelly and rocky soils too, as well as on hillsides and in mountains. This adaptability to a variety of soil and slope conditions is another feature that makes the mustard a nasty invader, simply because in the end it is not fussy about where it grows. But sand is the soil type it thrives best in.

A sandy plain in far northwestern Sonora, Mexico that has not yet become invaded by Sahara mustard. Sand verbenas and other wildflowers conduct their life business unimpeded.

In mid January and late February 2019 I took a pair of trips to Sonora, Mexico to view a significant mass display of desert wildflowers occurring there thanks to abundant October 2018 rains brought to the region by a pair of tropical cyclonic systems. The remnants of Hurricane Rosa crossed the northern Baja Peninsula and Colorado River Delta region on October 1, 2018 and dropped several inches of rain across this normally hyperarid ecozone. Less than two weeks later, Hurricane Sergio decayed in the eastern Pacific and pushed additional rainfall across the same area on October 12-13, 2018, albeit in lower amounts. This one-two punch of fall rains spawned by twin tropical cyclones set the stage for a mass wildflower bloom in these lowest and warmest parts of the Sonoran Desert ringing the head of the Gulf of California.

Adonis blazing star (Mentzelia multiflora) hoists golden flowers alongside sand verbena (Abronia villosa).

The reason so many annual wildflowers in mild-winter desert zones globally (such as the Sonoran in Arizona and Mexico, but also in other places such as South Africa, Chile, and Australia) respond best to fall rains as opposed to rains that occur at any other time of year is simply because the weather is cooler in the winter, which makes the moisture last longer. If an inch of rain falls in the monsoon season of July, for example, the long and hot summer days can quickly evaporate that beneficial rainfall within a week or two, especially if it is not followed by supporting rains shortly thereafter. But if that same quantity of rain falls in November, the short and cool days mean that it is likely to last for many weeks, which gives spring flowers a chance to sprout and grow.

If fall precipitation is followed by the usual winter rains as well, then that points towards a decent bloom season in spring of the following year since the seedlings are able to develop more robustly and set forth many flowers in carpets of color. If fall rain doesn’t occur, then little or no germination of native annuals will either, not even if winter rain  in January and February is adequate. If fall rain does occur, but is not followed on by adequate winter rain, then any good germination start will be severely reduced by dryness the next spring. So as one can see there are several factors of timing, temperature, and precipitation that all need to align in order to bring about a good wildflower season. This unpredictability of several factors spanning months of time is the major reason why mass wildflower carpets are so uncommon and notable, and why people are excited by them when they finally do occur.

Many people think this type of showing happens every year in the desert. It doesn’t.

Good bloom years (like the spring of 2019 was) are infrequent in the lowest and most arid regions of the Sonoran Desert. While some flowers may happen every 2 or 3 years, a very good show only happens once every 12 to 15 years, and truly legendary ones are spaced 25 to 50 years apart. Many dry years go by with zero flowering whatsoever. The seeds just lie dormant and wait, sometimes for decades if necessary. In fact the last time I witnessed a flower display comparable to 2019 was the spring of 2005 – fourteen years before! To be sure, there were flowers in intervening years, some of them fairly good ones; but none were as notable as 2005 was, which itself was ranked informally as a roughly once-in-50-year mass-flowering event. The same pattern occurred to set the show up 14 years ago – a number of wet and well-timed autumnal storms in October-December 2004, followed by record-setting winter rainfall continuing into April 2005.

I found a few mustard seedlings. There weren’t many but dammit, they have arrived nonetheless! Note that there are small rosettes of Mentzelia multiflora to the upper right that I did not pull. They look similar to mustard rosettes in that both have lobed leaves, but it is important to recognize native wildflowers in their pre-blooming state and let them be, while removing the invasive mustard weeds.

Unfortunately, the same precipitation pattern that benefits the desirable native wildflowers of the Sonoran and Mojave Deserts also enables the explosive growth of Sahara mustards and several other invasive weeds I’m not discussing here, but which have similar life cycles. These weeds include Eurasian cheatgrasses (Bromus rubens, B. madritensis, and others in that genus), Mediterranean grasses (Schismus arabicus and S. barbatus), different mustards such as London rocket (Sisymbrium irio) and tumbleweeds/tumble mustards (other Sisymbrium spp), and in the past 10 to 15 years, a new South African invader called stinknet or globe chamomile (Oncosiphon piluliferum). I recently made a post about the stinknet and discussed how individuals might help in controlling its spread by recognizing nascent populations before they get large and out of control. That led to this post on Sahara mustards, which I have learned a lot about in the past 20 years of trying to do that same type of manual control on still-small stands of the stuff.

Here’s a link to the blog post I wrote on the stinknet/globe chamomile:

A New Invasive Weed In Arizona: Globe Chamomile (aka Stinknet), Oncosiphon piluliferum

Badlands along the northern Sea of Cortez shoreline are blessedly free of weedy invaders of any type. Most of the vegetation here is intact and native.

I wanted to feature a few photos of the uninvaded regions of the Altar Desert, a vast and uninhabited region of sand plains and dunes in the Mexican state of Sonora, just south of the border with Arizona. It is part of the Lower Colorado River (LCR) subsection of the Sonoran Desert, but specifically refers to the zone of sand dunes, some of them fairly well stabilized by perennial vegetation, and others actively shifting and migrating. Virtually no one lives here aside from a few small settlements along the Sonoran coast that support themselves by fishing the northern Sea of Cortez. It is a wild and beautifully intact example of what the Sonoran Desert evolved to be in a region that averages only about 3 inches (75 mm) of rainfall annually.

An Ajo lily (aka desert lily, Hesperocallis undulata) stands regally on eroded bluffs overlooking the northern Gulf of California. Mountain ranges of the Baja Peninsula rise off in the distance across the water.

Unfortunately not every area of the Altar Desert (El Gran Desierto de Altar in Spanish) is as pristine as the ones featured in these photos are. Sahara mustard thrives in the company of human disturbance, and highways are one major avenue of their spread across the desert. When moistened by rainfall, the mustard’s seeds develop a sticky mucilage that clings readily to vehicle tires, construction and road maintenance equipment, and the soles of worker’s boots. This is how seeds can easily arrive at brand new areas, often many miles distant from the next nearest infestation. The soil disturbance inherent to road maintenance and construction is tailor-made for the successful establishment of brand new stands of mustard, as well as the other weeds mentioned a couple of paragraphs earlier.

Once a “spot fire” of new weed infestation has started, it only takes a few years of reproduction for them to start claiming ever increasing shares of the desert’s resources for themselves and depriving the native species of their portions. This ability to spread quickly into brand new areas with little or no human aid after an initial sowing of the seeds (however accidental) is a defining characteristic of what makes a given plant, or animal/other organism, “invasive”.

How it should look. This scene of a wide, sandy plain flanked by low, craggy mountain ranges covered in carpets of pink, white, gold, and blue flowers was once the norm across most of the Sonoran Desert’s lowest elevations in southwestern Arizona, southeastern California, and northwestern Sonora, Mexico. But due to the mustard invasion, scenes like this are becoming increasingly scarce.

Because highways are a primary, if not the greatest, avenue of initial weed invasion in the deserts and elsewhere, Sahara mustards have become widely established along most major desert thoroughfares on both sides of the border. This is true almost regardless of whether it is a major interstate or a smaller and more local paved highway, at least as long as the road in question receives a lot of vehicular traffic. There is a definite correlation between high-traffic corridors and high rates of weed invasion in most areas, and a lower invasion rate with lower traffic levels. Photos illustrating this are coming up a bit later in the post.

A blazing sunset serves as a backdrop for a blue sand lily (Triteleiopsis palmeri) and dune evening primroses and sand verbenas in the Gran Desierto de Altar.

The point about the linkage between roadways, human traffic, and the spread of invasive weeds having been made aside, there are also other ways for the Sahara mustard to spread that don’t involve humanity as much. The primary mode of spread of the mustard after a spot fire of new invasion is started by humans on their vehicle tires etc is via wind. The plants develop into large, nearly spherical tumbleweeds on brittle stems that break off at the ground level and then roll across the landscape, scattering seeds widely as they go. This is a dispersal tactic commonly utilized by a number of plants across several plant families, including various other tumble mustards (family Brassicaceae) and Russian thistle (Salsola kali, aka S. tragus, Amaranthaceae) for example, all of which have also become serious weeds in North America. This ability to roll across the landscape dropping thousands of seeds everywhere is what makes even the smallest starter population a threat to the entire area. The plants might start near highways, but they won’t stay there.

The full moon rises over the dunes of the Gran Desierto de Altar as Oenothera deltoides unfurls its nocturnal blossoms for the night.

Sahara mustards are also quite capable of spreading via water, since the seeds are readily carried away in flash floods coursing down normally dry wash channels. Obviously this is a one-way direction of spread, but it is indeed a major way that plants migrate from upper watersheds downstream from there, and then laterally out into the desert via wind after that. If a population gets started at a bridge crossing a wash channel, for example, and then seeds start getting into the water flows when they occur a few times a year, points downstream of the bridge will also become invaded over time.

The stabilized sand dune portions of the Altar Desert of northwestern Sonora contain a diversity of native wildflowers, desert shrubs, and wildlife. At least a dozen different plants are visible in this scene, unimpacted for now by mustard invasion.

One more mode of spread for Sahara mustard is via animals. Rodents do gather and cache the seeds, burying small piles of them in the soil, sometimes a few hundred feet away from where they were first growing. If you learn to pay attention to population dynamics in the mustards, sometimes you will see a tight cluster of plants germinating all in one spot, indicative of where a rodent buried a cache and then forgot about it. While this doesn’t disperse the seeds as far as vehicles, wind, and water, it is yet another mode to consider. More importantly, some seed is dispersed by more mobile and wide-ranging animals, such as birds or livestock or feral burros. If you come across a stand of mustard in a remote area, perhaps while hiking, fairly far away from roads and well up into the mountains towards the upper end of the local watershed, odds are good that it was a bird or large mammal that ingested some seeds and defecated them out – once again starting one of those “spot fires” I keep mentioning as an analogy.

The first signs of Sahara mustard invasion are starting to appear on a railroad embankment northwest of the seaside resort town of Puerto Peñasco, or Rocky Point in English. The plants are starting to flower amidst lupines overlooking a temporary freshwater lagoon created by heavy rainfall earlier in the year.

I am going into significant detail about how mustard seeds spread, because all of this is a lesson in understanding how the umbrella term of “invasive species” operates. Everything I’ve been saying about how mustards manage to invade large areas of remote desert also applies to how other invasive organisms invade different ecosystems. Invasive organisms – whether plant, animal, fungal, or bacterial – all share certain traits in common. These include:

1. Early reproduction with large numbers of offspring.
2. Multiple modes of rapid spread.
3. Adaptability to a wide range of ecological conditions and/or food sources.
4. A generalist species, as opposed to a narrow endemic or specialist species.
5. The ability to radically alter abiotic ecosystem dynamics such as fire frequency, nutrient cycling, and hydrology.
6. Competing with, crowding out, preying upon, or sickening native species.
7. Severe impacts upon the ability of native species to complete their life cycles.
8. Few or no natural controls upon the rapid spread into, and negative impact upon, the ecosystem and native ecological communities.

Understanding these traits common to species we deem “invasive” helps us to better manage rampant spread of undesirable organisms, although obviously each invasive species will have protocols best suited to handling it individually.

Look at the large, deep taproot of this Sahara mustard, reaching close to 30 inches long. This is one trait that makes them effective competitors, as they are able to reach deep soil moisture, grow larger, and produce seeds more prolifically with this access to extra water.

Even in largely uninvaded areas of the Altar Desert dunes, I was able to discover small numbers of Sahara mustards. They are here and may well continue to spread. I pull them out whenever I find them, focusing especially on small numbers where early intervention makes a real difference with fairly minimal effort. This segues into the part of the article where I discuss techniques to handle spot fire outbreaks, and why it’s worth doing when you discover them.

Fences are very hard to keep erect in dunes when the wind blows the supporting sand away and they topple over. In some places the opposite happens and the fences are buried while still upright by migrating dunes. Fences like these will neither keep out wandering cattle nor blowing mustard tumbleweeds….

The main highway leading out of Puerto Peñasco east towards Caborca, Sonora is fully invaded with Sahara mustards.

First off, let’s discuss when a Sahara mustard invasion has become too bad to be able to do anything about. A good example of this is along Mexican Highway 2, which runs along the USA border from Tijuana, Baja California and eastwards towards Ciudad Juarez, Chihuahua near El Paso, Texas. The portions of MX Hwy 2 that cross the Lower Colorado River Sonoran Desert have been heavily impacted by mustard invasion – more photos of that later. The fields of sand plain that used to be populated with colorful native wildflowers are now covered with a monotonous pale green carpet of ugly mustard covering tens of thousands of acres. Obviously this is an invasion run totally amuck, and aside from a biocontrol agent of some sort there is little chance that this situation will ever be resolved into better balance.

Once Sahara mustard reaches a closeout density like this, where a majority of open spaces in between the perennial shrubs have been entirely filled with the weeds, it is impossible to control via manual means any more. There is simply too much of it and it indicates a well-established thicket and the presence of a soil seed bank. 🙁

Detail of a flowering scape of Sahara mustard. This plant has no redeeming aesthetic value even – it’s not pretty, just weedy and aggressive with dull flowers.

One trait that makes Sahara mustard so successful at invading its new North American habitat is the ability to adjust its size to the environment and to the available resources, mostly water but also space and nutrients. This is called “phenotypic plasticity”, and it is a trait possessed by many invasive plants in particular. Basically what it means is that a mustard can become a large and rotund tumbleweed, and produce many thousands of seeds when it is in a favorable site with plenty of water and space. This massively increases its capacity to blow away and disperse numerous new offspring across potentially miles of wind-scoured sandy plains.

But in poor conditions, such as a year with low winter rainfall or crammed under a shrub or small desert tree that trapped the blowing parent tumbleweed, the plants will adjust their size to stay much smaller and produce only a few dozen seeds, maybe even less. While these small “tumbleweeds” are not likely to go far, they still mean that the population can stay active and reproduce itself at a pilot-light level, waiting for the next good rain year when the surviving seeds of the subsequent generation can become massive and explode in abundance. This capacity to be different sizes, and still reproduce successfully no matter what, is phenotypic plasticity in action.

Close up of the dull yellowish flowers of Brassica tournefortii.

Another trait that makes the mustard flexible at spreading quickly is the fact that flowers are self-fertile, meaning they can pollinate themselves and do not require another different individual to cross-breed with in order to set viable seeds. As a result, it is possible for one single seed, germinating in isolation all by itself in a brand new world full of potential, can start one of those “spot fires” that I keep mentioning. One seed, in a good year, can become a large tumbleweed that spreads thousands of new seeds all across the desert, leading to at least dozens to hundreds of new plants next year, which are of course also self-fertile and phenotypically plastic. It doesn’t take long for weeds with these characteristics to explode in density and coverage, and this is exactly what Sahara mustard has done and why it is such an ecological threat.

A few native sand plain wildflowers manage to struggle through groves of taller and highly competitive mustards east of Puerto Peñasco. But they are maybe only 10% as common as they would be in areas without mustards to compete with them.

This is why it is so damn important to stop Sahara mustards when populations are still small, limited to a few dozen or hundred or even a few thousand plants. They are annuals, and rely 100% upon seed reproduction. If you pull the weeds out before they can set any more seeds, you can totally smother that population in the cradle and protect the nearby ecosystem. If it is a first- or second-year outbreak, the odds of entirely eliminating all of the plants with one session of pulling them out is very high because there is not yet a “seed bank” of seeds waiting in the soil to germinate and replace the dead parents.

Farther southeast, there is a lightly-traveled state highway that runs roughly along the Sonoran coastline, where there are very few zones of Sahara mustard invasion. These are home to wildflower scenes such as this.

Even populations that are a bit more aged, such as those between 3 and 6 years old with a small soil seed bank, can be reduced by 90% the first year and fully extinguished by year two. The plants are not particularly difficult to pull out in the sandy terrain they favor, and they do not come back from the roots. Hand-weeding small and isolated populations of Sahara mustard is a highly effective mode of control. The same dynamic also applies to other annual invaders, including to the new globe chamomile/stinknet plant I referenced earlier. All it takes is one person to understand the threat and take effective action. For larger invasions a small group of committed weeders working for an hour or two can have essentially the same positive impact.

Orange globe mallows (Sphaeralcea sp, possibly S. ambigua) form thick meter-tall stands on many sandy areas in Sonora, Baja, and southwestern Arizona.

It’s important to once again highlight the threat that invasive weeds, of several types but especially the Sahara mustards, have posed to mass bloom displays of native desert annuals. While the wildflowers in question (verbenas, primroses, lupines, mallows, etc) are not necessarily threatened with extinction as species, there is something about ecosystem dynamics in mass blooming that goes beyond the sum of the individual floral parts. A mass bloom offers the ability of the natural sand dune, valley, bajada, or mountain slope ecological community to operate as a united whole, with numerous plants, insects, reptiles, rodents, birds, and larger mammals all able to conduct their life cycles freely in a rare good year. Some of these species must wait several years, or even a decade or longer, in between major successful reproductive events. In hyper-arid areas like this subsection of the LCR Sonoran Desert, good mass bloom years are even less frequent than in wetter areas such as the Arizona Uplands subsection of the Sonoran Desert, so these mass blooms take on greater relative importance. When this is shut down by nasty and ecologically useless invasive weeds, the entire wild community suffers and declines.

It’s so nice to see places where these wildflowers can still happen without competition from those stupid, hateful, domineering mustard weeds!

It’s also worth bringing up additional less-tangible factors associated with scenic mass blooming events.  Lately they are economically important for tourism, since some desert communities in Arizona and California depend upon an influx of visitors from nearby cities and international tourists for an economic boost during good spring seasons when wildflowers are happening. And just as importantly, but hard to quantify precisely, the sensory experience of all that color and fragrance and the sense of joyous life all around is critical towards experiencing the very soul of desert wilderness. All of this matters to us as humanity, aside from the equally valuable but human-independent ecological purposes to the wild animals and plants. This is why preserving the phenomenon of mass flowering is important, and why we need to do more to stop invasive weeds from shutting it down.

Mass blooms in the desert are memorable and get more than their fair share of botanical press, but they are actually globally uncommon events.

It’s important to grasp that mass blooming such as that which occurs in deserts is a fairly rare ecological phenomenon on a global basis. The earth has a couple dozen or so fairly distinct types of ecosystems, and hundreds of unique bioregions that exist as a subset within each broader category. For example, there is a large category called “desert”, and then there are the smaller bioregions called the Sonoran Desert, the Mojave Desert, the Atacama Desert, the Gobi Desert, and yes, the Sahara Desert. Out of these five deserts I just named (and obviously there are many more that I didn’t mention), only three of them experience mass blooms – the Sahara and the Gobi never flower with mass annuals like parts of the Sonoran, Mojave, and Atacama do.

Uh oh. I have unpleasantly discovered a particularly large and vigorous Sahara mustard in my otherwise pristine Mexican floral zone just featured above! And where there’s one, there are often more….

Meanwhile, other large bioregions of earth such as rainforests, grasslands, temperate forests, coniferous boreal forests, and tundra do not experience mass blooming of this nature whatsoever. This is not to say that those places don’t have wondrous and fascinating flora and miraculous biodiversity, because they do. But it’s a simple and basic fact that they do not erupt into landscape-wide displays of annual blossoms that go for miles and cover entire valleys and mountains. It is mainly deserts that do this, and then only some of them. This is why it’s important to research, understand, and work to preserve the mass-blooming natural phenomenon, because they truly are special in this regard.

Look at this bastard. One tiny seed the size of a grain of sand, and it turns into this monstrosity in less than 3 months.

So here’s the deal: If you happen to come across a few, a few dozen, or even a few hundred Sahara mustards in an otherwise uninvaded area with no others nearby, I would ask you to stop your hike or drive if possible and take some time to pull them out. Picture the valley nearby, or the mountain slope you’re on, teeming with beautiful natural flora and then replace it with ugly mustards. Do you want that? No, of course you don’t. So do your civic ecological duty and yank them out. Please dedicate some time to helping the natural world that ultimately supports your lifestyle. Just get the big ones if you can’t do it all. Get in the way of the invasion. Stamp out the spot fire. Thank you for caring.

Next, I have an entire plethora of tips to help you understand how to go about it and feel the sense of reward you will get for being a responsible ecological steward.

Note the developing flower stalk, rounding out into a large future tumbleweed. Once mature, a single plant like this will contain hundreds of pods (called siliques in the mustard family) and thousands of seeds. The invasive potential here is clearly enormous, so kill the plant before it can spread them!

Above is a photo of the mustard before I pulled it. It wasn’t that hard to do, since the loose sandy soil they prefer usually yields up the taproot quite easily. And even if it doesn’t come out, just pulling off all the top foliage and leaving the base level with the soil surface should kill the  plant. Remember, Sahara mustard is an annual and does not survive via the root system, so it truly isn’t that important to get the roots. Just remove all the top growth, yank off all the leaves if for some reason the taproot won’t pull free, and call it good. You have just eliminated the possibility of this gigantic tumbleweed blowing for miles across the desert spreading 5000 seeds as it bounces along! Feel proud of that, because it is a genuine victory!

Looking down into the crown of the plant. This particular mustard is about as large as the species is capable of becoming – roughly 3 to 4 feet tall and wide. Don’t forget that most are much smaller than this, but even average-size ones can each set several hundred to a thousand seeds. These infernal things are superb at replicating themselves, and we can’t let them go unchecked.

Upon discovering this massive individual mustard plant, I went seeking others up and down the road easement from where I had stopped to photograph the wildflowers that were my true purpose. And sadly I did find a bunch of them. But there is also good news: There were only a couple hundred, spaced widely, and all within the general zone of the roadways on both sides of the fencing. What this means is that the population was not old or well-established and it was likely within the first two or maybe three years of the first appearance. There is probably no soil seed bank from successive years of reproduction, and I found no big crowded clusters of mustards jammed into inconvenient places like underneath cacti or thorny mesquite or acacia trees which are a huge pain to extract. By spending an hour rooting out all the mustards I could find on both sides of the roadway within a quarter mile of my car, I probably eliminated about 95% to maybe 98% of the spot fire population. All because I took an hour of my time to try to protect the wildflowers growing nearby. That does feel useful. Noble even. I think I am permitted to pat myself on the back, and I would say the same to you if you were to do it instead of me. 😉

I decided to hang the plant on the barbed wire fence flanking the road so that viewers can observe how large it is. The fence posts are made of cement since metal posts tend to be stolen in Mexico, and they are very durable once installed.

After weeding the desert, I proceeded towards my destination for this particular trip, which was a grove of large cardon cacti (Pachycereus pringlei) I have visited before.

When I planned this particular wildflower-viewing trip in January 2019, I did so with the hope that I would be able to capture a majestic grove of Baja cardon cacti with wildflowers under a full moon lunar eclipse happening on January 20, 2019. I will add a few photos of this event here but the main post I wrote on the matter is at this link:

Lunar Eclipse In A Mexican Cactus Forest

I arrived an hour or so before sunset, stayed up much of the night photographing the lunar eclipse, and caught a beautiful dawn sunrise, shown above as the moon was setting.

The reddish eclipsed moon hangs in the sky during its totality phase at about 11:30 PM. This is why they call it a “Blood Moon”.

I also experimented a bit with firespinning steel wool. Again, for more on this visit the post at the link a couple of photos above.

The next morning, AACK! I found more mustards!

One thing I fully admit to is that I sometimes feel great frustration at discovering new populations of Sahara mustard at times and in places where I don’t expect to or want to. Well, it’s not like I ever actually want to find this crap growing somewhere, but the fact is that all too often I do and it’s frustrating and sometimes slightly despairing. But that’s when it’s important to remind myself that investing a bit of my time in stamping out this spot fire will yield important future benefits to the local ecosystem by preventing a wide-scale invasion. And no one else is going to do it. So I just suck it up, start bitching if that’s what is needed to sustain my energy, and get to it.

Yanking out all the mustards I could find (again, mostly limited to fairly near the roadside rest stop, and not in amongst the cardons themselves) took me roughly 3.5 to 4 hours. I felt I had to do it, because I love this lonesome grove of cardons and had just spent all night photographing them in a celestial eclipse. I had also visited this grove several times before and seen the wildflowers it could support amidst the stunning giant cacti. And I’ll be damned if this obnoxious goddamned weed was going to take all that away! So I just pulled and pulled and pulled, until I could find no more. When I return in the future to find very few or no new mustards, I can hopefully rest assured that my efforts did matter.
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I took a follow-up trip along Mexican Highway 2 between Organ Pipe Cactus National Monument and the border crossing south of Yuma, Arizona about 13 days later on Feb 4, 2019. The purpose was to view the ongoing spring 2019 wildflower show. And yes, mustards were an inevitable part of it too….

Over the past two decades and hundreds of hours spent pulling Sahara mustards from numerous wild locations, I have developed several ideas on how to best do it. If you read about the topic of how to dispose of weeds you pull online or in garden publications, the commonly proffered advice is to bag the weeds and dispose of them in the garbage. While that might be the solution for certain situations, it’s not that practical for most circumstances. After all, what if you happen across a spot fire of weeds while hiking, doing nature photography, birdwatching, or any other outdoor activity? Are you actually carrying a garbage bag or two with you? And even if by chance you are, are you also going to fill it and then carry that bag or two out with you all the way back to the car or wherever? What about if there are more than two garbage bags worth of material to remove? Do you just do nothing? Do you let the spot fire population go unaddressed, left alone to spread unchecked?

This scene is in Mexico but not much more than a couple of miles from the Arizona border, looking at the Cabeza Prieta Mountains. This was a small patch of wildflowers that were relatively mustard-free in an otherwise heavily-impacted zone covered in them. The gold ones are dune sunflowers (Helianthus niveus).

For me, the clear answer is no. You start pulling, even if you cannot remove the dead weeds you just killed. The reason bagging and landfill disposal is so recommended by weed control advocates is because the point is to remove all seed and inhibit future reproduction. Bagging and disposal certainly does that, but is hugely impractical or impossible in many situations, including most remote ones. But what about those numerous times you are likely to face once you become aware of the invasive concerns around this plant, where removal off-site is not feasible? What do you do then?

Traffic speeds by on MX Hwy 2 as I stop to photograph the wildflowers to the north of the road. Some mustards are visible in the distance, although right here at this spot there aren’t too many.

First of all, consider the actual age and phase of development for the mustards. If they are in rosette leaf form or just starting to flower, then there is no viable seed yet anyway. So why bother to remove bags of what basically amounts to salad greenery? Just pile the pulled plants in an open spot somewhere and let the drying desert sun dehydrate them to death. It only takes a day or two and the plants will simply compost down in place over the next year or so.

This criterion also applies to plants that are in the green-pod phase of their life cycle, where some siliques have been set but are not yet mature. In the Brassicaceae most pods split open laterally along a pair of seams, very similar to how one would shell peas and beans. If the pods are still immature or very green, they will not split and even if they did, the seeds are too immature to be viable for germination. Again, just pile the plants in an open spot and let them dehydrate to death. In each of these life cycle stages you have interrupted the amount of seed that can be set and subsequently dispersed by wind, water, or animals and don’t have to fear accidentally spreading them yourself.

Just a couple more miles down the road along MX Hwy 2 from where the previous wildflower images were captured, the weed scene is vastly different and much worse.

I’ll pause the “weeding the wilderness” discussion for a moment to illustrate how bad of an invasive weed Sahara mustard actually is. This sandy valley just west of the Cabeza Prieta Mountains is now very heavily dominated by Sahara mustards. Numerous plants are roughly 1 meter tall and they have reached a closeout density, to the point where very few native Sonoran wildflowers can compete and survive underneath the choking canopy of tangled mustard stems. Just gross. And most would agree that it is highly unappealing compared to the mosaic of colorful native flowers that should be there.

Not only are there few native plants left in this valley, but the high fuel load means wildfires are suddenly a threat. This is a highly unnatural situation for the desert.

Underneath the mustard canopy, equally invasive albeit much shorter Mediterranean grasses (Schismus spp) fill the spaces on the sandy plain. In between these two invasive weeds, both of them originating in the Mediterranean/Middle East, native Sonoran Desert plant seedlings are unable to effectively compete. The landscape has become dominated by two exotic weeds, as opposed to the diverse community of wildflowers and shrubs that should exist here and was shown earlier. Even worse, wildfires are now a threat. See some of my other blog posts on why wildfires are particularly ruinous to desert ecosystems, including these two articles:

Nevadagascar: The Link Between Invasive Weeds and Wildfires in the Joshua Tree Forests of the Mojave Desert

Sonoran Crisis: Wildfire Permanently Devastates The Saguaro Cactus Forest

As expected, the invasion is the worst close to the highway where it first began, but even some distance away it is rapidly deteriorating. This picture was taken with a telephoto zoom lens and shows large mustards mixed in with the native wildflowers. Over time the mustards are likely to win and take over, sadly.

Scanning out across the valley, one sees mostly mustards mixed in with a few struggling sand verbenas. In the 1980s and early 1990s, this scene would likely have been free of the weeds and instead been an expanse of pink, white, gold, and blue native annual wildflowers.

I will repost a photo from above that I captioned with, “How it should look.” This is to remind readers of what has been lost to the Sahara mustard invasion across much of the Sonoran Desert, particularly in Arizona and southeastern California. Tragically, the monotonous greenery of boring mustards is what most sand plains and intermontane valleys in so much of the LCR Sonoran Desert now look like. It makes me angry and sad, and is what drives me to stop the weed from spreading into otherwise intact regions.

Returning to “weeding the wilderness”, what does one do with mustards that have developed to a state of maturity where at least some, if not all, of the seed pods are filled with ripe and ready-to-grow seeds, but you cannot bag and remove them? I still dump all of them into piles, but modify the technique a bit in a couple of key ways. One is to be more careful and try to not scatter any seed while I pull the drying weeds, or to at least keep it to an absolute minimum. The second is to find something to pin the pile down with – a rock or tree branch will work, but in the absence of that I often just stomp the pile down as flat as it will go under my own weight. Sometimes I will toss some sand onto the pile as well just to add extra weight and keep the pile in place against windstorms. The purpose of the stomping and pinning is to prevent the pile from blowing away.

A bit farther away and several miles down a side road, a bunch of large mustards are in the process of taking over a zone occupied by brown-eyed evening primroses (Camissonia claviformis, aka Chylismia claviformis ssp rubescens).

Here’s why the “pile and pin” method actually works: By concentrating all of those tens of thousands of seeds into one spot and preventing them from blowing away across the landscape, you have just drastically reduced the areal coverage of what would have otherwise been hundreds or even thousands of large tumbleweeds next year. No longer can they freely scatter seeds all along whatever path the wind randomly chooses. You’ve just forced the entire seed crop of that spot fire population into just a few square meters. No, you didn’t bag and remove all those seeds, but you still effectively killed nearly all of them by making them into a compost pile. That’s a laudable effort, wouldn’t you agree?

I hated to walk away from this not-very bad infestation zone, but it was pretty close to much worse areas. Without a valley-wide control effort (or me living there permanently to plug away at it daily for weeks on end – a thing I actually did when I lived in the townsite of Yucca AZ and watched the invasion closing in around town) there was only so much I could do in a single afternoon while traveling through.

You can tell that the invasion in the above two photos is still manageable here, because the mustards are dispersed into relatively well-spaced individuals. If you were to wander around pulling the large, but comparatively few in number mustards, this particular invasion can be easily beaten back at this stage. It looks bad because the mustards that are there are huge, but in fact it isn’t all that unmanageable yet. After all, the density is far lower than in the photos of the Cabeza Prieta Mountains above. But who lives here to do it? No one. I couldn’t, so I had to let it be. There just was not enough time to do it all by myself….

Sadly, I had to move along and leave the valley to its fate of continuing to be converted over to a mustard field. After all, it was simply too close to previously pictured heavily invaded zones, where numerous new tumbleweeds could simply blow over from. Had this been just a few mustards in an otherwise pure region, like near the eclipse grove of cardons, I would definitely have made the effort. But with millions of acres across the desert, it’s not really possible for just one person to do it all. Tackling invasions that are too large is Sisyphean in nature. It is much better to prevent the spot fires from burning out of control in places where small preventative efforts can make a big difference.

I hate these weeds. Why can’t they just behave and NOT take over everything? But like it or not, they are here to stay.

One question that might arise is, “What about the plants that germinate in the ‘pinned piles’ of mustard with viable seeds? Not all of those seeds will die, so what you you do about the ones that survive?” This is a valid question, and my answer is that if you live close enough by to the zone you just weeded, or you pass by it regularly, then you can simply keep an eye out for escapees the next year and beyond that. By concentrating thousands of seeds into a pinned compost pile, it means that you have much less work to do next year. Rather than running around for miles across hundreds of acres of windblown desert pulling weeds out from under nearly every shrub, you just have to pull a few dozen tightly clustered survivors instead from wherever you made a pinned pile. It’s much easier. And again, doing anything at all to get in the way of a free-for-all invasion scenario with mustards spreading unchecked by the wind and water and critters, is way superior to doing nothing. So it’s not perfect, but it’s pretty dang great anyway. As the saying goes, “Don’t let the perfect become the enemy of the good.”

When selecting a spot in which to make a pile and pin it down, I try to opt for sites with the following characteristics: First, it should be open, and in between any nearby existing shrubs, which is fairly easy to find in most deserts. Try not to smother the living plants nearby when piling in the open spaces is just as easy. Second, make the spot as inhospitable as possible so that any viable seeds you might be depositing into the pile will have a harder  time germinating, surviving, and growing to large sizes with lots of new seeds. I like to choose harsh, rocky gravel flats if they exist, but open sand will do if desert pavement is not nearby.

Third, pick a spot away from even small wash channels, lest the next flood at some future point carry stray seeds downstream to germinate far away. Spots on stony rises, small hills, or barren rock outcrops are all pretty inhospitable to mustard seedlings. Keep the pile compact, stomp on it, and then pin it down with rocks or dead branches. Just make it as hard for stray seeds that might come up next year to survive and become sizable as possible.

And fourth, make fewer but larger piles when possible, rather than more but smaller ones. This is because the more concentrated all those seeds are into a handful of larger piles, the fewer weeds you will need to eliminate in future years. Following these four tips when “piling and pinning” will make your job of weeding that place in the future much easier, because there will be many fewer plants in the next year or two to yank out. The ultimate goal is completely extinguishing the spot fires of new invasions.

I have successfully eliminated a number of isolated Sahara mustard invasions over the course of 2 to 3 years by following these principles and being persistent. And remember that the first year you start pulling is always the toughest. It’s much easier, and vastly more gratifying, to follow up on the handful of escapees in years two and three and then to find no mustards whatsoever in years three or four, as the case may be. Hand pulling is labor intensive but it honestly works if you apply some effort to it.

Extracting Sahara mustards out from underneath chain fruit cholla cacti such as these is admittedly much more difficult than weeding out in the open.

If you live a long way from the pile and pin area you weeded, and cannot return in future years to monitor and pull escapees that survived, well at the very minimum what you did was set back the invasion process by a decade or two for that locality. Delaying invasions by even a few years does allow for some latitude, and maybe at some point an effective biocontrol is introduced. If not, then maybe if you return in 5 years you will discover that the invasion is still not that bad and then you can hit it again, keeping it suppressed. Perhaps you even mostly eliminated it? Once again, it’s a worthy task regardless of your ability to do it perfectly.

This extensive clonal patch of chainfruit chollas (Cylindropuntia fulgida) is one of the largest I have seen, covering at least several hundred acres. Mustards are present in the cholla “forest” but are not widespread.

Sahara mustards won’t likely spread as quickly in areas of taller or denser vegetation, primarily because the main mode of dispersal via wind is hampered by the larger plants. The tumbleweeds simply just won’t blow very far before they get stuck under the next tree, shrub, or cactus. This doesn’t mean that the mustards won’t spread down wash channels via flooding, or at random via animal activity, and it doesn’t mean there’s zero long-term threat. It does mean that many mustard colonies will be trapped underneath unpleasantly spiny vegetation, and those can be a real pain and a hazard to ream out. Sometimes, when I am planning on attacking an invasion with forethought and deliberation, as opposed to just happening upon one unexpectedly, I bring a long-handled rake or shovel or hoe with me to reach underneath the spiny plants. I grub out the mustards with the tool and a pair of gloves and long sleeves to protect myself, and let the weeds die uprooted but without piling and pinning. At the very least it knocks them way back for a few years, and if I return for followups I can entirely eliminate the issue in 2 to 3 years.

A different view of the extensive stand of 7 foot/2 meter tall chainfruit chollas along MX Hwy 2. These plants are sterile, meaning they can only reproduce clonally via dropped joint pieces and not via seeds. This entire ginormous population is entirely clonal, and they are all virtually genetically identical.

If you are trying to eliminate a new population of mustards in a zone of Arizona Uplands Sonoran Desert or Mojave Joshua Tree Woodland, for example, the immediate threat of rapid spread via wind is likely to be reduced due to the larger plants ensnaring the tumblers before they can go far. This means you can do an imperfect job of weed control one year, and come back the next and probably find things much improved or at minimum not worse than when you last left it. The population won’t have spread much in all probability.

Remember that you can tidy up in future years after doing 90% or more of it this year. Again, the perfect is the enemy of the good, so don’t be discouraged by a task that can only be partially done this season. Get started and usually your continued motivation and self-discipline will somehow materialize. As always try at least to grab the biggest, most seed-heavy mustards first if nothing else. Leave the little stupid ones stuck underneath the bushes since they don’t bear much seed when small. They can’t go far anyway, and snag them next year.

All this comparative discussion of how to handle mustards in different subsections of the desert does highlight how important it can be to stop the spread in more open sandy zones, where there is little to significantly impede windblown tumbleweeds. There they can spread the invasion for miles every single year!

A parting shot of white dune evening primroses and a scattering of sand verbenas and wicked Sahara mustards in the cholla stand at sunset, Feb 4, 2019.

Some final thoughts are that if more people understood how dangerously invasive plants can be, what the stakes of their unchecked spread are, learned to properly identify the problematic species, and put in the effort to control populations while they are still small and manageable, then we could collectively significantly mitigate their destructive potential. Some populations are simply too large now to effectively pull by hand anymore, but many community-based efforts can yield significant benefits in even fairly advanced invasions if people coordinate their responses and stay on it as a committed group.

Recruit help if you need to. Send potential participants in your weed quest a link to this post and have them educate themselves on why they should also become involved. Like community trash cleanups, this can actually become a constructive social event that does good for the planet and the people working to keep it healthy. Weeding parties can become a thing, as groups like the Sonoran Weed Whackers in Tucson (handling buffelgrass invasions, Pennisetum ciliare, in Saguaro National Park) and some of the residents and park employees living near Anza Borrego Desert State Park in California have done to manage Sahara mustards in the prime tourist-attracting flower zones outside the town of Borrego Springs have demonstrated.

Real progress can be made with education and concerted effort spanning 2 to 3 years of interrupting the seed formation and reproductive cycle of these annual weeds. The native wildflowers can quickly return once the weed-choked spaces are opened up again, and they absolutely will if given a chance.

Thanks for reading, and thanks for weeding!