Globe chamomile, also called stinknet in Afrikaans in its native South Africa, trying to come up in my garden from stray seeds imported in nursery pots.

If by any chance you see this plant coming up in your garden or anywhere else for that matter, pull it immediately! This is a South African invader called globe chamomile (Oncosiphon piluliferum). While it is actually reasonably attractive, that beauty belies the fact that it is explosive in terms of its ability to spread across large tracts of intact desert land. This is a winter growing annual in the sunflower family (Asteraceae) that spreads via small seeds, easily distributed by wind, flowing water, and foot and vehicular traffic. Seeds germinate in the fall with cool-season rains and grow all winter, blooming prolifically in March and drying out by late spring.

I remember a time back before the mid-2000s when the desert near Lake Pleasant and I-17 north of Phoenix, AZ was free of this particular invasive species. But in the decade-and-a-half since then, it has completely taken over the desert floor and covered thousands of acres without anybody knowing how dangerously fast-spreading it was. Not only does the plant outcompete native species for resources, but it also creates a tremendous fire hazard when it dries out. It has a pungent and often objectionable odor, leading to one of its common names of stinknet. The odor can even be smelled inside closed up homes and vehicles, and is particularly noticeable on rainy nights when the humidity is high. Some people are allergic to the smell, as well as the pollen and the plants if they are handled, which can cause irritation and rashes on the skin.

The offending plants, of which there are two here, plus a half dozen others elsewhere. All of them were pulled before they could set seeds and spread.

I admit that I find this modest annual appealing, but because it is so incredibly invasive, I cannot afford to try growing it. It was imported as a drought-tolerant annual wildflower from South Africa, intended to be used ornamentally in desert gardens, and it’s proven to be way too successful in the Sonoran Desert. It will likely also prove to be troublesome and highly invasive in the Mojave and Chihuahuan Deserts as well. This is yet another example of why extreme care must be taken when introducing ANY new plant (or animal) to a new ecosystem. So many highly invasive species were purposely introduced for their beauty in the landscape and horticultural trade, and then escaped to become huge problems for native flora and fauna.

I just found the first few globe chamomile plants trying to grow on my property, no doubt imported by accident as seed contaminants in nursery potting soil. They are inconspicuous until they bloom, and in low numbers it is possible to not even notice them mixed in with the general landscape if you aren’t well attuned to what your local ecosystem should contain. But once they reach a critical mass they become impossible to ignore. Unfortunately by that time they are often extremely hard to control, so it’s very important that people learn to recognize this plant and remove it by pulling it before they can set seed and before populations get out of control. Don’t be fooled into ignoring it until it’s too late!

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I was asked in the original Facebook post I made about this weed, “What is the best way to control it?” My answer is written below. It is a bit brief, relatively speaking for such an important issue, and I do intend to expand much more upon why more people should consider weeding the desert (or any other ecosystem where invasive weeds are a growing problem) and how they can help control new and small populations before they become too large to manage effectively anymore. That will be made in a new and thoroughly illustrated blog post, coming soon.

“The best way to handle new populations of globe chamomile, Sahara mustards, cheatgrass, and many other annual weeds of the desert is to monitor for new invasions that are only a few plants or a few hundred plants in size, and then hand-pull them before they can set seed. If you can recognize the globe chamomile  (or mustard, etc) invasion while the juvenile plants are still in their leaf-only form then pull them before they can go any farther. The distinctive foliage odor of globe chamomile/stinknet ought to help ID when not in flower. It’s also good to pull them while in bloom, but before any mature seed heads have started setting. If they are aborted in the flowering-but-not-yet-seeding stage, that entirely kills the plants too. Hand pulling is highly effective on small populations, but is difficult or impossible in large or extensive ones. I focus a lot of my effort on watching for new weed invasions and stamping them out before they can spread. It’s kind of like squelching a spot fire before it becomes a conflagration out of control.

“It is unnecessary to remove foliage or flowering-only plants from the site if you happen to be hiking outside and don’t have a plastic bag handy for removal – just make piles of them in an inhospitable, dry, open spot and pin the pile down with a rock or tree branch. The weeds will dry out quickly in the AZ sun and wind and be unable to reproduce. Because they were killed before they were able to set seeds, there is no need to remove them or carry out ungainly, heavy bags of leafy material. Just let it dry and decay in place. Globe chamomiles are annuals and do not have perennial roots, so if some plants break off at the soil line and you don’t get the roots, it is not a worry since they won’t regenerate from roots anyway if the top foliage is removed.

“Once they start dropping mature seeds, characterized by drying, flaky seed heads that shatter into dusty pieces loaded with tiny seeds, the best option is to carefully gather the plants into large plastic garbage bags, while trying to minimize the amount of incidental seed-droppage that happens. Remove the bags of seed-infested plants and discard into the landfill.

“If this is impossible, and I readily understand that it might well be, my policy is to gather all the seed-bearing plants into one of those piles (or several, as many as needed) and pin them down with a rock or dead branch again. When selecting a site to make a pile, I choose one that is wide open, barren sandy or rocky as the case may be, and spaced in between any other nearby plants. Many weed seeds have a much greater chance of survival if they germinate near or underneath established perennial vegetation, and by giving them a harsh, dry, sunny site their growth is stunted and they are much smaller and set many fewer seeds. This at least delays the invasion process by several more years.

“What this accomplishes is to concentrate all those seeds into a dense pile, rather than being spread out all over the desert by the wind. And while some number of those piled seeds will all germinate in one place, this actually makes them much easier to control the following year. Firstly, they all compete with one-another, and most will die if they are so crammed together. Competition alone reduces the number of viable survivors for next year, in addition to them being in a poor spot.

“By the way, you are not spreading the seeds by failing to bag and remove them, and you are not making the problem worse. What you are actually doing is stopping the spread by using the concentrate, pile, and pin method. It’s not perfect, but it is way, way superior to doing nothing. After all, doing nothing is precisely how these invasive weeds spread! Anything you do to get in the way of that is a benefit to the ecosystem.”

As stated I will write a better-illustrated blog post about weed control soon. There is a lot we can do to manage brand new and small invasions of weeds in otherwise uninfested areas, if we pay attention and use smart methods to stamp them out before they get out of control. I’ve been doing this with Sahara mustards and other weeds for years and it does work with some sweat equity and elbow grease and targeted effort.

Thanks for reading, and thanks for weeding!