During last week’s mid-January tour of Sonoran Desert wildflowers south of the border, I came across a wicked-looking, thick-stemmed, and short-stature grass growing upon the sand dunes amidst the friendlier and showier blooming plants. Growing about a foot across and 3 to 5 inches high, it was covered with large prickly burs reminiscent of a medieval mace weapon.
Cenchrus palmeri, the Sonoran sandbur.
This was a fine example of a Sonoran sandbur, Cenchrus palmeri. The genus Cenchrus is globally distributed and contains several dozen species of varying hostility, but a uniting characteristic is the presence of spiky-coated seeds. Some species such as field sandbur (Cenchrus longispinus) are widespread across the USA and are familiar to people in rural agricultural and rangeland areas, where they are encountered as an undesirable component of hay mixes for livestock or as pests in crop fields. Sandburs can also grow in lawns and weedy vacant lots, where the painful burs can easily puncture bare feet or be tracked into the house on the soles of shoes or by pets.
The fruiting bodies of Cenchrus palmeri are nearly an inch in diameter from spine tip to tip.
The burs of Sonoran sandbur are the largest in the genus, measuring between 3/4 inch and one inch (18-25 mm) across the ball, with dark, rigid spines that have a very sandpapery feel and a needle-sharp point. This rough texture is not dissimilar to the feeling of certain cactus spines, particularly ones of the genus Cylindropuntia (i.e. the chollas), and is likely caused by the same phenomenon of microscopic reversed surface barbs on each spine. The sharp-tipped, barbed, and spiny fruits of sandburs are adept at clinging to shoes, clothing, feet, and fur, which aids in dispersal. Just as it does with the unrelated cholla cacti.
An expansive field of Sonoran sandburs covering thousands of acres in northwestern Sonora, Mexico on January 20, 2019.
When I first encountered Cenchrus palmeri it was well past dark on the dunes of the Gran Desierto de Altar, or Great Altar Desert, which is the driest and sandiest subsection of the Sonoran Desert wrapping around the head of the Gulf of California (Sea of Cortez) just south of the border with southwestern Arizona. The first two photos of this blog showing the closeup details with my hand for scale were taken with a flash at night. While there were some sandbur plants on these dunes, they certainly were not extremely common either. It was only two days later when I headed farther southeast from the beach resort town of Puerto Penasco that I came across much more extensive carpets of it.
An impressive stand of Cenchrus palmeri in northwestern Sonora.
Once I recognized what I was seeing, I decided to stop the car and take some photos of this mass display. My true purpose on this several day long excursion was to find and document colorful desert mass blooms brought about by the heavy rains of October 2018, courtesy of Hurricanes Rosa and Sergio; and indeed I did find such displays, which will be treated in a separate post. But those heavy rains (anywhere from 3 to 6 inches, 75 to 150 mm) also encouraged abundant growth of the sandburs, in numbers that are probably not seen every year. I don’t see how anything can walk around in here without suffering – the dense mats of sandburs must discourage every sizable mammal (such as coyotes, kit foxes, jackrabbits, livestock) from attempting to cross.
Sandburs doing their sandbur thing: Trying to disperse.
It’s no secret that I actually like spiny plants, and that the spinier something is, the more partial I am likely to be to it. (This doesn’t mean that I dislike not-spiny plants, just that spiny things are pretty cool to me. 😉 ) So upon first seeing the Sonoran sandburs on the nighttime dunes, I considered collecting a few seeds to try growing back at the cactus ranch. I decided to pass for the time being, but may yet change my mind. However upon seeing how extensive and dominant they were farther south and east at this new locale which I was seeing by day, I thought, “Maybe I don’t actually want to introduce this plant to my gardens after all!” I didn’t collect any. Well, not on purpose at least….
An effective means of spreading themselves around. Being vicious, mean, and clingy are bad interpersonal relationship traits, but useful ones for surviving and dispersing in the desert.
However upon returning home and doing some identification and research on Cenchrus palmeri, I found that this plant is actually endemic to the Sonoran Desert, making it one of only five grass species that holds that designation. It has been found in low desert and coastal zones of only four Mexican states (Sonora, Sinaloa, Baja California, and Baja California Sur) and one single small region southeast of Yuma, Arizona, all centered around the Sea of Cortez.
Range map of Cenchrus palmeri, Sonoran sandbur, courtesy of SEINet, the Southwestern Biodiversity Network.
I also found out online that the lone Arizona population was discovered very recently, only five years ago, on January 19, 2013, on sand dunes about 12 miles southeast of Yuma by botanist Jim Malusa. Located on the Barry M. Goldwater Range, a unit of the U.S. Military, the small population there was estimated to be at between 500 and 1000 individual plants, and is suspected to possibly have been brought in by undocumented immigrant activities in that sector of the border. Given how clingy and persistent the sandburs are to shoes and clothing, this is not an unreasonable hypothesis.
A screenshot of the document I found online from Jim Malusa’s discovery of the Sonoran sandbur in Arizona for the first time in January 2013.
Jim Malusa says in his document online that the plant is frost-sensitive and dies with cold weather, which if true (as seems probable) would explain why it is not found farther north or more inland at higher elevations away from the warming influences of the coastal waters. After all, if it is a Sonoran Desert endemic that could tolerate some freezing, it would almost surely be found more widely in interior Sonora, Arizona, and southeastern California which are all Sonoran Desert regions, albeit slightly frostier ones upon occasion.
Well, maybe in that case I can’t grow it in Yucca AZ regardless, because it definitely freezes hard here every winter, about 200 miles north of and 2500 feet higher than its natural range. Despite the fact that as a child and teenager I detested stepping barefoot upon the smaller but otherwise equally painful sandbur fruits of the weedy Cenchrus longispinus growing up in Colorado, there’s something about this dark-tinted, relatively gigantic, and grudgingly charismatic desert grass that intrigues me enough to maybe give it a try anyway. Maybe one will voluntarily hitchhike along with me on my next trip?
Thank you for the educational (and at times entertaining) information. I wonder if the rabbit and other small rodents and reptiles are more sparse where there are concentrations of the “The Meanest Grass Imaginable.” Walking on it with appropriate attire would present some degree of satisfaction … sort of like crunching on Doritos!
I wonder if rodent populations are affected too. It could go either way. If the seeds are a good food source, and predators are discouraged by the numerous sharp spiky spheres, then their populations could be higher in zones dominated by Cenchrus. If they aren’t the best food source, then perhaps they might be reduced? I don’t think the size of the rodents themselves will matter that much because they are probably light enough in weight to not suffer from the spines, in the same way that they can perch upon cholla plants and not be impaled. If anything I suspect that discouraging land-based predators, primarily coyotes, bobcats, and kit/gray foxes, that the abundant seeds would result in a better than average rodent population.