…and Barnyard Full of Superweeds, ee-i-ee-i-oooooo
Sow Thistle (Sonchus oleraceus)
Canadian Horse Weed (Conyza canadensis)
Goose Grass (Eleusine indica)
But first a note on our Native Plants On-Line MOOC. The class has filled with 60 apprehensive victims, and is therefore closed. We expect to offer it again maybe in the winter.
A pretty weed distributed throughout all of sunny Florida has earned the distinction of being the newest addition to the International Survey of Herbicide Resistant Weeds worldwide list of glyphosate-resistant weeds. Glyphosate is the active ingredient in the world’s top herbicide, better known as Round-Up. Our newly crowned Round-Up-Resistant (R-U-R) superweed is number 28 on the Survey’s hit parade dating back to 1996. And it is Common Sowthistle (Sonchus oleraceus).
Not that Florida is under-represented by prior R-U-R title holders. Also in our state natively or just visiting are: Spiny Amaranth (Amaranthus spinosus), Tropical Sprangletop Grass (Leptochloa virgata), Jungle Rice Grass (Echinochloa colona), Sour Grass (Digitaria insularis), Johnson Grass (Sorghum halepense), Ragweed (Ambrosia artemisiifolia), Narrow Leaf Plantain (Plantago lanceolata), Canadian Horseweed (Conyza canadensis and a related resistant cousin), Goose Grass (Eleusine indica), and a couple of bit-players. In other words, a big chunk of the world’s R-U-R species are right here in our own back yards. Literally.
And that’s where it gets extra-interesting. If you do not want these in your own back yard, the day may be fast approaching where your turf grass is a Round-Up-Resistant selection, so all you have to do is lay on the Round-Up to eliminate any weeds that have not joined La Résistance. There exist some GMO Round-Up-Resistant turf grasses now, although there have been annoying problems with the resistance genes spreading a few miles from test plots, even to other species. To my very limited Google-influenced knowledge these turf species have not come onto the market, and they are not for South Florida anyhow.
Of course Round-Up Ready commercial crops, including a lot of cotton, have been around a generation, leading to massive Round-Up applications, helping us get those R-U-R weeds. (Interestingly, btw, whatever you think about GMOs, herbicide contamination, and whatnot, did you know Round-Up adds unwelcome yucky-poo phosphate to the environment?) Many weeds have evolved resistance¹ to many other herbicides; in fact, efforts are in progress to develop crops resistant to additional weed-killers, even nasty old infamous 2,4-D and more. But today’s topic is Round-Up Resistance so let’s stay on topic.
I don’t want to delve into the politics of Round-Up and GMOs. Doing that would take all night, and the blog world is loaded with it. If you are interested, there is a relevant article in this week’s New Yorker magazine (8/25/14, p. 46, “Seeds of Doubt”). So just a few more words in our plant blog on the green things.
My first brush with Goose Grass, on the R-U-R list, was in approximately 1995 as a golf course weed. At that time and place, at an expensive Caribbean resort, the control measure of choice was the herbicide MSMA, where the A stands for arsonate, as in arsenic. Arsenic is not nice stuff, unless you want to murder somebody gradually, and arsenic in the soil, presumably from MSMA, caused a flap in Wellington not long ago. That flap was odd—we spew it all over the golf course over umpteen years, then it makes the papers when somebody !!!OMG!!! detects arsenic in the soil. That’s like dumping molasses on the floor and detecting sticky. Back to plants:
Canadian Horseweed is a superweed to know and love just about everywhere. We usually just step on it in passing, or mow it down, or spray it, or curse it, but please show it some respect. You don’t tug on Superman’s cape. Horse Weed was ahead of its time achieving R-U-R superweed status in 1999, and merely 5 years later the resistant strain had spread to a dozen states. Another decade has gone by, and the R-U-R Horse Weed spans a dozen nations from Brazil to China. That’s just one weed and one herbicide and one decade. Use your imagination for the rest of the story.
¹How do you evolve a superweed? It’s Darwinian survival of the fittest. Take a big cornfield, douse it with Round-Up to kill 99.9% of the weeds. The remaining 0.1%, hmmmmm, seem to have some natural Round-Up resistance. (Round-Up interferes with an enzyme, so the tiny minority of resistant plants may have genetically a slightly different enzyme or a slightly different cellular physiological environment or slightly less ability to allow the poison to penetrate.) That small number of naturally resistant plants can make a lot of seeds of course, so the 0.1% expands. Repeated years of applying Round-Up to the same field with the ever-growing resistant population gives repeated years of such intense artificial selection for resistance. It builds up. One day the weed-killer just isn’t doing the trick. An approach to delay that day is to leave some areas near the crop fields unsprayed, so the non-resistant weeds there will cross and dilute the resistance of the spray-survivors. The mechanism for evolving superweeds is how you wind up with antibiotic resistant bacteria too.