Conyza canadensis (aka Erigeron canadensis)

(Conyza is an ancient name for flea-repellent powder from a similar plant.   Conyza canadensis grows from Canada to the tropics.)

Asteraceae

The point here today is to look at the “world’s fastest evolution,” flashing right before our eyes with the native plant Canadian Horseweed.  The species is arguably sort of attractive sometimes.  John Bradford’s close-ups of the flowerheads bolster the iffy case for a little pretty.

The plant starts out as a rosette sprawled on the ground, then rises, often symmetrically and branch-ily, to, oh say, 4-5 feet with leaves at right angles from the stem, and a million little flower heads, white with yellow centers.   The fruit looks like a tiny seed on a parachute.   They blow forth and multiply,  and to help take hold far afield, the flowers can self-pollinate.

Saying Canadian Horseweed is native is an understatement.    It grows across most of North America, and is distributed further, naturally or unnaturally, pretty much around the green earth from near-Arctic to the tropics.   It is everywhere, lots and lots of it, welcome or not.  

Canadian Horseweed is a fearsome pest in farms fields, plant nurseries, orchards, vineyards, and everywhere plants are grown.   They thrive under any conditions: wet or dry? Yep, even desert. Acid? Yes. Alkaline? Sure. Sun? Of course. Shade? OK.  Terrible soil?  No worries. Rich soil? Even better.  I could go on even more boringly, but you get the idea.   Hey, if you have trouble making plants grow, try a Horseweed!  The species is allelopathic, that is, it can suppress other plants, sometimes reducing crop yields as much as 90% by some combination of competition, altering the soil ecology, and allelopathy.

Canadian H-weed got real interesting with the advent of Round-Up Ready Soybeans.   Around 1996, soybeans resistant to the famous herbicide Round-Up came into common use.  The idea was if the crop is immune to the herbicide, then spray a lot, spare the beans, and smite the weeds.  Just three years later, in 1999 there were Round-Up resistant strains of Canadian Horseweed in multiple states.  And we thought evolution takes eons!

If I spray a field inhabited by 10,000 Horseweeds and 9,999 die, that sole survivor has some mutation that renders it immune.  And remember they are self-pollinating, so that lonely mutant can spawn a documented 200,000 seeds, many, most, or all with the resistance mutation.   Repeat that a few generations, and poof!, we have just evolved a whole new strain untouched by Round-Up.  Such untouchable strains are now widespread.

No problem we say, if Round-Up won’t work, switch poisons.   That occurred, and now Horseweeds (not all of them everywhere) are resistant as follows:

Speaking of mass tort attorneys trolling TV for clients, Horseweed is also resistant to Paraquat (and its cousin Diquat).  Atrazine got into litigation when a University of California Professor raised an alarm about deformed frogs, implying deformed humans you might say.   Horseweed resistant?  Yes, as is so for the similar Simazine.

A whole family of herbicides is based on similarity to the plant nutrient compound urea.  Horseweed is resistant to several of these, including Diuron.  A comparatively recent family of weed killers, the sulfonylureas may be the solution?…oops, nope, them too.

All this instant evolution suggests a remarkable ability for the seeds to wander and mix genes with other Canadian Horseweeds.     Not only are the seeds wind-dispersed like so many plants, they are outta this world.   The teensie parachutes zoom at 60 feet/second over flyover country in the atmospheric “Planetary Boundary Layer.”    And, come to think of it, a plant does not have to disperse seeds to disperse its genes, given that pollen grains can have the same effect.  Although the research is preliminary, it seems that may happen too, even if CHW is mostly insect-pollinated.

Why called “horseweed”?  Among many historical uses, the plant was a medicine for horses, probably the reason for the name.   Of those many recorded uses, and I have not tried it personally, Seminole men wishing to be rid of their wife rubbed it on their bodies (on their own bodies, I think)….I don’t know if it worked the other way around too.