It is getting closer to that time. Allergy season. If you are not horribly allergic to the Giant Ragweed, you probably know someone who is. This plant has caused so much suffering that it is nearly universally despised.
Last year I decided to champion the lowly hackberry (Celtis). This year’s project: ragweed, a much tougher sell. Like most people you probably hate ragweed, but maybe I can help you admire just how well adapted it is for survival. And though it is the bane of so many people, it is extremely important for wildlife.
Why Is Giant Ragweed So Invasive?
Remember when Terence McKenna said plants invented animals to do their work?
Well, Giant Ragweed must have invented two powerful friends: Humans and Earthworms. The two organisms work in perfect harmony to ensure that Giant Ragweed can grow happily and abundantly ALL OVER THE PLACE.
Naturalized European earthworms (Lumbricus terrestris) think Giant Ragweed seeds are exceedingly delicious. They will tirelessly hunt the seeds down by smell. Earthworms won’t eat those seeds right away — they can’t. Instead, they cache the seeds deep underground and wait until the hulls decompose into a tasty slime. That might not sound appetizing to you but earthworms are picky eaters. Decomposed is the way they like things; in fact, they think the inner seed is disgusting and inedible so they just leave those bits behind. They kind of remind me of those people who need to cut the crusts off their bread.
This behaviour creates a handy seed bank in the soil. Those deposits will bide their time waiting for humans to disturb the earth. And humans love to dig. They dig into their gardens, they plow up fields and bulldoze construction sites. All that turned earth brings the banked seeds up to the proper germination depth. Next thing you know weeds seem to be growing everywhere.
Here’s a little movie that shows an earthworm gathering ragweed seeds. I haven’t decided if I find the accompanying music or the comments more amusing.
Giant Ragweed seed hulls are hard and need to be weathered away. So hard that the seeds can remain viable for several years. The embryo can also remain dormant. Those strategies give the plant many opportunities to wait out bad growing conditions.
Multiple Strategies Ensure Success
The plant is just as tough as the seeds. Shade. Sun. Who cares? And as long as it can get some moisture it seems to be able to grow in most soil types. Ragweed is native to continental North America and seems to grow pretty much everywhere with the possible exception of Nevada. And it seems to be spreading across the globe. I once saw a Giant Ragweed complaint that came from Korea.
Giant Ragweed plants don’t need pollinators, a breeze will do the trick.
And Giant Ragweed is one of the infamous resistant superweeds everyone was talking about awhile ago. Dosed with poison (even at 5x the recommended application) a genetically resistant plant will just grow extra shoots. At maturity that resistant plant will produce just as much leaf mass, pollen and seed as a plant left untouched.
What is it good for?
Yeah, I know. You probably channelled Henry Rollins’ emphatic: ABSOLUTELY NOTHING!
Ecology of Giant Ragweed
As an early succession plant, Giant Ragweed is important for stabilizing disturbed soil. It is one of those annuals that can grow quickly and abundantly. I have seen it grow more than ten feet tall in some places. The roots hold onto soil in danger of erosion. While alive the leaves offer shade and wildlife habitat. When they die, those leaves return organic material and nutrients to the soil. Giant Ragweed is sort of like a soil band-aid.
Because it is tall enough to stand above the snow pack Giant Ragweed is especially important as a winter food source in colder areas. Besides earthworms, many animals use it as a food source.
- Insects: moth caterpillars, aphids, the bordered patch imago, hover flies and various grasshoppers. Honey bees love ragweed pollen.
- Birds: the Greater Prairie Chicken, Dark-eyed Junco, Brown-headed Cowbird, Northern Bobwhite, Purple Finch, Mourning Dove, American Goldfinch and the Red-bellied Woodpecker
- Mammals include the Eastern Cottontail, Meadow Vole, deer, horses, sheep and domesticated dogs. Long ago it was a source of food and medicine for humans. The oil content of the seeds compares to soy. The resin is anti-bacterial and anti-viral
The most interesting thing I learned about Giant Ragweed is that it is self limiting. When the plant dies the roots release a toxin that only seems to affect ragweed itself. When the concentration gets high enough the ragweed dies out making way for new plants to take its place. So really. Please keep the herbicide locked up. It really does make things worse.
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