A supervillain has invaded our back yard. Its name is MUGWORT.
Mugwort, for those who don't already know it and curse its name, is the weed that makes all other weeds look cute and cuddly. As far as I can tell, it can grow just about everywhere and outcompete just about everything. It starts out fairly short and sprawling, but if left to itself, it will grow knee-high or taller, turning the whole yard into a jungle that has to be beaten back with a machete. But the worst thing about it is that once it's taken root, it's all but impossible to remove. It spreads by long, underground runners, so a single plant can send up dozens of new shoots. When you try to yank out a mugwort plant, instead of resisting until it finally comes loose, it will seem to come up right away—but then you'll find yourself dragging up a long attached tendril of root, like an umbilical cord, that seems to have no end to it. You just keep pulling out more and more of it until it eventually breaks, leaving a living end from which a new plant can grow. This makes trying to pull up these weeds by hand like battling a hydra: every time you remove one head, two more pop up in its place.
Unfortunately there's not much else you can do to remove them, either. A Google search on "how to control mugwort" yields a bunch of documents of varying credibility, but the gist of all of them is, "Well, it's not easy." This document from the Cornell Cooperative Extension, after noting that hand-pulling doesn't work well because "new plants quickly emerge from rhizome fragments, just like the brooms in 'The Sorcerer’s Apprentice,'" goes on to say that most herbicides won't work on the stuff either, and the few that do will kill everything else in sight as well. It suggests "mulching beds to reduce their growth," but based on my own experience, I don't think much of that strategy. See this fine specimen, here? It's growing up out of the slope on the northern side of our yard, which we pretty much smothered under a huge pile of dirt last year as part of our patio project. The grass and the other, tamer weeds haven't reappeared, but the mugwort has already pushed its way blithely to the surface, stretching its little green arms and singing, "Oh, What a Beautiful Morning!"
The only remotely helpful suggestion I've seen was in this article from the National Gardeners' Association. It says there are two effective ways to deal with perennial weeds like mugwort: smother them or crowd them out. Smothering the mugwort under heavy plastic won't really work for us, since it's spread all throughout the yard and we'd have to smother everything else along with it. However, the suggestion of "cover cropping" with another plant to keep mugwort out of garden beds caught my attention, because there actually are a couple of small spots in our yard where there isn't any mugwort visible. One is in a shady spot where most of the ground is covered by wild strawberries, and the other is in a sunnier area where all the microclover blend we planted last fall seems to have settled. I was initially disappointed with this stuff, thinking that it had only taken root in patches, leaving other areas bare—but now, seeing how thick is is in this low-lying patch, I suspect what happened is that the seed all washed off the slope and settled in this one area, where it eventually grew in thickly enough to crowd out even the mugwort.
So I suspect our best hope of dealing with this supervillain weed in the long term may be to try and wipe it out by attrition. Each spring and fall, we pull up all the large mugwort plants we can find, leaving bare patches behind; then we fill in all the bare patches with the microclover seed, spreading it as thickly as possible and giving it plenty of water and compost to encourage it to take root as quickly as possible, ideally before new mugwort plants can grow up from the broken-off runners. If we keep diligently following this program over the course of several years, we can at least hope that the microclover will eventually become well enough established to crowd out the mugwort completely. If it also crowds out some of the other weeds—the dandelions, the purple deadnettles, or even the wild strawberries—then we'll consider that a nice bonus, but frankly, at this point, we're not even going to worry about them. In the face of the Merciless Mugwort, all other enemies must be ignored as mere distractions.