Q. About five or six years ago, we noticed some ground cover that was not there before. It turned out to be Bishop's weed. We do not want Bishop's weed in our garden. I've been trying to pull out as much of the root as possible, but this has gotten harder every year, and it's already coming up all over—in between everything else, which makes it even harder to pull. It's out of control, and we're concerned that it will kill the plants we do want. Is there any easy-on-my back, non-chemical way to get rid of this weed that some people call ground cover?
---Abraham in Burtonsville, MD
I've been fighting the noxious plant known as Bishop's Weed for many years. I'm convinced it came with some iris and daylilies from my grandmother's garden; apparently it was once a popular bedding plant. It is just now emerging from the ground, and I'd really like to get a jump on it this spring. I have won small battles forking the ground and then sifting through the soil to get every piece of the white roots. My aunt, who also had this problem from the same source, swears she eradicated the pesky plant through diligent weeding— preventing the leaves from feeding the roots. I don't want to use Roundup because the plant is intermingled with a particularly wonderful border of native shrubs. It has been least successful in the vegetable garden where I attribute the cyclical turning of soil to keeping it at bay.
---Jill in Unionville, PA
A. No—frequent tilling makes weed problems worse. It's probably more that this incredibly invasive ground cover thrives in wet shade, and one hopes that your vegetable plots are well-drained and sunny.
Now, this plant—like many so-called invasives—did get its start as a very popular, deliberately planted ground cover. Also known as goutweed (which would lead you to believe it's a folk medicine, but the name is actually a corruption of 'goatweed', as goats love to eat it) and Snow on the Mountain, it has attractive leaves and umbeliferous flowers, like those of Queen Anne's Lace. Those flowers are great attractors of beneficial insects, and the plant itself thrives in the shade, spreads to cover bare ground at a rate of three feet a year, needs no care or feeding…
…and like virtually all 'fast growing, no-care' plants, quickly becomes a problem for gardeners who didn't understand the true nature of this otherwise-excellent ground cover (and/or received it accidentally).
Now, you two are right on the money in your timing; the Federal Bureau of Land Management's Plant Conservation Alliance notes that you have the best chance of control when the weed first wakes up in Spring. In an excellent web page on this weed with many names, they note that:
"Preventing goutweed from photosynthesizing in early spring (at the time of leaf-out) can control the plant by depleting its carbohydrate reserves. This can be accomplished by cutting all plants once they've fully leafed out with a mower, scythe, or weed-whacker, and then covering the area with plastic." Attacking the plants later in the season, after they have acquired substantial food reserves, is much less effective, they warn.
This advice—which supports the theory of Jill's victorious aunt—is much like my favorite plan for controlling the Godzilla-level invasive plant, running bamboo: Allow the plants to grow a little bit and use up some of their root energy, then cut off all the leaves before they can collect any solar energy. Covering the area with plastic afterwards isn't a viable option when the weed is next to wanted plants, so I would instead prescribe a steady schedule of cutting—say weekly—especially early in the season. Maybe hire and train a neighborhood kid to cut down Bishops for an hour or two once a week. Over time, this technique is potent enough to eradicate the notorious running bamboo, and I suspect the Bishop will only hit the canvas faster.
Another option is digging out every last bit of rhizome, but I prefer the somewhat lazier method of death by a thousand cuts. It may take a couple of seasons to be completely done, but it's less back-breaking work and gentler on nearby plants.
An alternative method would be to cut the young plants back a few times early in the season and then spray or paint new shoots with a non-chemical plant-killer, like herbicidal soap or one of the new herbicides whose active ingredient is iron. Do this when the plants and soil are dry (because this weed likes it wet).
And thank you for saying no to Roundup, which is deadly to pest-eating amphibians like toads and frogs and may well wreak havoc with the hormonal systems of people and pets..
Q. I have been battling Bishop's Weed for several years and have had no success. In fact, it's more widely spread than ever! I've put lots of it in my compost pile. Could the pulled-up roots still be viable after months in the pile? Have I contaminated the pile?
---Pat in Havertown, PA
A. More like you've been using that compost pile as a nursery for invasive plants. 'Normal' weeds can be safely composted as long as the pile contains lots of well-shredded dry brown material and the weeds haven't set seed yet. But trying to compost aggressive invasive rhizomes is bound to come back and bite you on the Bishop.
The book says to destroy any life in the roots of pulled plants by laying them out on concrete in hot sun for a week. And then throw them in the trash.