Q: A few years ago, I threw in the towel when it came to fighting bishop's weed (Aegopodium podagraria), aka snow-on-the-mountain, and, to my surprise, the subsequent growth has helped to give our home a woodsy cottage look. The only thing is, every midsummer the plants begin to wither and thin dramatically, in spite of weekly watering. Is there any way to keep the plants green and vibrant until the fall?
A: Bishop's weed, also sometimes called goutweed, is touted as a fast-growing, low-maintenance ground cover. Reaching 18 to 24 inches in height with white, umbrella-like flowers in mid-summer, this plant may sound like a gardener's dream. But here is a perfect example of a plant backed by expert marketing rather than common sense. As you discovered, bishop's weed is extremely aggressive, crowding out more desirable plants, spreading into the lawn, and even popping up in driveway cracks and sidewalks. This is not a plant I suggest you intentionally grow.
Your throwing in the towel is a common practice among gardeners facing a battle with such an invasive species. I don't blame you and have thrown in the towel myself on plants like pink evening primrose, ribbon grass and goose-neck loosestrife, all of which have similar qualities. I strive to manage them rather than eradicate them since the latter practice is often a frustrating and never-ending battle.
Since you have decided to live with what you've got, lets work on what to do to keep the plant in better shape through the summer months. Bishop's weed doesn't like to be dry. It's great that you water it, but you cannot allow the plants to dry out, even once, after the hot weather sets in. A single brief period of drought is enough to brown out the plants and prevent them from looking "fresh" for the rest of the season. Another decision to take into account is whether or not the excessive use of water they require is worth the cost and effort. Though the plants may look a bit rough around the edges during the summer months, it won't hurt them. Trust me, they will always return next spring.
Another option you have is to regularly trim the plants back to the ground. Once the foliage has browned, cut it off. The plants will go into a sort of dormancy if the weather remains hot and dry. After a few good rains, new, green growth will appear. Again, this practice will not hurt the plants and you'll see them return next spring just as aggressive as ever.
