I've had no practical experience of ground elder weed, but I'll pass on the info that's in a couple of my gardening books. The main thing about ground elder is that it really needs to be dug up, and every fragment of the roots must be removed. The roots snap easily, and any little bit that's left behind means you get a new plant. If the weed has worked its roots into a plant that you can't, or don't want to, lift to get at it, hoe it and hoe it again and again until the weed dies from lack of nutrients that it gets from the air. One of my books was published in 1969, so the next bit of advice might be illegal by now (!). Try a hormone weedkiller of the 2,4-D type. Two or three dressings in a season are necessary. That's it from my books. Best of luck.