I have been dealing with this invasive plant for 4 years since we moved into our current home. It got worse each year. I finally have to use RoundUp as it has become unmanageable. My question is : Should I dig up the dead ones before I plant over the site? Please advise.
Hello – I can sympathize as I have been dealing with lily of the valley in my garden for many years. As you know, if you dig them up, you often break the root into pieces and from these pieces grow more plants. You have to remove all of the root. You will have a similar issue with the use of Roundup. Roundup is a systemic fertilizer which is absorbed by the foliage and translocated to other parts of the plant. So the issue will be whether the herbicide had reached and killed all of the root.
My own method when I’m creating or extending a garden bed is to dig them up and then cover the area with several layers of overlapping newspaper, wet the newspaper and then cover with 10 cm of mulch. I let this sit for a season or if I’m doing this in the spring, I’ll plant in the fall. I’ll still get the odd lily of the valley popping up but these I just cut off just below the soil surface to avoid disturbing other plants.
So I would recommend digging out what are hopefully dead plants. Removing that fibrous root mass will make replanting easier in any case.
Note that we would never recommend the use of Roundup for removing lily of the valley. The use of herbicides such as Roundup for cosmetic purposes in lawns and gardens has been illegal in Ontario since 2009 because of the related health issues.