I put a dark plastic over the Tiger Lilies last Sept. that I no longer want, but it looks like they insist on growing. They are coming up yellow though not green. Of course, there are a few that still are coming up on the sides of the plastic. Should I still keep the plastic on them and for how much longer?
Thank you for your question. Tiger lilies are an invasive plant in Ontario.
The most effective way to eliminate the lilies is to dig up the bulbs making sure to get all of the clumps. This would involve digging up all of the bulbs which are probably 6-8 inches below the surface.
This article from a Peterborough Master Gardener provides a lot of detail on this invasive plant. Ditch Lilies – A Cautionary Tale
If the area is very large and you are unable to dig up the bulbs, you could try removing the dark plastic you have currently in place and cut down to the ground the lilies that have emerged under the plastic. Then put your dark plastic back on and weight it down with a mulch so that there is no chance of light and oxygen reaching the lilies. You might want to use a slightly larger piece of plastic to ensure that none creep out from under the edges.