I put a dark plastic over the Tiger Lillies last Sept. 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 not easy to kill off and patience is required. I suggest that you remove 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.
It will take this growing season and possibly until next spring to successfully eliminate your tiger lilies.