Will milkweed shoot through cardboard?
I have a beautiful, large patch of showy milkweed (Asclepias speciosa) that I started from seed about five years ago. It has propagated well, expands every year, and even gets pollinated so new seed is entering the patch most years as well. The downside is that it is surrounded by quack grass and vetch which are invasive in my region.
I have recently dug up as much of it as I can, including roots. I live in zone 5 and it snows each winter (though we are getting less and less snow each year). I want to cover the newly exposed soil where the vetch and grass were with something this year to try and kill it off. I can see the majority of milkweed shoots that are going to come up for the season so I can easily leave them poking out of whatever I use.
If I use cardboard and covered it with mulch, would new milkweed shoots be able to push through next spring? Knowing they send out lateral rhizomes and new growth won't come out at the exact same spot each time, I'm not sure. I would prefer this method than plastic as it's nicer, and I would definitely have to remove the plastic in spring to let new growth of the milkweed up. I also plan on planting some more native perennials in the area once the grass and vetch is dealt with. Will this kind of coverage harm the milkweed roots underground?
Another caveat is that my soil is contaminated with lead. The milkweed patch actually has the lowest levels in the garden because I removed the sod when I originally planted this area, but it's still there. I don't need to replace my soil, I just don't like to keep it exposed for long periods of time.
Thanks!!