|I am in the process of lifting all my plants out of a 16x20 raised rock bed that has serious witch grass problems. What is the best type of plastic to use and how long should it take here in Maine? Is there much difference between clear and crystal clear for this application? Thank you|
|Soil solarization does work, but it takes several months and needs to be done during the hottest months of the summer. Clear plastic is the best type of plastic to use for solarization. It allows sunlight to penetrate and heat to build up, effectively killing existing weeds and weed seeds in the top few inches of soil. The problem with witch grass (also called quackgrass) is that it grows from underground rhizomes, some of which are 12-18 inches deep. Solarization probably won't kill the rhizomes that are that deep. A broadleaf weed killer such as glyphosate (Round Up) can be applied to the actively growing weed. It will be translocated down to the roots and will kill off that part of the rhizome. The hitch is that there are many latent growth buds within the rhizome and not all of them will be affected by a single application of the herbicide. This means multiple applications will have to be made. Grass B Gon is another herbicide that does a decent job of controlling quackgrass. It targets only grassy weeds so it is reasonably safe to use in a flower bed, as long as you target the quackgrass and not your flowers. Again, repeat applications may have to be made to completely eradicate the weedy growth.
Another method of suppressing weeds is to lay an organic mulch over the bare soil to keep sunlight from penetrating, and keep weed seeds from sprouting. If you apply 3-4 inches of compost, leaf mold, or other organic material now, you can move it to the side when you plant this spring. Laying several thicknesses of newspaper first, then covering with organic matter is another method of killing existing weeds and keeping the area weed free.
Whichever method you use, I wish you good luck with your flower beds.