Adding plastic mulch AFTER planting?

We have a young permaculture orchard, which was mulched with composted hay around each tree last year. However, the quackgrass and agrimony have grown up so tall around each tree this summer, through the mulch, you can hardly see them :-/ Not to mention, we have a yellow jacket hive IN one of the mulch piles, making it impossible to get near that tree. I've watched Stefan's Permaculture Orchard more times than I can count and after reading his responses on this forum post, I'm strongly considering getting black plastic mulch to lay down next spring.

My question is: how would I lay down the plastic around the trees/shrubs/horseradish that's already growing? Cut a slit all the way to the outside edge and slide it around the tree base? It seems like that slit would be constantly blowing open in the wind and letting grass grow through it. It also probably doesn't help that some of the shrubs were planted offset of the line of trees, the idea being they will hopefully get more sunlight; so it won't be as simple as running two strips of black plastic down, with them meeting/overlapping in the middle around each tree. Our local Menard's also has an 8 mil, UV stabilized sheeting that comes in 8' wide rolls, so we could, mathematically, just run one strip down each row of trees.

hau Lindsay, yes you would cut a slit and slide it around the trunk, then you would pin it down with some wire pins which you can make by taking a length of 10 gauge wire, bending it into equal halves and pushing it in the soil to hold the plastic down.
Don't forget that you will still need to put a mulch layer on top of this barrier but it can be a thin layer.

Our local Menard's also has an 8 mil, UV stabilized sheeting that comes in 8' wide rolls, so we could, mathematically, just run one strip down each row of trees.  
 

That will work nicely if you aren't wanting anything between the trees. Of course you and always plant things in poked holes in the spring too.

When I mulch trees I use enough to get a 4 -6 inch thick layer once it is compressed tightly together.
Don't forget to give the trunks some breathing space (about 6" away from the trunk is good) so you don't create disease issues for your trees.

For the yellow jackets, waiting until a good cold snap is always the easiest way to take care of those, they won't be able to move so much and a lidded container is helpful if you don't plan on extermination.
If you plan to exterminate then you will still want them to not be moving a lot, after dark is a good time to get them, they will all be on the nest.

Redhawk