Mulching to suppress grass between trees

We planted 46 dappled willow trees in a zigzag pattern about two years ago. The trees have a six foot spacing with quite a healthy lawn in between. We would like to place mulch over the grass between the trees…

How can we keep weeds from regrowing?

The backyard was cleared of weeds last fall but the roots are still in the ground. What is the best way to cover the ground to avoid regrowth of all the weeds – black plastic or other mulch?    …