Q: We planted a maple tree in our front yard a few years ago. It's gotten big, and the grass doesn't grow well in its shade. I'd like to just remove that grass and put down mulch rather than dealing with the frustration of trying to grow grass in the shade, but how do I kill the grass without hurting the tree? I see some websites that suggest laying sheets of wet newspaper over it and then adding wood chip mulch. Will this kill grass without harming tree roots? Would it be better to put a ring of landscape bricks around it and fill it with topsoil?
A: It's a good idea to surround trees with mulch rather than making them compete with grass. This is especially true for young trees. Grass takes up nutrients and water that the tree needs, and the things we often do to turfgrass - adding herbicides, mowing, and weed whipping - can result in injuries to trees.
Mulch helps retain moisture, keep temperatures even, and suppress weeds. Trees surrounded by mulch grow faster than trees surrounded by turf. So it's a good idea to remove grass before planting a tree, and put down mulch once it's planted. But what to do if a tree is already established, as yours is?
Let's start with a couple of things not to do:
Don't till around a tree, as this will damage its roots. Tree roots are close to the surface.
Don't surround the tree with landscaping bricks and backfill with soil. Burying the roots deeper than they are naturally can kill them, and soil should never be piled around the trunk of a tree. The root flare at the bottom of the tree should be visible.
The newspaper trick you mention can be effective in killing grass, but layers of newspaper don't breathe well and don't let enough moisture through to keep your tree happy.
A better approach is to put down a thick layer of wood chip mulch. Washington State horticulture professor Linda Chalker-Scott suggests 8 inches or more. This is more mulch than you'd normally put around a tree. Once the grass is dead, after several months, you can remove some of it. Eventually, what you want is a layer 3 to 4 inches deep.
When mulching around your tree, be sure to keep the mulch several inches from the trunk. Sometimes you see mulch piled up around the trunk of a tree, but this is a bad idea. Mulch "volcanoes" can cause tree bark to rot, and can even kill trees.