GARDEN CAPSULE
The challenge: You know mulching trees is supposed to be good for them, but you’re not sure how much to use. Some people use just a shallow blanket of wood chips, while others heap mountains of mulch piled against tree trunks.
The solution: Tree experts recommend applying a blanket of mulch only 2 to 4 inches deep and never allowing the mulch to be in direct contact with the tree’s trunk.
Pluses: When mulching guidelines are followed, the mulch helps by suppressing weeds, keeping the soil from drying out too fast and moderating soil temperatures. Mulched trees are freed from having to compete with the lawn, and their trunks are protected from accidental damage by a lawn mower or string trimmer. Limiting the mulch layer to no more than 4 inches deep is also a good first step toward preventing an annoying problem with artillery fungus. This fungus grows in deep wood-chip mulch, and its sticky spores can shoot 20 feet through the air, covering house siding, cars or even the foliage of plants with what looks like black tar spots.
Minuses: Too much of a good thing isn’t better: if mulch is piled on too deep and too close to the trunk, the bark will stay too wet, inviting disease and decay. Insects and rodents are more likely to attack the wet bark. Needed oxygen can’t reach the tree’s roots or covered bark. Rains may not be able to penetrate the thick, matted mulch layer to reach the soil or conversely, after heavy rains, the soil might stay too wet for too long.
Feeder roots grow in the mulch instead of the ground, where they belong. A big, decaying mulch mound can even heat up so much it “cooks” the delicate tissues in young tree trunks.