Autumn is a good time to spread mulch around trees

Fall is an excellent time to spread mulch over the roots of trees, shrubs and perennials. “It will benefit plants at any time of the year,” said Julie Janoski, Plant Clinic manager at The Morton Arboretum in Lisle. “It’s just convenient to spread mulch in autumn when there are fewer other garden tasks to be done.”

Mulch does plants good in many ways. Basically, it imitates the accumulation of leaves and other debris that fall to the ground in nature. Like that layer of dead leaves in the woods, mulch protects and enriches the soil where plants’ roots live.

Over time, a mulch layer made of wood chips, shredded wood, leaves or other plant materials will decay into the soil, feeding beneficial organisms and adding nutrients that plants need. Studies have shown that trees surrounded by an area with a layer of mulch grow better and stay healthier.

The mulch layer insulates the soil against extremes of either hot or cold. It keeps moisture from evaporating from the soil surface, so more water stays in the soil where plants’ roots can absorb it.

A zone of mulch around the base of a tree fends off lawn mowers and string trimmers, which can seriously damage a tree by cutting into the bark. Where there is mulch, there is no grass, so the mulch zone reduces competition between turf and tree roots.

However, it’s important to spread mulch the right way. “Mulch can actually cause problems if you apply it without understanding how to do it correctly,” Janoski said. “Don’t just do what you see your neighbors do, because there’s a lot of confusion about applying mulch.”

Here are some tips for applying mulch so it helps your trees and other plants.

Make a wide, even layer. It’s better to cover a larger area of soil and roots than to pile the mulch deeper. Around trees and shrubs, mulch should be no more than 3 or 4 inches deep. In perennial beds, 1 to 2 inches is sufficient. “If you pile mulch too deep, it will block air and water that need to reach the soil so roots can absorb them,” Janoski said.

Don’t make a pile. It’s common to see a mound of mulch around the trunk of a tree. “That’s doing the tree more harm than good,” she said. Mulch heaped up against the bark will trap moisture and encourage disease and decay. It will also give cover to insects and burrowing animals such as voles that eat the bark. Animal damage can lead to wounds that admit disease-carrying organisms into the wood. “Piling up mulch is a very common mistake,” Janoski said. “People see heaps and mounds around trees so often that they assume it’s the right way to use mulch. It’s not.”

Keep mulch clear of the bark. After you spread a wide, even layer, use a rake or your hands to pull the mulch away from the bark all around the base of a tree or shrub. “You should actually be able to see a band of bare soil 3 to 5 inches wide between the mulch and the tree,” Janoski said.

Think beyond a ring.

You don’t need to make a perfect circle of mulch around each tree. “Plants don’t care about geometry,” Janoski said. “As far as they’re concerned, the more of their roots are covered, the better, but the shape doesn’t matter.” Mulch can cover a wide area that includes multiple trees and other plants. “It’s fine to grow shade-tolerant species of ground covers, perennials and even small shrubs under a tree,” she said. “They will appreciate a layer of mulch as much as the tree does.”

Expand your idea of mulch. Bags labeled as mulch usually contain shredded or chipped-up wood, typically byproducts of lumber production. You can also buy that type of mulch in bulk by the cubic yard from landscape suppliers. “If you like a tidy look, you may want to use the same kind of commercial mulch all over your property,” Janoski said. “But as far as soil and roots are concerned, anything made of dead plants makes a beneficial mulch.” Good mulches include fallen autumn leaves, compost, and chipped-up wood from pruning or tree-removal crews.

“All kinds of plants, from perennials to trees, grow best in soil that’s covered by a layer of decaying organic matter,” Janoski said. “It can be mulch bought in bags or the leaves that fall from your trees, just as it is in the woods. The plants aren’t particular.”