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Why and How to Spread Mulch

The base of a tree trunk surrounded by dark brown mulch
Mulch protects trees in a variety of ways.
June 5, 2018

A layer of mulch, if properly applied, is a big help to trees, shrubs, and perennials, according to the Plant Clinic at The Morton Arboretum.

However, it doesn’t necessarily need to be one of the products sold in bags labeled “mulch” at home centers. Those are generally shredded wood or wood chips, byproducts of the lumber industry and the landscaping business.

Many kinds of plant matter can be used for mulch. The Arboretum uses partially decayed leaves for an attractive, dark brown mulch in its perennial beds. Vegetable gardeners often use straw. Compost also makes excellent mulch, especially in garden containers.

Whatever kind of plant matter you use, it’s important not to apply the mulch too deeply. Spread it in a wide, even layer over the roots of a tree or shrub, in a circle that extends at least three feet from the trunk. It should be no more than 3 to 4 inches deep.

Never pile mulch against a tree trunk or the stems of a shrub. Piled-up mulch traps moisture, which encourages insects, disease, and rot that can damage the bark. Small animals such as voles can live in the mulch and dine on the bark. It’s best to keep mulch an inch or more away from the trunk.

Too-deep mulch, especially bagged wood products, can prevent needed water and air from penetrating the soil and reaching the roots.

In perennial beds, mulch should be only 1 or 2 inches deep. A fine-textured material such as shredded leaves is more appropriate in perennial beds than chunky wood mulch.

The mulch will decompose over time. To replenish it or freshen the look of your garden, you can reapply new mulch right over old. Just be sure not to pile it too deep.

Mulch has many benefits. It prevents moisture from evaporating from the soil, so plants’ roots are less likely to dry out in hot weather. It insulates against extremes of cold and heat, such as the hot summer sun and warm spells in late winter. Your plants will appreciate reduced temperature swings. And as mulch decays, it improves the soil.

A circle of mulch around a tree has another important function: It protects the bark from lawnmowers or string trimmers. Cuts in the bark do serious harm to a tree, preventing it from drawing water and nutrients up from the roots to the leaves. Since you don’t mow mulch, the mulch circle creates a buffer zone to keep machines well away.

When we spread mulch, we are imitating the layer of decaying dead leaves, stalks, and branches that accumulate on the ground in nature. Microbes, fungi, and other organisms will break down the mulch, helping to supply nutrients to plant roots. Studies have shown that mulched areas have more soil organisms and are healthier for trees and other plants.

Mulch can deter weeds by keeping weed seeds in the shade so they don’t get enough sunlight to sprout. However, it won’t kill existing weeds that already have roots, so be sure to weed the area thoroughly before applying mulch. Expect to weed a mulched area occasionally, because airborne weed seeds can still land on top.

The Arboretum advises against using landscape fabric beneath a mulch layer. The fabric barrier reduces the flow of air and water to plants’ roots and keeps them mulch from decaying into the soil to improve it. It does not permanently stop weeds, which can still sprout from seeds that land from the air.