Too Mulch Of A Good Thing: How Much Mulch Is Too Much?

A circle of brown mulch around a white tree, surrounded by grass
(Image credit: sanfel)

The benefits of mulch are so well known that the current question is no longer “should I use mulch?” but rather “how much mulch should I use?” This is a valid inquiry, since using too much mulch can hurt trees and plants. How much mulch is too much? Can too much mulch kill a tree? Read on for information on the disadvantages of mulching too much.

How Much Mulch Is Too Much?

Every article about mulch extols its virtues, and there are more than a few. A layer of organic mulch not only keeps down weeds and locks in moisture, it also insulates the ground from cold and hot weather. Finally, it disintegrates into the soil, providing both nutrients and organic content.

But too much mulch around trees or too much mulch in flower beds can create serious problems. A thick layer of much can smother small plants and even trees. The amount that will be detrimental depends on the plant. In warmer climates, too much mulch can prevent a plant from experiencing a hardening off process that prepares them for winter.

Mulch Volcanoes

One detrimental practice in applying mulch is descriptively termed “mulch volcanoes.” This is when the gardener piles mulch up against the trunks of trees and shrubs in a pyramid shape. This is always bad since mulch holds in moisture. That means that the bark beneath the mulch is constantly moist. The practice results in decay to the tree or shrub and can also provide a point for diseases and pests to enter.

This can be easily solved, however. Never pile mulch up against the trunk or stem of a plant. Rather, keep it back by at least 3 inches (7.6 cm).

Disadvantages of Mulching Too Much

Excess mulch can smother plants in a number of ways in addition to the damage done by mulch volcanoes. For example, if plant roots grow into thick mulch when it is wet, they will die once it dries out. Likewise, too thick a layer of mulch can deprive a plant of water if the gardener only waters lightly, since water may not penetrate beyond the mulch. And deep mulch can cause roots seeking oxygen to circle around in the mulch and girdle the tree or shrub.

Another disadvantage is mulch's tendency to attract rodents. Too much mulch makes an excellent hiding place for voles or other rodents that gnaw on the shrub bark. It provides a hiding place for voles, a rodent that gnaws on bark, and girdles trunks and the lower stems on shrubs.

Proper Mulch Thickness

There are several thickness rules for mulch that are useful to keep in mind. First, never apply more than 2 or 3 inches (5 or 7.6 cm) of organic mulch to any plant. Spread it to the tree line, no further out.

Second, when a plant has shallow roots, apply even less, perhaps 1 inch (2.5 cm) or so. This applies to shallow-rooted shrubs too, like boxwood. One exception is dry leaves, which decompose rapidly and can be piled up a bit thicker.

Teo Spengler

Teo Spengler has been gardening for 30 years. She is a docent at the San Francisco Botanical Garden. Her passion is trees, 250 of which she has planted on her land in France.