Applying mulch is one of the easiest and best ways to help your trees and other plants, but doing it the wrong way can lead to trouble.
“Mulch should be a flat, even layer over the soil,” said Spencer Campbell, Plant Clinic manager at The Morton Arboretum in Lisle. “It should never be heaped up, especially around a tree.”
When mulch is piled up against a tree’s trunk, it can trap moisture against the bark, which encourages decay. “If the bark rots away, the tree’s circulatory system will be cut off,” Campbell said. “It can lead to the death of the tree.”
A heap of mulch also can shelter harmful insects and bark-chewing small animals such as voles, giving them perfect cover to nibble away. Young trees have bark that is thin, tender, easy to chew and quick to rot, making them especially vulnerable to the dangers of piled-up mulch.
To avoid these problems, spread the mulch in an even layer over a wide area, and make sure that it stays at least 2 inches away from the bark all around the tree’s trunk.
Spread the right way, mulch plants a world of good because it protects and feeds the soil where their roots live. Mulch insulates the soil against extremes of heat and cold. In hot weather, it reduces evaporation to keep soil moist. As the mulch decays, it releases nutrients to your soil and feeds microorganisms that improve the conditions for your plants’ roots. Arboretum research has shown that trees surrounded by a wide area of mulch grow faster and more vigorously.
That mulched area protects the tree in another important way: It will keep lawnmowers and string trimmers away from the bark so they don’t damage it.
Mulch is powerful for all plants, not just trees, because it emulates the layer of leaf litter their roots live beneath in nature. Through the seasons, fallen leaves, dry stalks and dead branches accumulate. Gradually, soil-dwelling organisms, including insects, fungi and bacteria, consume that litter to break it down and mix it up to make rich, light soil. “When we spread mulch, we’re just trying to imitate what plants have been doing for themselves for millions of years,” Campbell said.
The commercial mulch sold in bags or by the cubic yard consists of shredded wood, but any plant material can be used as a mulch layer. “The leaves that fall every autumn make wonderful mulch,” he said. “That’s what trees in the forest depend on.”
Larger chunks, such as commercial shredded wood mulch or wood chips from tree work, will break down more slowly. Thinner or smaller kinds of mulch, such as straw or dried leaves, will decay faster. “They’ll improve your soil more quickly, but you’ll also have to renew the mulch more often,” Campbell said.
These lighter mulches are the best choice for vegetable gardens that are replanted annually. “For vegetables, you want a mulch that will break down over the course of the year,” he said. Spread mulch in an even layer about 4 to 5 inches deep around trees and shrubs, and remember to keep it away from their bark. Make the mulched area as large as you can, although it doesn’t need to be a perfect circle. In perennial and vegetable beds, a layer 1 inch deep is sufficient.
Don’t forget to spread a light mulch layer around annual flowers and even in containers. “It will help keep their potting mix from drying out so quickly,” Campbell said. “All plants benefit from a nice layer of mulch.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
