How to choose the right mulch for your garden, trees and plants

A layer of mulch is a powerful way to defend your plants against summer heat, drought and overbearing weeds, while improving the soil to nourish their roots. But what is the best kind of mulch?

“There are many materials you can use to create a mulch layer,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “The critical thing is that it should always be made from plants.”

Plant-based mulch is powerful because it emulates the way plants live in nature. When leaves, stalks, dead branches, and other plant materials fall to the ground, they form a protective layer over the soil. That layer of leaf litter keeps soil moist around plants’ roots and insulates them against extremes of heat and cold. Over time, a wide array of soil-dwelling organisms, including insects, fungi and bacteria, will consume the litter, breaking it down to improve the soil.

What you choose for mulch in your garden may depend on price, availability, where you’re using the mulch and how long you want it to last. “In general, the larger the pieces of material in mulch, the longer it will take to decay,” Yiesla said.

Shredded wood. This is the most widely available kind of mulch in the Midwest. A bag labeled mulch at the home center most likely contains shredded byproducts from the lumber industry. Shredded wood mulch lasts a year or two. It works well around trees, shrubs and perennials, but may be too coarse and lumpy for beds where you will dig often, such as vegetable gardens.

This kind of mulch also can be ordered in bulk from landscape supply yards and some garden centers. It is sold by the cubic yard.

Dyed mulch. Shredded wood that is dyed black, brown or red is common. The dyes themselves won’t hurt plants. They are used to give a uniform appearance to a mixture of shredded wood from demolition sites, old pallets, and other reclaimed wood. “It’s not necessarily harmful, but you just don’t know what’s in it,” Yiesla said. The Mulch and Soil Council, an industry group, has guidelines for the content of mulch. Look for its seal on bags.

Fresh wood chips. Tree-trimming crews use shredders to reduce branches to coarse wood chips. Sometimes homeowners can get a load of those chips for free from tree companies, utility crews, or municipalities. Since the chunks are large, this kind of mulch is best around trees and shrubs, where it is likely to take several years to break down. “You probably don’t want those big, awkward lumps in the soil of your garden beds where you’re likely to be working,” Yiesla said. To seek out free wood chips, start by asking your local municipality. Some have piles of mulch available to residents.

Composted wood chips. The Arboretum uses wood chips that have been allowed to break down slightly to mulch its trees and shrubs and on its unpaved trails. A similar material, available from landscape supply companies, is dark brown with a less raw appearance than fresh wood chips.

Leaves. An excellent and abundant mulch, leaves are free, if you remember to collect them in autumn when they fall from the trees. The thin, light leaves will break down fairly quickly, which makes them good for beds of annuals, vegetables and perennials. “You do need to renew leaf mulch fairly often,” Yiesla said, “but after all, nature renews it every year.” The Arboretum uses leaf mulch in all its garden beds, after the leaves have been allowed to compost for a few months.

To prevent leaves from blowing around or forming an impenetrable mat, shred them by raking them into a pile on the grass and running the lawn mower over them. Some gardeners stow away a supply of leaves each fall, whole or shredded, to use for mulch or to add to the compost pile throughout the year.

Straw. Stalks from wheat and other grains are a traditional mulch in vegetable gardens since they break down over the course of a growing season. In the city and suburbs, straw may be hard to find and relatively expensive. It also may contain weed seeds.

Cocoa mulch. Shells from roasted cocoa beans are sometimes available. They make an attractive, fine-textured mulch. Pet owners should be aware that they may contain compounds dangerous to dogs.

“Any plant-based mulch will benefit your plants, but if appearance is important to you, decide on a material and use it consistently,” Yiesla said. That way, when mulch becomes thin because the lower layers are breaking down, you can simply add more of the same type to the top.

Just be careful not to make it too deep. Mulch should be 1 to 2 inches deep around perennials, annuals and vegetables, and 3 to 4 inches deep around trees and shrubs.

One material to avoid is gravel. “We don’t consider that mulch,” Yiesla said. “Rocks can’t do for your plants what a real, plant-based mulch will do.”

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.