Fallen leaves make a fine mulch for garden beds and trees and shrubs, but they aren’t the only possibility.
“Any kind of organic matter can make a good mulch,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. “Your plants just want a layer of something that also comes from plants to cover their roots.”
That layer will insulate the roots against the sudden swings between freezing and thawing that are such a risk to plants in a Chicago winter. The mulch also will hold moisture in the soil. It can help reduce weeds by shading out weed seeds so they don’t get enough sunlight to sprout. A ring of mulch around trees will keep lawnmowers from getting too close and damaging the trunk and roots. Over time, the mulch will be broken down by soil organisms to improve your soil.
“Every kind of plant, from vegetables to trees, does better with a layer of mulch over its roots, and many people like the tidy look of mulch in the garden,” Yiesla said. “All plants ask is that the mulch layer be made from plants.”
Plant-based mulch works because it emulates the way plants live in nature. In the wild, trees and other plants are naturally mulched by leaves, stalks, dead branches and other plant materials that fall to the ground every year and form a layer over their roots.
Spread any kind of mulch evenly 1 to 2 inches deep over beds used for perennials, annuals and vegetables. Around trees and shrubs, make it into a wide, even ring 3 to 4 inches deep. Be sure to keep the mulch on the ground, not piled against a tree’s trunk. It should stop 2 to 3 inches away from the bark.
Your choice of mulch may depend on price, availability, how you will use it and how long you want it to last. In general, large chunks break down more slowly; small pieces decay faster, but as they do, they add more nutrients to the soil.
Shredded wood. 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 the pieces may be too coarse and lumpy for beds where you will dig often, such as vegetable gardens. It can also be ordered by the cubic yard from landscape supply yards and some garden centers.
Dyed mulch. In these popular products, dyes give a uniform appearance to a mixture of shredded wood from demolition sites, old pallets and other reclaimed wood. The red, brown or black dyes themselves won’t hurt plants, Yiesla said, but they make it harder to be sure what the mulch is made from. 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 dumped for free by tree companies, utility crews or municipalities. This kind of mulch has big chunks, so it is best used around trees and shrubs rather than on perennial or garden beds. To seek out free wood chips, start by asking your local municipality or forest preserve district. Some have piles of mulch available to residents.
Composted wood chips. Composted wood chips are dark brown and look less raw than fresh ones. The Morton Arboretum uses these partially decayed wood chips to mulch around its trees and give a good walking surface to its trails. Similar materials are available from landscape supply companies.
Straw. Stalks from wheat and other grains are a traditional mulch for 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.
Leaves. “They’re free, and they’re already right there in your yard,” Yiesla said. Because leaves are thin and light, they 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,” she said, “but after all, nature renews it every year.” 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. 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.