A layer of mulch is an effective way to protect your plants against summer heat, drought and rampant weeds, while improving the soil to nourish their roots. But what is the best mulch?
“There are many materials you can use to create a layer of mulch,” said Sharon Yiesla, a plant knowledge specialist at the plant clinic at Morton Arboretum in Lisle. “The bottom line is that it should always be made from plants.”
Plant-based mulch is strong because it mimics the way plants live in nature. When leaves, stems, dead branches, and other plant matter fall to the ground, they form a protective layer over the soil. This layer of foliage keeps the soil around the plant roots moist and insulates them from extremes of heat and cold. Over time, a variety of soil-dwelling organisms, including insects, fungi and bacteria, will consume the litter and break it down to improve the soil.
What you choose for mulch in your garden can depend on price, availability, where the mulch is used, and how long you want it to last. “In general, the larger the pieces of material in the mulch, the longer it takes for them to break down,” Yiesla said.
Crushed wood. This is the most common type of mulch in the Midwest. A bag labeled Mulch at the home center most likely contains shredded by-products from the lumber industry. Shredded wood mulch will last a year or two. It works well near trees, shrubs and perennials, but can be too coarse and clumpy for beds where you dig up frequently, such as in gardens. B. Vegetable gardens.
This type of mulch can also be ordered in bulk from landscape supply yards and some garden centers. It is sold per cubic meter.
Colored mulch. Shredded wood colored black, brown, or red is common. The dyes themselves do not harm the plants. They are used to give a uniform look to a mix of chipped wood from demolition sites, old pallets and other scrap wood. “It’s not necessarily harmful, but you just don’t know what’s inside,” Yiesla said. The Mulch and Soil Council, an industry group, has guidelines for the content of mulch. Look for the seal on the bags.
Fresh wood chips. Arborists use chippers to break down tree branches into coarse wood chips. Sometimes homeowners can get a batch of these chips for free from tree companies, utility companies, or municipalities. Because the chunks are large, this type of mulch works best around trees and shrubs, where it will likely take several years to break down. “You probably don’t want those big, nasty clumps in the soil of your garden beds, which is where you’re likely to be working,” Yiesla said. To look for free woodchips, first check with your community. Some have piles of mulch available for residents to use.
Composted wood chips. The arboretum uses wood chips that have decayed easily to mulch its trees and shrubs and on its dirt paths. A similar material available from landscape supply companies is dark brown and looks less raw than fresh wood chips.
Leaves. An excellent and plentiful mulch, leaves are complimentary if you remember to collect them in the fall as they fall from the trees. The thin, light leaves fall apart fairly quickly, making them good for beds of annuals, vegetables, and perennials. “You have to renew the leaf mulch quite often,” Yiesla said, “but nature eventually renews it every year.” The arboretum uses leaf mulch in all of its garden beds after the leaves have been composted for a few months.
To prevent leaves from flying around or forming an impenetrable mat, break them up by raking them into a heap on the grass and mowing over them. Some gardeners stash away a supply of leaves each fall, whole or shredded, to use as mulch or to add to the compost heap year-round.
Straw. Stalks of wheat and other grains are a traditional mulch in vegetable gardens as they break down over the course of a growing season. In the city and suburbs, thatch can be hard to find and relatively expensive. It may also contain weed seeds.
cocoa mulch. Roasted cocoa bean shells are sometimes available. They form an attractive, finely 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 one material and use it consistently,” Yiesla said. That way, when the mulch gets thin because the bottom layers are breaking down, you can just add more of the same kind on top.
Just make sure it doesn’t get 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’re not considering this mulch,” Yiesla said. “Stones can’t do for your plants what a real, plant-based mulch does.”
For tree and plant advice, contact the Plant Clinic at Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or email@example.com). Beth Botts is a staff writer at the Arboretum.