Mulch has become a darling of the gardening world, embraced for its many advantages by homeowners and landscapers alike.
But there can be too much of this good thing. Mulch deeper than 4 inches - especially right up against stems and trunks, where it keeps them wet - makes plants targets for diseases and other problems.
“Unfortunately, a lot of people are overdoing it and applying it too high on stems of plants,” says Dave Roberts, director of the Michigan State University Extension Plant and Pest Diagnostic Clinic.
As he travels the state, Roberts is seeing mulch piled as high as 12 and even 16 inches right up against tree trunks. Apparently, people are applying new mulch on top of the old every spring, possibly because they like that fresh-mulch look.
“Consumers out there believe the more they mulch, the more they’re getting for their money. But it can be very detrimental,” Roberts says.
Wood chips, straw, pine needles, shredded bark - organic mulch can be many things, but it is not a panacea. The goal is to protect plants, not smother them.
A layer of organic mulch 2 to 4 inches thick retains moisture and moderates soil temperature. Mulch also shades the soil’s surface, which prevents some weed seeds from germinating.
Organic mulches break down at different speeds. As they decompose, some mulches will make the soil more crumbly and even add nutrients. Many people judge mulched beds and plants as looking neat and well-tended.
But a too-deep layer of mulch around trees can cause collar rot, verticillium wilt and other diseases. It also encourages root growth right in the mulch. Those roots may start encircling the tree trunk and eventually cut into the trunk or girdle it, Roberts says. Overmulching can also prevent shallow plant roots from getting oxygen.
Roberts recommends keeping only the thinnest layer of mulch against the stems or trunks of trees and shrubs, gradually increasing the depth at the outer edge of the mulched area, where mulch should be no more than 4 inches deep.
In mulched flower beds, keep several inches around the base of plants open so air can circulate and the plants are not constantly wet.
Although it has many advantages, mulch - even applied properly - has a downside. By keeping soil cool and moist, organic mulch around hostas is a red-carpet invitation to slugs, especially in wet weather. And mulch next to the house also makes it easier for earwigs to sneak indoors. Keep a mulch-free zone so the soil can dry out next to the building.
Certain organic mulches - including wood chips, sawdust, shredded bark and straw - can tie up nitrogen. MSU Extension recommends compensating by applying fertilizer before putting down the mulch, or in early spring before adding new mulch. Leaves that turn yellow in the growing season may indicate the plant needs more nitrogen.
Mulch doesn’t always have to be reapplied every year. Some mulches break down more slowly. Let the depth of the mulch, rather than its age, be the deciding factor.
Still, the drawbacks of mulch are minor compared to its advantages. Just don’t go off the deep end.
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