how mulch do you know?

Using mulch can help suppress weeds, make landscapes more beautiful

by Amy Beth Graves 

A sign that spring planting season is just around the corner is whenever mulch starts showing up everywhere: piled high in mulch or gardening centers, in residential driveways or even in gas stations.

Mulch is any type of material that is spread on the surface of soil. It comes in various forms and colors. Organic mulches are derived from plants or animals and include wood chips, pine needles, tree bark, grass clippings, leaves, peat moss, ground corn cobs, recycled paper and even composted sewage sludge. Inorganic types usually don’t decay and include pebbles, crushed gravel, marble chips and woven ground cloth.

Interest in using mulch appears to be on the rise with more varieties being offered, said Hannah Mathers, an assistant professor and Extension specialist in nursery and landscape crops at Ohio State University.

“In Ohio it seems to me that people are pretty mulch happy,” said Mathers who lived in Oregon before coming to the Midwest a few years ago. 

Why do you need mulch?

Used properly, mulch can help suppress weeds, reduce soil erosion, decrease water loss from the soil and keep the soil cool with strong sunlight or warm during cold weather. Often it is used by backyard gardeners to make landscapes more beautiful. Some gardeners use it to return the soil to a better condition.

A popular type of mulch is dark-colored hardwood, which decays quickly and can be worked into the soil to add nutrients, Mathers said. Another favorite is ground-up wood pallets that are dyed different colors. But Mathers cautions that using the wood pallets without adding fertilizer to the soil can cause nitrogen deficiency and possibly harm plants. 

How to apply

In Ohio the recommended depth for applying mulch is 2 to 21/2 inches. To help prevent termites or other insects from getting inside a building, place the mulch 6 inches to 3 feet away from a structure, Mathers said.

Piling mulch up around a tree trunk is harmful, Mathers said. During wet weather, the mulch keeps the trunk wet, and the tree can die from rot or from not getting enough oxygen, she said. If the weather is dry, the tree will start to shoot roots into the mulch in an attempt to get oxygen, and the roots often will die. It’s best to keep mulch at least 6 inches away from the trunk.

Before buying your mulch, you should check it first. Mulch that hasn’t been properly composted can harm plants or even cause them to die, Mathers said. Smell the mulch before you buy it; good mulch should have a strong earthy or fresh-cut wood smell. If your mulch has a strong ammonia odor, let it sit for a couple of days and turn it because it can burn plants if applied too soon.

Test the mulch for weeds before you buy a lot. Get a sample of the mulch and put it in a pot. If weeds come up in a couple of weeks, it’s not good mulch, Mathers said.

“It’s best to test because haste makes waste,” she said.

Other mulch problems

Improper use of mulch can cause problems. Putting mulch on top of heavy soils, such as clay, can cause plants or trees to stay too wet. Too much mulch can decrease soil oxygen levels, attract insects such as slugs and earwigs or cause shallow rooting.

“People are continually adding mulch and if it doesn’t break down that fast, the next thing you know, you have a foot of mulch,” said Pam Bennett, Ohio State Extension horticulture educator in Clark County. “Your mulch — old or new —should only be 2 to 21/2 inches deep.”

Using mulch also can cause problems with nuisance fungus. The spores from the commonly called shotgun fungus (Sphaerobolus stellatus) can be as difficult to remove as tar. They stick not only to plants but houses and cars. Slime molds are harmless but ugly and can grow up to a foot in diameter. Raking or putting water on them helps with their removal.

Applying mulch too soon also can cause problems if the soil hasn’t had time to warm up. Except for trees and shrubs, it’s best to wait until the end of May in Ohio to mulch, Bennett said. If you had a tree cut up, avoid using the chips around young plants or vegetables for a year because they can rob nitrogen from the soil and kill the plants. Add nitrogen if you put the fresh bark chips around trees or shrubs.

Mulch can become compacted after heavy rainfall and even repel water, Bennett said. Loosen the mulch after it dries. As for herbicides, put them on the soil first and then put mulch down.

“The benefits of mulch do outweigh the disadvantages,” Bennett said. “The key is proper mulching.” 

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