Soil & Roots

Fall is a Great Time to Mulch

“I know we’re supposed to do something to our trees in the fall, but what?”

Tree owners often feel compelled to spray, prune, or apply something to their trees and landscape plants on a regular basis. But, unless there is a specific reason to spray, prune or apply things to landscape trees, the best thing to “do” to keep your trees healthy is to apply a layer of composted mulch.

“Fall is a great time to be out in the yard spreading shovels-full of composted woodchip mulch under your trees,” says Tchukki Andersen, BCMA, CTSP* and staff arborist with the Tree Care Industry Association. “Trees with mulched root zones are usually larger, healthier, develop faster, and have higher rates of survival than plants surrounded by turf grass or bare dirt. Mulches retain soil moisture and nutrients and reduce erosion and soil compaction.”

Mulched trees also have fewer weeds growing near the trunk, which reduces the need for the roots to compete for limited resources. The soil under the mulch also likely stays warmer longer into the winter and also warms faster in the spring, helping extend the growing season for plants in colder regions.

Natural mulches are a favorite among professional arborists, who view wood chips as excellent, attractive mulch for trees. Other natural mulches include bark chips, ground bark, composted lawn clippings, leaves, and straw. These mulches are high in cellulose and low in nitrogen and should be free of weed seeds.

Good mulching

How Wide is Wide?

A good mulch bed should extend out at least three feet from a tree’s trunk in all directions, though extending out to the dripline is preferred. This is where the fine, absorbing tree roots extend out into the soil, and mulch provides many health-related benefits for those roots. Keep all mulches several inches away from the base of the tree to avoid rot and diseases.

How Deep is Deep?

The mulch bed depth should be maintained at 2 to 4 inches.
Go Ahead, Cover the Grass! If there is grass in the area that needs to be mulched, put a five-page layer of newspaper over the grass, get it wet, then add mulch on top (this will help keep the grass from growing up through the mulch).

Use the Right Mulch

For poor soils, use a well-composted mulch to build up the nutrients. Soils that are healthy will do fine with a highly stable softwood bark (such as cypress bark), which doesn’t break down as easily. Measure the pH content. Checking the pH content of the mulch ensures it is compatible with the tree and soil.

Bad mulching

No Volcanoes, Please!

The biggest no-no when mulching is to create a “mulch volcano” that is piled high around the base of the tree. This practice traps moisture around the tree trunk and root flare leading to decay and, eventually, structural failure.

Avoid Fine Mulch

Thick blankets of fine mulch can become matted and prevent the penetration of water and air.

Don’t Let Mulch Sour

Low oxygen levels (from packed mulch) create a toxic “sour” mulch – which may give off pungent odors. Even worse, the compounds produced during the souring process (methanol and acetic acid) can kill young plants.

Don’t Keep Adding New Mulch on Top of the Old

While mulch does decompose, you do not want to accumulate excessive mulch year after year by adding fresh mulch every spring. If you want the look of fresh mulch, break up the old with a rake, and only add a layer of new on top if there is less than 4 inches in depth.

What can you do?

A professional arborist can assess your landscape and work with you to determine the best trees to plant. Contact the Tree Care Industry Association (TCIA), a public and professional resource on trees and arboriculture since 1938. It has more than 2,300 member tree care firms and affiliated companies who recognize stringent safety and performance standards and who are required to carry liability insurance. TCIA has the nation’s only Accreditation program that helps consumers find tree care companies that have been inspected and accredited based on: adherence to industry standards for quality and safety; maintenance of trained, professional staff; and dedication to ethics and quality in business practices. An easy way to find a tree care service provider in your area is to use the “Locate Your Local TCIA Member Companies” program. You can use this service by doing a ZIP Code search on

* Board Certified Master Arborist, Certified Treecare Safety Professional

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