Oaks and other species of shade trees are commonly improperly mulched in commercial landscape plantings. (Photo by Allen Owings, LSU AgCenter)

Landscape mulches being researched at the LSU AgCenter Hammond Research Station. (Photo by Allen Owings, LSU AgCenter)

News Release Distributed 09/18/15

By Allen Owings
LSU AgCenter horticulturist

HAMMOND, La. – Fall is the time of year when we include gardening maintenance and upkeep in our list of outdoor activities. Fall is also a great time to add new plants to the landscape. Many home gardeners also mulch new plantings or add new mulch to older, established plantings before winter.

All ornamental plants greatly benefit from mulching when it’s done correctly. Good mulching can have several beneficial effects on plants, the soil and the surrounding area. These benefits include conserving soil moisture, preventing the soil surface from being crusty, maintaining ideal soil temperatures, reducing weed seed germination and subsequent growth, preventing splashing soil fungus during irrigation and rain, lessening cold damage, slowing soil erosion, reducing soil compaction and adding aesthetic beauty to the landscape.

The LSU AgCenter recommends mulching annual bedding plants and herbaceous perennials to a depth of 1 inch, shrubs to a depth of 2 inches and trees to a depth of 3-4 inches. You can add new mulch on top of old mulch, and you don’t have to use the same mulch material each time. Just maintain the mulch depth at the recommended levels.

We generally recommend using pine straw and pine bark for mulching. You also can also find synthetic pine straw, eucalyptus mulch, dyed wood chips, shredded wood mulch that looks like pine straw and much more. All of these have positive benefits in the landscape when used correctly. The AgCenter does not recommend using cypress mulch due to environmental concerns pertaining to harvesting issues.

It’s discouraging to see mulch used improperly in Louisiana, especially with excessive mulching around trees and shrubs. This is a practice that is becoming more common by home gardeners and professional landscapers alike. We probably have more landscapes improperly mulched than properly mulched.

Excessive mulching generally means the mulch materials are placed too deep around plants and are being placed over the lower trunks of trees and shrubs. Mulch is going “up” the trunk instead of out from the trunk. Sometimes people refer to this practice as “volcano mulching.” This is not a recommended practice. It creates health-related problems for plants. Small, flowering trees, such as crape myrtles, in residential and commercial landscapes are typically mulched improperly. We also see this with oaks, bald cypress, pines, elms and other popular shade trees.

Research has shown that mulching deeper than 4 inches is not healthy for most landscape plants.

Problems from over-mulching or piling mulch around the base of trees include starving the shallow roots of oxygen, dying phloem tissue that moves nutrients through the vascular cambium of the plant, and more fungal and bacterial infections because of increased moisture around the trunk, lower stem and in the root zone. In addition, heat buildup from mulch decomposition can kill stem and trunk tissue.

Mulch out to a tree’s drip line, the edge of the branches, or beyond – at least an 8-foot-diameter area around the tree. Mulching young trees can protect them from string trimmer, mower and other mechanical damage. Remember that in a forest environment, a tree’s entire root system, which usually extends well beyond the drip line, would be mulched naturally with fallen leaves and forest debris.

Be aware that thick blankets of fine mulch under trees can become matted over time and may prevent water and air from seeping through or become like potting soil and support weed growth. Rake old mulch a few times each year in spring and fall to break up any matted layers and to refresh the appearance.

Mulching with some materials can influence the acidity of the underlying soil. Mulches have a pH just like soil and water have a pH level. Know your mulch pH. When mulch is excessive, microbial organisms can compete with tree roots for nutrition. Also, rodents can live in mulch and feed on the plants, chewing on lower stems and surface roots.

The AgCenter has been conducting mulch studies, and what we learn should help in understanding how to use mulch properly in the landscape.

Fall is a great time to add new mulch to landscapes and remulch older plantings to provide all the benefits.

You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website. Also, like us on Facebook. You can find an abundance of landscape information for both home gardeners and industry professionals at both sites.