This landscape mulch demonstration project at the LSU AgCenter Hammond Research Station shows how different materials withstand weather conditions. (Photo by Allen Owings. Click on photo for downloadable image; right click to save.)

News Release Distributed 07/19/13

By Allen Owings
LSU AgCenter horticulturist

HAMMOND, La. – All ornamental plants benefit from mulching when it’s done correctly. Good mulching can have several beneficial effects on plants, 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. It is important to remember to “go out” instead of “go up” with mulch – spread mulch horizontally instead of piling it vertically.

Recommended mulches include pine straw and pine bark. You now can 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.

We continue to see problems with excessive mulching around trees and shrubs – a practice that is becoming more common by home gardeners and professional landscapers alike. Research has shown that mulching deeper than 4 inches is not healthy for most landscape plants. One of the tendencies in landscapes now is to make piles of mulch – sometimes resembling the shape of a volcano or fire ant mound – around the bases of trees, especially small, flowering trees, such as crape myrtles, in residential landscapes. We also see this with oaks, bald cypress, pines, elms and other popular shade trees.

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 and lower stem and in the root zone. In addition, heat buildup from mulch decomposition can kill stem and trunk tissue.

Mulching with some materials can influence the pH of the underlying soil. 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.

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.