All ornamental plants greatly benefit from mulching when it’s done correctly, says LSU AgCenter horticulturist Allen Owings.
Good mulching can have several beneficial effects on plants, the soil and the surrounding area. Mulch can conserve soil moisture, maintain ideal soil temperatures, reduce weed seed germination and growth, prevent soil fungus from splashing on leaves, lessen cold damage, slow soil erosion, reduce soil compaction and add 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 to 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,” Owings says. “Just maintain the mulch depth at the recommended levels.”
Owings generally recommends using pine straw and pine bark for mulching, but 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 have positive benefits when used correctly.
For trees, mulch out to the drip line — the edge of the branches or beyond — at least an 8-foot-diameter 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 would be mulched naturally with fallen leaves and forest debris.
“It’s discouraging to see excessive mulching around trees and shrubs,” Owings says. “We probably have more landscapes improperly mulched than properly mulched.”
Excessive mulching generally means the mulch materials are placed too deep around plants and cover the lower trunks of trees and shrubs. Mulch going up the trunk instead of out from the trunk is sometimes called “volcano mulching,” Owings says.
Research has shown that mulching deeper than 4 inches is not healthy for most landscape plants. Problems include starving the shallow roots of oxygen, dying phloem tissue that moves nutrients through the plant, and more fungal and bacterial infections.
In addition, heat buildup from mulch decomposition can kill stem and trunk tissue.
“Fall is a great time to add new mulch to landscapes and remulch older plantings to provide all the benefits,” Owings says.
Got a gardening question? Write to GardenNews@agcenter.lsu.edu.