Ferguson: Mulch in moderation — Avoid too much, as well as too little

Published 11:05 am Friday, April 8, 2022

Mulch serves important purposes in landscapes and other sites where plants are grown. It suppresses growth of weeds that compete for water and nutrients, helps keep the soil moisture level and temperature more consistent, and can reduce erosion, soil compaction, and the likelihood of string trimmer or mower injury to plants.

Also, unlike synthetic landscape fabrics, organic mulches materials such as pine straw, bark, shredded hardwood mulch, and coarsely shredded leaves help build up soil organic matter and release nutrients over time.

Pine straw is a good choice for many situations, especially sites with a slope, as the needles become intertwined and don’t readily wash away. However, it settles quickly and will need to be replenished often.

Pine bark mulches tend to settle more slowly than most organic mulches. Weeds will occasionally take root in pine bark. After all, it’s often a major component of potting mixes used to grow plants in containers.

Shredded hardwood mulch tends maintain its thickness longer than pine straw but not as long as pine bark. Mulch that includes wood (and thus cellulose) occasionally will host an organism called the artillery fungus. This microbe ejects spore masses that can stain siding, cars, and other surfaces. It’s not a very common problem but occasionally occurs.

Two mistakes observed frequently in landscapes are on opposite ends of a spectrum: Either grass and other weeds are allowed to grow too close to the bases of trees and shrubs, or mulch is piled up at, and in direct contact with, the base of the trunk.

We don’t want turfgrass and other weeds competing with the root systems of young plants for water and nutrients. One suggestion is to mulch a circle at least three to four feet wide around newly planted trees. For older trees, try to keep the area under the canopy — and preferably some distance beyond the drip line — mulched.

At the same time, having mulch directly against the base of a trunk contributes to wet conditions and greater likelihood of disease and other pest issues. The practice of “volcano mulching” is not recommended. Try to keep mulch two to three inches away from the bases of tree trunks and shrub stems.

Likewise, don’t spread mulch right up to the foundation of a house. Retaining moisture near buildings is not desirable, and we don’t want to help termites cross the barrier where insecticide has been applied to the soil. Keep mulch at least six to 12 inches away from the foundation.

We want mulch to be deep enough to suppress weeds and perform the other functions that mulches have but not so deep that it prevents oxygen-rich air from reaching plant roots. Two to three inches of mulch is appropriate for most situations. When mulch breaks down to the point that only a couple of inches are left, reapply it to maintain sufficient depth.

Be sure to acquire enough mulch for the landscaping you want to do. To cover 100 square feet to a depth of three inches, you’ll need 25 cubic feet of mulch, or a little less than one cubic yard (27 cubic feet). A circle three feet deep and three inches thick around a newly planted tree will require about 1.7 cubic feet of mulch.

Let me know if you have questions.

 Dr. Mary Helen Ferguson is an Associate Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu, 985-839-7855 (Franklinton) or 985-277-1850 (Hammond).