Barbara J. Bromley, Mercer County Horticulturist 1998
Over the last several years some landscapers and grounds maintenance professionals have mounded shredded organic mulches at the bases of trees and shrubs. Thinking this practice to be both attractive and acceptable, the public started copying it. Such applications may cause damage if improperly applied. The use of mulch has gained acceptance over the past 20 years to improve the appearance of a landscape planting and can be very useful. In addition to looking nice, properly applied organic mulches help to:
- conserve soil moisture
- reduce -extreme soil temperature fluctuations
- add organic matter to the soil
- deter weed growth
- mask protruding or buttressing roots
- keep grass away from the trunks of trees or shrubs. This helps prevent damage from mowers and nylon string trimmers
However, piling mulch on the trunks and stems of woody plants and heaped to excessive depths is worse than using no mulch at all. This detrimental “mulch mounding” practice is causing some of the following problems:
- Voles, or pine mice, can gain access to the bark of trunks, stems, and roots by tunneling through the mulch layer. The mulch provides needed protection by shielding the vole from predatory birds and other animals. This is a problem primarily in winter.
- Mulch piled on a trunk or the stems of shrubs keeps bark too wet for too long and allows or encourages the entrance of disease and rot organisms.
- An excessive mulch layer encourages the development of weak adventitious roots directly off the trunk or woody stems into the mulch at the expense of development of roots in the soil. The adventitious roots, which often provide enough water and nutrients to keep the canopy of leaves looking reasonably healthy, die when the mulch dries out, leaving the plant at risk in times of drought. Some shallow rooted plants such as azalea and red maple often send roots into this moist organic matter.
- Excessive mulch may trap irrigation and/or rain water in the mulch layer, thus preventing moisture from getting to the main roots in the soil.
A maximum (less is OK too) of 3 inches of mulch on silt loam or other heavy clay soils or 4 inches on sandier soils is enough to be beneficial and to look nice. Any depth exceeding 3-4 inches, even away from the trunk, has the potential to keep the soil, especially heavy clay, too wet. As the mulch decays it will eventually raise the grade around the plant if more is added each year.
There should be a 2 to 6 inch gap between the flare of the trunk and the start of the mulch. (This is sometimes called “doughnut” mulching, because the trunk is in the hole.) This gap reduces the incidence of vole damage, of adventitious root development, of stem canker, and rot diseases, and the possibility that insects will invade water softened tissue.
Professionals owe it to the public to set a good example. Homeowners owe it to themselves to stop allowing this practice to persist on their own properties and at their public buildings and parks.