Mulch Overuse Problems

Over-mulching is a waste of time and money and is a major cause of death of many common bushes and trees. Knowing what kinds of plants are planted, in addition to knowing the soil's drainage, is imperative to avoid over-mulching. If there are shallowrooted species growing on poorly-drained soils (clays) mulch depths generally should not exceed 2 inches. On the other hand, if there are more deeply-rooted species growing on better-drained loams or sandy soils the plants would benefit from 3–4 inches of mulch. Coarser-textured mulches (large nuggets) can go a bit deeper due to the better oxygen diffusion through the mulch and into the soil. However, more caution is required with finer, double-shredded mulches: a 1–2 inch layer may be all that is needed to keep weeds down and prevent unnecessary soil drying in the summer. Also, a mixture of mulch particle sizes is recommended for a more continual process of mulch breakdown and nutrient release.

The best way to determine whether there is a mulch problem is simply to dig through the mulch layer to see how thick it really is. Sometimes, existing mulch needs only to be raked over to give it a "finished" landscape appearance.

As a rule-of-thumb, for optimum plant health, keep mulch a minimum of 3–5 inches away from trunks and stems of young trees and shrubs and 8–12 inches away from mature tree trunks, and spread the mulch evenly to the outer branch tips of the plant. If done properly, mulching is one of the best things that can be done to landscape plants, but it must be done properly.

Some of the many reasons why over-mulched trees, with mulch piled high against the stem or trunk, die are discussed below:

Excessive Moisture and Root Rot

This is the number one cause of death by over-mulching. Repeated applications of mulch can result in a waterlogged soil and root zone, causing root suffocation, that is, they can no longer take in oxygen to respire. The problems that are caused from yearly over-mulching are not immediate…symptoms may take three to five years to develop, and sometimes longer, depending on species and soil type. Unfortunately, by the time symptoms (off-color foliage, abnormally small leaves, poor growth and die-back of older branches) are recognized, it is generally too late to apply corrective measures. At this point the plant has gone into an irreversible decline - when roots decline and die, so does the plant.

Inner Bark Tissue Death

Above-ground stem tissue of most trees, shrubs, and perennials is morphologically different from roots and must be able to freely exchange oxygen and carbon dioxide. Mulch that is piled onto trunks and stems decreases gas exchange, with inner bark tissue eventually dying. When the inner bark dies, roots no longer receive the energy produced by leaves through the process of photosynthesis, and the plant dies.

Canker Diseases

Another mortality factor that is associated with the application of mulch next to stem tissue involves fungal and bacterial "canker" diseases. Most plant diseases require moisture to grow and reproduce, and these lethal trunk diseases are no exception. They gain entry into the stressed, decaying bark tissue and once established, the cankers will eventually encircle the tree, killing the inner bark, starving the roots, and ultimately killing the plant.

Excessive Heat or Preventing "Hardening"

Thick mulch layers that are placed against stems will begin to decay and can produce excessive heat. Similar to composting, where inner layers may reach 120-140o F, the heat may directly kill young trees and shrubs or may prevent the natural fall "hardening" process that plants must undergo to prepare for winter. If root flare tissue does not adequately harden before freezing weather arrives, the tissue will die, the roots will starve, and the plant will go into decline.

Rodent Chewing and Stem Girdling

Placing piles of mulch adjacent to tree trunks and plant stems can kill plants by providing cover and habitat for chewing rodents, such as mice, voles, etc., where these critters will usually live under warm conditions in winter and chew on the tender and nutritious inner bark. Often this chewing is not noticed until the following spring or summer when the plant no longer looks healthy. If chewing is extensive (more than 50 percent of stem circumference) or goes around the whole trunk (girdles it completely), there is little that can be done to save the plant or tree.

Nutrient Deficiencies and Toxicities

Continuously using the same type of mulch may cause plant death by changing the soil's pH, that is, the acidity level. Acid mulches like pine bark may have a pH of 3.5 to 4.5 and when applied continuously can cause the soil to become too acidic for most plants to grow. Conversely, hardwood bark mulch, although initially acidic, may cause the soil to become too basic or alkaline, causing acid-loving plants to quickly decline. Soil pH above 6.5 usually creates micronutrient deficiencies of iron and manganese for many common landscape plants. This can be avoided by periodically rotating the type of mulch used. Finally, non-composted, "fresh" or non-aged mulches may cause nitrogen deficiencies in many young trees, shrubs, and flowers. Decomposing bacteria and fungi that ultimately break down mulch must have an ample supply of nitrogen to do their job. Most landscaping mulches are comprised of bark or wood that has very little nitrogen available for the decomposing bacteria. Hence, bacteria in the soil utilize existing nitrogen to break down the mulch. This process may cause nitrogen deficiencies and yellow leaves as a result of the excessive mulch.

By Dr. Brian Maynard, Associate Professor of Horticulture, University of Rhode Island