Arbor Business: Mulch Matters

The symptoms are easy to spot: piles of mulch against tree trunks or mulch mounds with the center carefully scooped out, but still piled
8 to 16 inches over the soil surrounding the plant.

This recent epidemic is known as “volcano” mulching and has become a problem in the tree care industry. To understand how we got to this point, let’s spend a moment considering the reasons for using mulch.

MUCH ADO ABOUT MULCH. Mulch provides the landscape with valuable benefits. Whether organic or inorganic, mulch controls weeds, conserves moisture, increases water permeability, reduces runoff and moderates soil temperatures. Mulch acts as a barrier between trees and mowers or trimmers, preventing injury. And mulch is attractive, setting plantings off from the turf and creating clean landscape lines.

But let’s start with best practices, which should be tailored to the site, plant and mulch type. Contractors should apply mulch to a minimum 3-foot circle around individual trees. This means a minimum of 3 feet from the trunk to the edge of the mulch or 6 feet across. Modify this to at least 12 inches beyond the periphery of the root ball for newly planted trees, as well as out to the drip line of established trees, whenpossible. The goal is to maximize the soil area under the mulch so roots are encouraged to explore, particularly if site stresses limit growth.

As a rule of thumb, keep mulch 3 to 6 inches away from young tree trunks and 8 to 12 inches away from more mature tree trunks (corresponding to the width of the root flare). In wet, poorly drained sites, avoid fine-textured mulches. Coarse mulches might help improve the porosity of the soil over time.
Most importantly, do not apply mulch more than 2 to 4 inches deep. This means 2 to 4 inches of mulch above the soil, measured from the proper soil level.

Freshen or replace mulch every two to three years. Apply fine-textured mulches, such as sawdust, double-shredded bark and buckwheat hulls, only 2 to 3 inches thick, and pile coarsely textured mulches, such as nuggets or wood chips, up to 4 inches.

Take care not to add these amounts to what is already in place. Measure and rake mulch off the surface of old mulch to improve its appearance and then add fresh mulch to a total depth of 2 to 4 inches. Remove old mulch only if it has compacted and become gydrophobic (sheds water). If needed, rake away and dispose of old, decayed mulch, then put down fresh product to the proper final depth.

LEVEL THE VOLCANO. We have all seen examples of volcano mulching. Why is this practice a problem? The most obvious issue concerns placing mulch against the trunk. This practice keeps the bark moist, but bark is not designed to tolerate continuous moisture. Wet bark is subject to fungus or insect attack. Mulch piled around the bark also restricts the exchange of carbon dioxide and oxygen between the living bark (phloem) and the atmosphere, suffocating the bark.

This increases the risk of decay. In addition, the heat of the decomposing mulch also warms the trunk, delaying dormancy into the fall and increasing cold injury risk. Microbes in the damp mulch may decompose the bark along with the organic matter in the mulch. Finally, placing mulch against the trunk might even provide shelter for rodents who could end up feeding on the bark during a long winter.

Many professionals go to great lengths to ensure mulch does not touch the trunk. This, however, is how we end up with the caldera of the mulch volcano. The center of the volcano is created by pulling mulch away from the trunk, while retaining the basic cone-shaped mound. This seldom works in the long term, as wind, rain and snow eventually cause the mulch to fill in again. In the meantime, there is still a foot-tall mulch mountain around the plant.

On a positive note, most mulch volcanoes contain enough materials to provide a nice, wide mulch apron. So, grab a rake and break down some volcanoes. – Thomas Smiley and Brian Maynard

The authors are plant pathologist/soil scientist, Bartlett Tree Research Laboratories, Charlotte, N.C., and associate professor of horticulture, department of plant sciences, University of Rhode Island, Kingston, R.I., respectively.