Released May 8, 2008

DURHAM, N.H -- Spring is finally here. The days are longer and warmer, daffodils are popping up, and the mulch volcanoes are erupting.

The mulch volcanoes? You know, those piles of bark mulch mounded up beneath many landscape trees. I don't know where or why this practice started, but too much mulch under a tree or shrub actually does more harm than good, and is a waste of mulch and money.

Mulch serves several purposes besides dressing up your landscape. It helps retain soil moisture, important because most "feeder" roots (the roots that obtain water, air and nutrients for the tree) grow in the top 12 inches of the soil.

Mulch that's deeper than three inches can smother feeder roots and compromise the tree's health. Roots may actually grow up through a too-thick mulch past the base of the tree and encircle the trunk, a condition called girdling. As the tree grows, these roots could choke the tree and cause a slow death.

Mulch should never touch the trunk of the tree. Because it holds moisture, mulch will keep the trunk damp, encouraging bark rot and disease. Also, insects and rodents may find homes in the mulch volcano and feed on the bark of the tree.

Spread the mulch in a uniform layer two to three inches deep and out at least to the drip line (the area beneath the outermost branch tips of a tree). You want the natural flare at the base of the trunk, where it meets the ground, to be exposed. You can renew mulch each year by adding a little to the surface; there's no need to remove old mulch, but don't allow it to become deeper than four inches.

Because mulch helps prevent roots from drying out, especially when newly planted, use a quality, natural mulch. Untreated plant-based mulches are best, since they help improve the soil as they decompose. These include wood chips, bark mulch, straw, leaves, and pine needles. Avoid wood-chip mulches made from construction materials, pressure-treated lumber and old telephone poles, as these might contain harmful chemicals.

Mulch can also prevent weeds from growing around the base of the tree. Particularly when the tree is growing in a small or confined area, unwanted plants (or even flowers) may compete with the tree or nutrients and water.

Finally, mulch provides a "safe zone" between your lawn and the tree. This makes lawn mowing easier, and protects the bark from mower or trimmer damage.

If you'd like to remove the mulch volcano from your tree, do it gradually over a few months. The trunk has been kept shaded and damp from the mulch, and sudden exposure to the sun could cause it to crack. If the feeder roots have grown up through the mulch, they could dry out and lose their ability to absorb water. Remove the mulch a little at a time, remembering to water the roots thoroughly for 20 minutes each time.

You may lose some roots to exposure during this process, but in the end your tree will benefit from having deeper roots and a healthier growing environment.

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http://extension.unh.edu/news/2008/05/too_much_mulch.html