Mulch Mounds Are Killing Trees

By Scott Aker
Special to The Washington Post
Thursday, June 8, 2006

Q What are your views on mulching trees? In our subdivision, the landscaper hired by the homeowners association follows the common practice of dressing the base of the trees with a mound of shredded hardwood mulch. Besides looking absurd, this mulch cannot be healthy for the trees, I believe. I am tired of my dues paying to replace sick trees that have died or fallen over. I've tried contacting the authorities in charge, only to be told that I should write a letter explaining why it is bad for the trees when it "looks so nice," and they will "consider the matter." Am I right?

AIf I had the choice of eliminating one harmful gardening practice, it would be the mulch volcanoes that appear around trees. To some, it may look neat and tidy, but it ultimately results in tree decline and mortality. That "professionals" are doing it and giving it credence boggles the mind. The aim may be to preserve soil moisture in dry spells; the effect is exactly the opposite: No rain can penetrate a foot-thick layer of mulch, and because the mulch is sloped away from the tree, it runs off. Thick mulch also promotes the growth of mats of fungi that shed water. In effect, the mound functions as an umbrella over the root zone.

A trunk wound covered in mulch doesn't heal and becomes an entryway for pests and disease. Also, many trees actually send roots out into the mulch over time, in a desperate search for moisture. These often grow near the surface of the mulch and are subject to drought stress. A heavy layer of mulch may also harbor rodents that can injure the bark and major roots.

Often trees mulched in this way fail to develop a normal root flare, which works to buttress the trunk as the tree grows to maturity. Heavily mulched trees are more likely to be uprooted in a windstorm.

Mulch volcanoes are most injurious to young trees, and, unfortunately, that is where they are seen most often. If a newly planted tree is subjected to this treatment, the root ball may be completely shielded from any moisture, even if the area is irrigated heavily. Such trees may struggle to become established and often die within five years of planting.

The correct method is to form a circular ridge of soil at the edge of the root zone and then lightly mulch this saucer so that the soil and mulch together trap rainwater and feed it to the roots. Mulch should not touch the trunk.

Your association should consider the effect of mulch volcanoes not only on the trees, but also on its finances: All that mulch and the labor to apply it costs money, as do the replacement trees.

Last summer, large patches of our pachysandra turned brown and died. My neighbor's suffered the same problem. Do you know what the disease is and how to treat it?

The disease is caused by a fungus, Volutella pachysandrae , and appears on pachysandra that has suffered some winter injury or drought, or has had to grow in conditions that are marginal. It is common on pachysandra grown in a site that is really too sunny for its needs and on pachysandra that has been mulched heavily. It has been more prevalent in the past two years because of wet weather that has favored new infections.

Fortunately, the disease does not typically kill the underground portions of the plant, and bare patches often quickly fill in as long as the growing conditions are favorable. This may not be the case if the area is too dry, too sunny or too heavily mulched. Pull out any diseased portions of the planting. Fungicides can be a help but must be applied as soon as new growth emerges and thereafter on a weekly basis while top growth continues.

You should also take care to water pachysandra infrequently. Although it likes evenly moist conditions, it does not care for frequent waterlogging. Scale insects, particularly fern scale and euonymus scale, sometimes attack pachysandra, most typically if the site is sunny. Both are difficult to control, and if they are present with the fungal blight, you should consider a different type of ground cover. You might try growing hypericum or epimedium in this space if it is too sunny or stressful for the pachysandra.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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