Volume 13, Issue 22 ~June 2 - 8, 2005

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Dr. Gouin's Bay Gardener

Too Mulch of a Good Thing
Part 2. Death by Asphyxiation

The roots of plants need air. Some plants — among them azaleas, rhododendrons, boxwoods, dogwoods and mountain laurel — have extremely shallow roots. The roots of these plants are near the surface of the soil because they need lots of oxygen, like we do.

Other plants — such as oaks, yews, forsythia and viburnums — grow roots deeper into the soil because their roots can tolerate growing where there is less oxygen. Yet other plants — including cypress, sweetbay magnolia and cattails — can grow submerged in water because they can tolerate growing where there is still less oxygen because they have other means of breathing and surviving.

Yet numerous research studies have concluded that 90 percent of all tree roots can be found growing in the upper 10 inches of soil, where there is sufficient oxygen for root growth and development. For shallow rooted species, 90 percent of their roots can be found in the upper three inches of soil.

Yearly applications of thick layers of mulch around shrubs and in gardens results in an accumulation of organic matter on the surface of the soil. As these mulches decompose, they leave behind a layer of fine particles called humus. Unless this layer of humus is incorporated into the existing soils by shallow cultivation, it seals the surface of the soil, restricting the movement of oxygen and carbon dioxide in and out.

Yearly applications of mulch over a five- to 10-year period, without soil incorporation, may add up to three inches of rotted organic waste, which shuts off the oxygen supply to shallow rooted species. Unless these species can initiate new roots from the stems and develop new roots into the mulch layer, they will die.

Death or stunting by asphyxiation of the roots caused by repeated application of mulch or over-mulching is a very real problem. The problem can be solved by removing the old mulch and replacing it with new mulch each year; freshening the existing mulch with a steel rake and applying new mulch only at two- to three-year intervals; or applying only one inch of compost per year — which will not only look great but will supply nutrients to the plants at the same time.

Q Even the editor can’t ignore the doctor’s orders. I now want to have my soil tested. Do I gather a different test bag for each area of lawn (front and back) and garden (fern, vegetable, azalea) that has a different purpose?

—Sandra Martin, Fairhaven Cliffs

A One bag for the lawn, one bag for the vegetable garden and one bag for the azaleas. Ferns are not fussy.

Professor Emeritus Francis Gouin retired from the University of Maryland, where he was the state’s extension specialist in ornamental horticulture. Follow his column of practical gardening and plant advice every week, only in Bay Weekly. Ask Dr. Gouin your questions at [email protected].

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