Volume 13, Issue 24 ~ June 16 - 22, 2005

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Dr. Gouin's Bay Gardener

Too Mulch of a Good Thing
Part 4: Beneath a Pretty Face
Many home gardeners like to use double-shredded hardwood bark as a mulch because it has an almost black-brown color. Experience and research has demonstrated that repeated yearly applications, after eight to 10 years of this mulch in layers of two inches or more, results in the accumulation of manganese. Manganese is an essential plant nutrient, but excess levels of 100 pounds per acre or more in soils becomes toxic to the roots of plants. Also repeated applications of double-shredded hardwood bark cause the pH of soils to increase, making the soil less acid, which can be detrimental to azaleas and related species.

Many home gardeners have killed their plants with double-shredded hardwood bark from mulch containing high levels of alcohol and acetic acid. If not properly stored and bagged, when wet this mulch will develop wood alcohol and acetic acid at rather high concentrations. You can generally detect these odors when you open the bags. Sometimes there is sufficient alcohol to ignite. If there is alcohol, there will also be sufficient acetic acid to kill any succulent plants when they come in direct contact. Smell the contents of the bag before spreading the mulch in the garden.

If you have azaleas whose leaves have become abnormally small, and reddish-green and whose small branches keep dying,and you have been using double-shredded hardwood bark, you should have your soil tested for manganese (Mn). Over the years, I have seen soil test results reading 200 to 400 pounds per acre. In such instances, the only recommendation I can make is removing and replacing the soil.

Also avoid using marble chips and blue stone around such acid-loving plants as azaleas, rhododendrons, mountain laurel and andromeda. Marble chips and blue stone are low grades of limestone that will cause an increase in soil pH, making the soil alkaline.

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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