Volume XVII, Issue 16 - April 16 - April 22, 2009

The Bay Gardener

by Dr. Frank Gouin


Too Mulch!

Beneath the pretty surface, it’s killing your plants

Many organizations are now taking orders for bagged mulch. Most is double shredded hardwood bark. This is the second-worst mulch sold.

Over-mulching is a national disease. Over the years, I have probably diagnosed more problems associated with over-mulching than any other cause. Every year hundreds of plants are killed or harmed because of over-mulching. It is not uncommon to find six inches to a foot of mulch over shallow-rooted plants such as azaleas, rhododendrons, yews, boxwoods and hollies.

Stop before it’s too late! Every time you apply another layer, you are suffocating the roots. Each layer makes it more difficult for oxygen to penetrate down into the soil. Layer by layer, carbon dioxide accumulates around the roots, stunting or killing plants. As mulches decompose, they leave behind a thin layer of fine particles that seal out oxygen and seal in carbon dioxide. The more mulch you have covering the roots, the more water it takes to reach the roots, especially in drought.

The excuse is generally that mulch controls weeds and makes the landscape look nice. Do sick-looking and dead plants look nice in the landscape?

The worst mulch on the market is Big Red or Big Brown. These mulches are made of ground pallets or wood waste dyed with artificial color. They rob nutrients from the soil. Shallow-rooted plants such as azaleas, rhododendrons, boxwoods, herbaceous perennials and annuals are unable to compete. The problems I predicted when these mulches were introduced continue to occur.

Double-shredded hardwood bark causes the same problem. When it does eventually decompose, it leaves behind high levels of manganese, which accumulate from repeated applications. When manganese levels accumulate above 200 pounds per acre, plants begin to exhibit iron deficiency and stunted growth. They eventually die. Double-shredded hardwood bark mulch also tends to raise the pH of the soil, making it more alkaline, which is opposite of what azaleas and rhododendrons need. I have seen situations where manganese levels were so high that the only solution was replacing the plants and the soil.


Ask Dr. Gouin your questions at frgouin@erols.com. All questions will appear in Bay Weekly.
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