Murder by Mulch
Are you guilty?
Looking out from the window of my room at Heritage Harbor Rehabilitation Center, I see mountains of mulch suffocating the trees. The sight is enough to undo my promising rehab after falling off a ladder while cleaning gutters on May 13.
Over the 30 years I served as Extension Specialist in Ornamental Horticulture with statewide responsibilities, I concluded that most problems associated with landscapes were related to over-mulching.
The appearance of a fresh layer of rich brown mulch around foundation plantings and flower gardens is appealing. The sharp contrast gives the landscape a finished quality.
What you may not realize is that layer upon layer of mulch results in suffocation of roots. Roots require oxygen. The more mulch you spread over the roots, the less oxygen there is. Roots of shallow-rooted species such as azaleas, rhododendrons, mountain laurel, andromeda, leucothoe, blueberries, dogwood, Japanese and American holly as well as boxwoods have a high oxygen requirement and are easily damaged or killed by repeated applications of mulch.
If the plants are unable to generate new roots in the mulch, they will show symptoms of decline, including short new growth, smaller leaves, discolored leaves often with purplish leaf margins, dead and dying branches and general decline.
If shredded hardwood bark is applied year after year, the result can be an accumulation of manganese, which prevents the absorption of iron by the roots of the plants. There is no cure for this problem other than replacing the soil.
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Hardwood bark mulch also tends to raise the pH, making the soil less acidic. This can be a problem for species that perform best in acidic soils. The problem can be resolved by applying acid-base fertilizers. However, acid-based fertilizers result in releasing more manganese into the soil, thus further reducing the absorption of iron by the roots.
To avoid murder by mulch, first remove last year’s layer of mulch before applying new mulch. In soils that already have high levels of manganese, use pine bark mulch or pine needles. Although pine bark does contain manganese, it also contains compounds that reduce the solubility of the manganese, making it less damaging.
You may already have enough mulch on the ground, so all you need to do is use a steel rake to loosen it and break up the layers. If that deep brown color is what you want, consider spraying liquid mulch onto your existing mulch.
Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.