Root of the problem: mounds of mulch
Mulch is a standard form of soil and winter protection for many shallow-rooted plants. However, too much mulch can be too much of a good thing.
The availability and low cost of bark-type mulch materials has created an over-mulching disease in our landscapes. Gardeners have come to think of mulch materials as wonder products that provide endless benefit to our plants.
Misuse of mulch materials decreases a landscape's aesthetic value by changing the appearance of a plant. Misuse of mulch can also kill many of the materials it is intended to help and protect.
Shade and flowering trees that have mounds of mulch surrounding their trunks are unsightly. They look like ant hills with a plant in the middle. Such use of mulch detracts from the quality of the landscape and spells death for the plant.
Some plants can withstand such abuse, but mulching is often the leading cause of death to azaleas, rhododendrons, dogwoods, pieris, boxwoods, mountain laurel, hollies, cherry trees, ash, linden, maples and any number of narrow-leaf evergreen trees.
The over-mulching of a plant not only wastes material, but it can smother the root system or provide optimum conditions for the development of cankers on the lower trunk. In either case, the damage cannot be reversed and the plants can die within a matter of seasons.
Not all mulch material has the same potential for injury or damage to the plants. However, no matter what material is selected, try to maintain a two- to three-inch layer and no more.
When decomposition results in a loss of mulch material, replace it to return to the depth of two to three inches. All too often, gardeners apply a fresh layer each season to retain the specific look achieved with the fresh product.
Most organic mulches, except for the bark of coniferous trees, are high in cellulose and low in lignins. Cellulose decomposes rapidly and leaves little residue on the soil surface.
The bark of coniferous trees like pine and spruce are low in cellulose and high in lignin. Such bark mulches are more resistant to decomposition and last longer on the soil surface.
The ground below these mulches appear to accumulate high amounts of fine material as the material decomposes. As additional layers are added each season to keep a fresh look, you are actually increasing that fine layer below the visible layer of mulch. This accumulation of bark fibers tends to remain wet and acts as a seal over the surface of the soil.
Roots under a sealed surface layer are often starved of oxygen necessary for optimum growth and development. With a weakened root system, plants develop poor foliage color, flower slowly and may even collapse in the first warm weather. Foliage that is present may be chlorotic which resembles iron deficiency, and abnormally small, and stem growth is weak and there can be considerable die-back of older stems.
Well-established plants can also be injured with an over-application of mulch. There are many instances where a perfectly healthy tree has been killed with this practice. The injury occurs when the plant bark begins to decompose under the layer of mulch. The resulting canker (or open wound) in the bark does not heal.
The essential connection between the food-producing foliage and the nutrient and water-absorbing root system is broken. Basically, the top and bottom of the plant starve from the lack of supplies. Flowering cherry, dogwood and ash seem to very susceptible to this type of injury. Other trees can establish roots into the mulch layer, but these roots do little to sustain the plant under stressful periods.
If you apply mulch every spring, remember that most ornamental plantings only need to be mulched every two to three years for an adequate layer. A light raking will loosen it and increase its effectiveness. Raking will also make mulch material look fresh again for the rest of the season. When adding a new layer of material, incorporate the older material into the soil surface to reduce compaction of the original layer's decomposition products.
Some gardeners tend to apply a new layer of mulch in spring in hopes of killing out a crop of weeds in the existing mulch. Once your mulch layer is infested with weed seeds, it is next to impossible to control them with new mulch.
Mulch is still one of the most effective gardening tools, but like anything else, when carried to extremes or used for the wrong purpose, injury is very possible.
For answers to your garden questions, call the Lehigh County Cooperative Extension Office, 610-391-9840, or Northampton County Cooperative Extension Office, 610-746-1970, and ask to speak with a Master Gardener. Volunteers staff phone lines several days a week, Monday - Friday.
Growing Green is contributed by Lehigh County Cooperative Extension Office Staff and Master Gardeners.