Over-mulching of trees and shrubs can best be described as a plague that is slowly and methodically annihilating ornamental tees and shrubs in established landscapes. Over-mulching is a serious problem responsible for the decline and death of many shallow rooted shrubs as well as large coniferous and deciduous trees.
Nurserymen and landscape contractors have long been aware of the importance of not placing excessive amounts of soil over the roots of established trees and shrubs. However, they fail to understand that excessive use of mulch around shallow rooted plants has the same detrimental effect. Heavy and repeated applications of mulch around established azaleas, rhododendrons, mountain laurel, leucothoe, andromeda, boxwoods, hollies, yews, camelias, etc., suffocates their roots. If the species is able to root readily, it will often initiate new roots from the stems into the mulch, but produce little top growth. Shallow rooted plants growing on sandy soils appear to tolerate more mulching than similar plants growing in heavy soils but even they eventually succumb to suffocation.
Death to plants from over-mulching is slow and agonizing. There is generally a gradual decline in the plant vigor. The annual rate of growth becomes less each year and the leaves do not grow to mature size. Symptoms of iron chlorosis on the newly emerging leaves appear in late spring and later summer. Approximately a year before the plants die, the new growth exhibits a severe iron chlorosis and there is often considerable winter die-back of branches. There is generally little response to foliar applications of chelated iron or fertilizers. During this last stage of decline, there is little chance for recovery and the plants become susceptible to attack by insects and diseases.
Boxwood decline can often be attributed to over-mulching. Simply removing the mulch from around the plants exhibiting early symptoms of decline has resulted in total recovery. During the winters of 1976-77 and 1981-82 many over-mulched boxwoods and Japanese hollies were killed. Because these species root readily, their only healthy roots were growing in the mulch layer. The original root systems in the soil were dead from suffocation. With no snow on the ground to insulate, temperatures in the abnormally dry mulch dropped below root killing temperatures for these species. Similar plants growing in the same vicinity, but without mulch or only lightly mulched survived without difficulty.
Many coniferous and hardwood trees are killed because mulch is piled high around the stems. Organisms that cause stem rots thrive in the cool moist environment of heavy mulch. Most species of spruce appear to be especially susceptible to injury from mulch piled around the stems.
Although it is profitable to apply mulch annually to landscape plantings, it is also detrimental. Many irreplaceable trees and shrubs in landscapes are being killed because organic mulches are being overused. One to two inches of mulch applied every 2 to 3 years is adequate to keep the soil cool, reduce water lost by evaporation, give the landscape a neat appearance and to allow easy penetration of water to the soil. Shallow raking of existing mulch will give the landscape planting that freshly mulched appearance. Fresh mulch should not be applied until the existing mulch is nearly decomposed and lightly incorporated into the soil.
Mulches should not be used exclusively to control weeds. Organic mulches decompose and enrich the soil, making conditions favorable for seed germination. Weeds in landscape plantings should be controlled mechanically by hand weeding or with pre-and/or post-emergence herbicides. Wick or directed spray applications of post-emergence herbicides can be used to kill existing weeds while the pre-emergence herbicides will control germinating weed seeds,
Happy Gardening!
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