How to Mulch Trees

Every spring, like the return of the robin, mulch mountains begin to appear in the landscape. If a little mulch is beneficial for tree health, then a mountain — carefully sculpted against the trunk — must be very beneficial. Right?

Wrong!

Mulch Mountains Harm Trees

“A 2 to 4 inch mulch layer over the tree’s root system has many benefits,” explains Robert Rouse, staff arborist with the National Arborist Association. “Never pile mulch more than 4 inches high. Homeowners should avoid over-mulching, because excessive mulch can smother the tree’s root system. And do not allow mulch to touch tree trunks, where it can serve as a conduit for harmful insects and disease.” Rouse suggests keeping mulch back about 6 to 12 inches.

When applied properly, mulch protects the base of the tree from mower damage and reduces competition with turf or weeds. Lawn grass, especially when well maintained, robs trees of valuable nutrients and moisture.

Mulch tends to stabilize soil temperatures and increase the soil’s water holding capacity, which translates into less watering. Finally, mulch can increase soil microbial activity and loosen the soil, which can reduce the need for aeration and fertilization. In short, mulch emulates the tree’s natural habitat.

Homeowners and professional arborists depend on mulches in landscapes for several reasons. Functionally, mulches discourage weeds from growing, conserve moisture during drought periods, and allow better use of water by controlling runoff and increasing water holding capacity of light, sandy soils.

Many organic materials can be used as a mulch. Bark mulches and wood chips are the two most commonly used mulches in most of the country. In the South, pine needles are included in that list. Mulch can be applied just about any time of the year when trees and shrubs are being planted. The best time to apply mulch in established bed areas, however, would be in mid-spring, when soil temperature has warmed up enough for sufficient root growth. If applied earlier, the mulch could lower soil temperature and delay root growth.

Most arborists consider organic mulches as the most compatible with trees. There are, however, several inorganic materials used as mulches, including weed barriers. Black plastic is sometimes used to discourage weeds, though it interferes with the normal oxygen and water supply to the tree’s roots. Plastic barriers can contribute to the creation of a very shallow root system, which, during drought periods, make plants less capable of obtaining water or withstanding heat stress. Therefore, it is not recommended to use black plastic around trees. There are, however, several landscape fabric mulch products available that will function in the same way as plastic but allow for normal water and oxygen exchange. These materials are placed on bare soil around trees and shrubs with the mulches used on top. There are many brands and types of materials from which to choose that have proven to be beneficial in discouraging weeds and holding soil moisture.

REMEMBER:

Keep it low! Mulch pile should be lower than 4 inches.

Back off! Do not allow mulch to touch the tree trunk, where it can make the bark rot and allow pests in.

Go organic! Many arborists find organic mulches work best with trees.


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