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Mulching
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Overmulching Problems
The number one cause of death to trees
and shrubs in the landscape is from overmulching the root zone. A too-deep mulch keeps the soils waterlogged, slowing
evaporation in the root zone. Roots must breathe and take in oxygen. When soil oxygen drops below 10 percent, roots decline
and will die in time. It can take 3 to 5 years, depending on the size of the plants and how much of the root system is
covered by mulch.
The number two cause of death is from mulch being in contact with the stems and trunks of
plants. Plants need to freely exchange oxygen and cabon dioxide through lenticels. When mulch is piled against the trunks, the
gas exchange decreases with phloem tissue declining and dying. When phloem dies, water and nutrient uptake is reduced,
weakening the whole plant.
A third cause of stress in plants also is from the mulch in contact with the trunks of plants. Fungal and
bacterial disease invade the bark tissue caused by the moist environment from the mulch being piled up against the trunks of plants.
A fourth problem is that deep mulch can become a home for rodents that feed on the inner bark of plants in winter.
This is a problem because this goes unnoticed until the spring when the snow melts. If the chewing goes 50% or more around the trunk, the plant
will decline and become stressed.
All of these problems cause the plants to become stressed. A stressed plant attracts bark beetles and borers. These
insects will cause further decline of the plant. As the plant becomes more stressed, fungal diseases attack the plant, dealing it the final death blow.
Proper Mulching
Mulch 2" - 4" deep
2" for heavy soils
4" for sandy soils
Keep mulch 3" - 12" from the trunks of trees
Keep mulch 3" from shrubs and ground covers
The mulch should reach 1' - 2' beyond the drip line of the tree
For shrubs and ground covers 1' - 2' is also the proper way to mulch.
Never lift and place mulch under shrubs and ground covers that have branches that come in contact with the ground.
Mulch wide, not deep.
Mulched Tree
Mulched Shrub
Mulched Ground Cover
The reason for mulching trees and shrubs beyond the drip line is that they grow in a fungal base soil.
Grass grows in a bacterial base soil. Trees and shrubs should not be grown together. By keeping the root system free of grass, a stronger root system is created.
This helps the trees and shrubs to stay strong. A strong plant will be free of most disease and insects.
Mulch Volcanoes
A mulch volcano is when the mulch is piled up the side of the tree so the trunk looks like it is coming out
of a volcano.
The number one problem in newly planted trees is from water being shed off the mulch. The root ball does not
receive the necessary moisture. Many newly planted trees die because the mulch has kept moisture from the root ball.
A second problem is from the roots growing in a mulch that is kept too moist. Roots will grow into the mulch
and some may cause girdling roots. They grow around the trunk, restricting the growth of the trunk. Water and nutrients will not flow properly, stressing the tree.
A third problem is that roots in mulch can be killed by frost in a snowless winter -- a problem that will not be noticed until the following
growing season.
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