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Mulches are materials placed over the soil surface to maintain moisture
and improve soil conditions. Mulching is one of the most beneficial
things a home owner can do for the health of a tree. Mulch can reduce
water loss from the soil, minimize weed competition, and improve soil
structure. Properly applied, mulch can give landscapes a handsome,
well-groomed appearance. Mulch must be applied properly, if it is too
deep or if the wrong material is used, it can actually cause significant
harm to trees and other landscape plants.
Benefits of Proper Mulching
• Helps maintain soil moisture. Evaporation is reduced, and the need for
watering can be minimized.
• Helps control weeds. A 2-4 inch layer of mulch will reduce the
germination and growth of weeds.
• Mulch serves as nature’s insulating blanket. Mulch keeps soils
warmer in the winter and cooler in the summer.
• Many types of mulch can improve soil aeration, structure (aggregation
of soil particles), and drainage over time.
• Some mulches can improve soil fertility.
• A layer of mulch can inhibit certain plant diseases.
• Mulching around trees helps facilitate maintenance, and can reduce
the likelihood of damage from "weed whackers" or the dreaded
"lawnmower blight."
• Mulch can give planting beds a uniform well-cared-for look.
Trees growing in a natural forest environment have their roots
anchored in a rich, well-aerated soil full of essential nutrients. The soil
is blanketed by leaves and organic materials that replenish nutrients
and provide an optimal environment for root growth and mineral
uptake. Urban landscapes, however, are typically a much harsher
environment with poor soils, little organic matter, and big fluctuations in
temperature and moisture. Applying a 2-4 inch layer of organic mulch
can mimic a more natural environment and improve plant health.
The root system of a tree is not a mirror image of the top. The roots of
most trees can extend out a significant distance from the tree trunk.
Although the guideline for many maintenance practices is the drip line
the outermost extension of the canopy the roots can grow many times
that distance. In addition, most of the fine absorbing roots are located
within inches of the soil surface. These roots, which are essential for
taking up water and minerals, require oxygen to survive. A thin layer of
mulch, applied as broadly as practical, can improve the soil structure,
oxygen levels, temperature, and moisture availability where these roots
grow.
Types of Mulch
Mulches are available commercially in many forms. The two major types
of mulch are inorganic and organic. Inorganic mulches include various
types of stone, lava rock, pulverized rubber, geotextile fabrics, and
other materials. Inorganic mulches do not decompose and do not need
to be replenished often. On the other hand, they do not improve soil
structure, add organic materials, or provide nutrients. For these
reasons, most horticulturists and arborists prefer organic mulches.
Organic mulches include wood chips, pine needles, hardwood and
softwood bark, cocoa hulls, leaves, compost mixes, and a variety of
other products usually derived from plants. Organic mulches
decompose in the landscape at different rates depending on the
material. Those that decompose faster must be replenished more often.
Because the decomposition process improves soil quality and fertility,
many arborists and other landscape professionals consider this a
positive characteristic, despite the added maintenance.
Not Too Much!
As beneficial as mulch is, too much can be harmful. The generally
recommended mulching depth is 2 to 4 inches. Unfortunately, North
American landscapes are falling victim to a plague of overmulching. A
new term, "mulch volcanoes," has emerged to describe mulch that has
been piled up around the base of trees. Most organic mulches must be
replenished, but the rate of decomposition varies. Some mulches, such
as cypress mulch, remain intact for many years. Top dressing with new
mulch annually (often for the sake of refreshing the color) creates a
buildup to depths that can be unhealthy. Deep mulch can be effective in
suppressing weeds and reducing maintenance, but it often causes
additional problems.
Problems Associated with Improper Mulching
• Deep mulch can lead to excess moisture in the root zone, which can
stress the plant and cause root rot.
• Piling mulch against the trunk or stems of plants can stress stem
tissues and may lead to insect and disease problems.
• Some mulches, especially those containing cut grass, can affect soil
pH. Continued use of certain mulches over long periods can lead to
micronutrient deficiencies or toxicities.
• Mulch piled high against the trunks of young trees may create habitats
for rodents that chew the bark and can girdle the trees.
• Thick blankets of fine mulch can become matted, and may prevent the
penetration of water and air. In addition, a thick layer of fine mulch can
become like potting soil and may support weed growth.
• Anaerobic "sour" mulch may give off pungent odors, and the alcohols
and organic acids that build up may be toxic to young plants.
Proper Mulching
It is clear that the choice of mulch and the method of application can be
important to the health of landscape plants. The following are some
guidelines to use when applying mulch.
• Inspect plants and soil in the area to be mulched. Determine whether
drainage is adequate. Determine whether there are plants that may be
affected by the choice of mulch. Most commonly available mulches work
well in most landscapes. Some plants may benefit from the use of a
slightly acidifying mulch such as pine bark.
• If mulch is already present, check the depth. Do not add mulch if there
is a sufficient layer in place. Rake the old mulch to break up any matted
layers and to refresh the appearance. Some landscape maintenance
companies spray mulch with a water soluble vegetable-based dye to
improve the appearance.
• If mulch is piled against the stems or tree trunks, pull it back several
inches so that the base of the trunk and the root crown is exposed.
• Organic mulches are usually preferred to inorganic materials due to
their soil-enhancing properties. If organic mulch is used, it should be
well aerated, and preferably, composted. Avoid sour-smelling mulch.
• Composted wood chips can make good mulch, especially when they
contain a blend of leaves, bark, and wood. Fresh wood chips may also
be used around established trees and shrubs. Avoid using non-
composted wood chips that have been piled deeply without exposure
to oxygen.
• For well-drained sites, apply a 2-4 inch layer. If there are drainage
problems, a thinner layer should be used. Avoid placing mulch against
the tree trunks. Place mulch out to the tree’s drip line or beyond.
Remember: if the tree had a say in the matter, its entire root system
(which usually extends well beyond the drip line) would be mulched.
For Additional Information
This brochure is one in a series published by the International Society
of Arboriculture as part of its Consumer Information Program.
Why Hire an Arborist
Tree Benefits
Tree Values
Tree Selection
Buying High Quality Trees
Avoiding Tree & Utility Conflicts
New Tree Planting
Mature Tree Care
Plant Health Care
Trees and Turf
Proper Mulching Techniques
Pruning Young Trees
Pruning Mature Trees
Why Topping Hurts Trees
Insect & Disease Problems
Avoiding Tree Damage During Construction
Treatment of Trees Damaged by Construction
