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
Benefits of Proper Mulching
- Helps maintain soil moisture. Evaporation is reduced, and the need
for watering can be minimized.
- Helps control weeds. A 2- to 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 “lawn
- 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 large
fluctuations in temperature and moisture. Applying a 2- to 4-inch layer
of organic mulch can mimic a more natural environment and improve plant
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 that characteristic
a positive one, 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 over
mulching. 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
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.
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 are
- Organic mulches usually are 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
- Composted wood chips can make good mulch, especially when they
contain a blend of leaves, bark, and wood. Fresh wood chips also may
be used around established trees and shrubs. Avoid using noncomposted
wood chips that have been piled deeply without exposure to oxygen.
- For well-drained sites, apply a 2- to 4-inch layer of mulch. 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.
International Society of Arboriculture