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- 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 grass cutting
- 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 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,
geo-textile 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
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 colour) creates a
build-up 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
- 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
- Anaerobic “sour” mulch may give off pungent odors, and the
alcohols and organic acids that build up may be toxic to young
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
- 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
- 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
- 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
non-composted 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