is one of the most important ways to maintain healthy landscape plants.
A mulch is any material applied to the soil surface for protection or
improvement of the area covered. Mulching is really nature’s idea.
Nature produces large quantities of mulch all the time with fallen
leaves, needles, twigs, pieces of bark, spent flower blossoms, fallen
fruit and other organic material.
Benefits of Mulching
- When applied
correctly, mulch has the following beneficial effects on plants and
- Mulches prevent
loss of water from the soil by evaporation.
- Mulches reduce
the growth of weeds, when the mulch material itself is weed-free and
applied deeply enough to prevent weed germination or to smother
- Mulches keep the
soil cooler in the summer and warmer in the winter, thus maintaining a
more even soil temperature.
- Mulches prevent
soil splashing, which not only stops erosion but keeps soil-borne
diseases from splashing up onto the plants.
- Organic mulches
can improve the soil structure. As the mulch decays, the material
becomes topsoil. Decaying mulch also adds nutrients to the soil.
- Mulches prevent
crusting of the soil surface, thus improving the absorption and
movement of water into the soil.
- Mulches prevent
the trunks of trees and shrubs from damage by lawn equipment.
- Mulches help
prevent soil compaction.
- Mulches can add
to the beauty of the landscape by providing a cover of uniform color
and interesting texture to the surface.
- Mulched plants
have more roots than plants that are not mulched, because mulched
plants will produce additional roots in the mulch that surrounds them.
Types of Mulches
There are basically
two types of mulches: organic and inorganic. Both types may have their
place in the garden.
An organic mulch is
a mulch made of natural substances such as bark, wood chips, leaves, pine
needles, or grass clippings. Organic mulches attract insects, slugs,
cutworms and the birds that eat them. They decompose over time and need to
be replaced after several years.
such as gravel, pebbles, black plastic and landscape fabrics, do not
attract pests and they do not decompose.
Your yard "trash" can be recycled as mulch with the advantage of
retaining the nutrients found in these organic materials, in addition to
saving money otherwise spent in transporting and disposing of the yard
The best use for grass clippings is to leave them on the lawn. Grass
clippings will decompose rapidly, adding nutrients back into the soil. A
two-inch layer of grass clippings provides weed control if they are not
full of weed seeds. It is best to build up the layer gradually using dry
grass, not fresh clippings, to prevent the formation of a solid mat. Be
careful not to use clippings from lawns that have been treated with
Hay and Straw
Never use hay for
mulch since it contains too many weed seeds. Straw decomposes rapidly, so
you will have to replenish it to keep the weeds down. Straw is not very
ornamental and is best for a vegetable garden or over newly sown lawns.
Straw will improve the soil as it decays.
Leaf mold has a tendency to form a crust, preventing water from
penetrating into the soil. It is better to use leaf mold as a soil
amendment than as a mulch.
A 2- to 3- inch layer of leaves provides good weed control. It is best to
shred the leaves coarsely, using a shredder or your lawn mower. Whole
leaves have a tendency to blow away, while finely shredded leaves do not
allow water to penetrate. Oak and beech leaves help to acidify the soil
for acid-loving plants. Leaves are usually easy to get, attractive as a
mulch, and they will improve the soil once they decompose. After the
leaves decompose, dig them into the soil and add a new layer of mulch on
A 2- to 3- inch layer of pine bark is good for weed control. Pine bark
makes an attractive, usually dark-colored mulch. It can be purchased in
various particle sizes, from shredded to large-sized particles, called
nuggets. Large pine bark nuggets float in water and may not stay in place
during a heavy rain. They may also attract termites and other insects.
A 2- inch layer of pine needles makes an excellent mulch for acid-loving
trees and shrubs. This mulch is very attractive and allows water to
Shredded Hardwood Mulch
This mulch is good at suppressing weeds. It does not wash away easily. It
decomposes relatively slowly, and it is very attractive.
This material contains bark and pieces of wood of various sizes and makes
an attractive mulch. A 2- to 3- inch layer of wood chips provides good
weed control. Small wood chips decompose very rapidly using nitrogen from
the soil, which needs to be replaced by nitrogen fertilizer. Wood chips
may attract termites and other insects.
Pecan shells make a long-lasting, attractive, dark brown mulch that is
effective in retaining moisture in the soil. Availability is usually
limited to areas where pecans are processed.
Many perennial ground cover plants, such as ivy, periwinkle, pachysandra,
mondo grass and liriope, will cover the soil and act as a mulch.
Inorganic Mulch Materials:
Gravel, Pebbles and Crushed
These materials are permanent and are best used for permanent plantings
such as foundation plants. A 1- inch layer of small rocks will provide
good weed control. Do not use them around acid-loving plants since the
rocks may add alkaline elements and minerals to the soil. These materials
reflect solar radiation and can create a very hot landscape environment
during the summer months.
polyethylene film is very effective in preventing weed growth. It also
holds water in the soil. Therefore, plastic is not recommended for
poorly-drained areas as it may cause the soil to remain too wet, which
could result in root disease problems. You may have to cut holes in the
plastic if water does not go through it. There is black plastic available
that has small holes in it to help with drainage. If exposed to sunlight,
black plastic is broken down fast, losing its effectiveness as a mulch.
However, if you bury black plastic in the soil, it will last for many
years. Covering the black plastic with a layer of wood chips or pine
needles will reduce heat absorption and mask its artificial appearance.
Clear plastic will not suppress weed growth because light penetrates the
film and raises the soil temperature, which may result in an increased
growth of weeds in early spring.
Landscape Cloth or Woven
Materials woven of fabric, plastic or paper are available in various
lengths and widths. The materials are treated to resist decomposition.
Unlike plastic films, woven materials allow water and air to move through
them. They are very effective in controlling most weeds, although some
grasses may grow up through the holes in the fabric. Landscape cloth needs
to be fastened down so it will not be pushed up by perennial weeds. Better
moisture, temperature and weed control will be obtained by adding several
inches of another mulching material on top of the landscape cloth.
Aluminum-coated plastic and
One layer of
either one of these materials provides excellent weed control. These
materials decompose very slowly, but they are very expensive and quite
Ground Rubber Tires
Mulches made of
ground rubber tires do not decompose and therefore, never need to be
replaced. The use of ground rubber tires is relatively new and its
effectiveness as a mulch is still being evaluated.
Where to Use Mulch
Mulching is a very important
practice for establishing new plantings, because it helps to conserve
moisture in the root ball of the new plant until the roots have grown out
into the surrounding soil. The growth rate and health of trees and shrubs
increases when there is no competition for water and nutrients from weeds.
Mulch also helps to prevent tree trunk injury by mowers and trimmers.
Newly planted trees require a circle of mulch 3 to 4 feet in diameter.
Maintain this for five years. Mulch entire beds of shrubs, trees, annuals,
herbaceous perennials and ground covers.
Mulch can also be used to
cover trails, driveways, and play and natural areas.
Light-weight mulch such as
dried grass clippings and pine straw can be used temporarily to cover
low-growing tender plants to protect them from frost injury.
When and How Often to Mulch
The best time to mulch new
plantings is right after you plant them. Around established plants mulch
is best applied in early spring. This is when plants are beginning to grow
and before weed seeds start to germinate.
How often mulch needs to be
replenished depends on the mulching material. Grass clippings and leaves
decompose very fast and need to be replenished frequently. Inorganic
mulches such as gravel and pebbles rarely need replenishing. As the plants
grow and fill in the bed areas, less and less mulch is needed.
How to Apply Mulch
Before applying any type of
mulch to an area, it is best to weed the area. Spread a layer of mulching
materials over the entire plant bed. Keep mulch 2 to 3 inches away from
the stems of woody plants. This will prevent decay caused by wet mulch and
rodent damage during the winter. Keep mulch 6 to 12 inches away from the
walls of buildings.
Subterranean termites nest in
the soil and feed on materials that contain cellulose. Termite treatments
are applied to the soil around buildings, so keeping mulch away from walls
will prevent termites from using it as a bridge to cross treated soil.
Newly planted trees require a
circle of mulch 3 to 4 feet in diameter. Maintain this for at least three
years. Do not pile mulch against the trunk. For established trees in lawns
create a circle of mulch about 2 feet in diameter for each inch of trunk
diameter. Increase the size of the mulched area as the tree grows. Try to
apply the mulch at least 6 to 12 inches beyond the drip-line of the tree.
Because the root system can extend two to three times the crown spread of
the tree, mulch as large an area as possible.
How Deep to Mulch
The amount of mulch to apply
depends on the texture and density of the mulch material. Many wood and
bark mulches are composed of fine particles and should not be more than 2
to 3 inches deep. Excessive amounts of these fine-textured mulches can
suffocate plant roots, resulting in yellowing of the leaves and poor
Coarse-textured mulches such
as pine bark nuggets allow good air movement through them and can be as
deep as 4 inches.
Mulches composed of grass
clippings or shredded leaves should never be deeper than 2 inches, because
these materials tend to mat together, restricting the water and air supply
to plant roots.
How to Calculate the Amount
of Mulch Needed
To determine how many cubic
feet of mulch is needed, you need to calculate the surface area and the
desired depth of coverage. There are 27 cubic feet in a cubic yard. One
cubic yard will cover a 324-square-foot area with an inch of mulch. Figure
out the square footage of your bed, that is the width times the length for
square or rectangular shaped beds. The square footage of a circular bed is
the distance from the middle of the circle to the outside, multiplied by
itself and then multiplied by 3.14 (which is pi).
Multiply your square footage
by the depth desired (in inches) and divide by 324 square feet. This will
tell you how many cubic yards you will need.
Though mulch benefits
plants, "sour" mulch can quickly damage plant tissue and lower
the soil pH causing injury or death. Bedding and low-growing woody plants
are most easily damaged. Symptoms include yellowing of the leaf margins,
scorching or dropping of leaves and occasionally entire plant death.
Although it may be several days before symptoms appear, spreading sour
mulch can damage plants immediately.
Sour or "acid"
mulch is caused by poor handling or storing of mulch resulting in
anaerobic (without air) conditions. Mulch piles need to
"breathe" to prevent anaerobic conditions from occurring. In the
absence of air, microbes in the mulch (mostly bacteria) produce toxic
substances such as methanol, acetic acid, ammonia gas, and hydrogen
Sour mulch smells like
vinegar, ammonia, sulfur or silage. Good mulch smells like freshly cut
wood or has the earthy smell of a good garden soil. Another way to
determine if mulch is sour is to test its pH. Toxic mulch will have a pH
of 1.8 to 2.5.
To prevent mulch from turning
sour or to cure sour mulch, you need to turn your pile once or twice a
month, more frequently if the pile is very wet. Do not let the pile get
larger than 4 feet thick in any dimension if you are not turning the pile
regularly. A good aeration will eliminate the toxic compounds in 24 hours,
but to be safe allow three days.
The Artillery Fungus
organisms in mulches can become a nuisance. The shotgun or artillery
fungus (Sphaerobolus) may cause serious problems. While it decays
the mulch, it also produces fruiting structures that resemble tiny cream
or orange-brown cups that hold a spore mass resembling a tiny black egg
(1/10 inch in diameter). This fungus shoots these spore masses high into
the air. They stick to any surface and resemble small tar spots on leaves
of plants, on cars or on the siding of homes. They are very difficult to
To avoid damage to cars and
houses do not use mulches that contain cellulose (wood). Use pure bark
mulches, especially pine, or if the mulch is already in place, cover the
hardwood mulch with pine needles.
Slime molds are
another type of nuisance fungus. They first appear as bright yellow or
orange slimy masses that may be several inches to a foot or more in
diameter. They are harmless but unsightly. Some fungi in mulches produce
toad stools (mushrooms), and some of these are toxic to humans. It is a
good idea to destroy them when small children have access to the mulched
Prepared by Marjan Kluepfel, HGIC Information Specialist, and Bob
Polomski, Extension Consumer Horticulturist, Clemson University.