To Mulch or Not To
Mulch
Susan Ware, KC Master Gardener 2001
That is the question!
Over the last several years, the practice of mulching vegetable and
flower gardens has grown. Years ago,
many vegetable and flower gardeners in general did not mulch, with the
exception of utilizing materials close at hand.
Today, there is a large variety of materials available to us. The question, however, is whether mulch is
truly beneficial and under what circumstances.
We know mulching is beneficial in maintaining healthy
landscape plants by sustaining soil moisture and stabilizing soil temperature,
and it is aesthetically pleasing. Nature
produces large quantities of fallen leaves, needles, twigs, spent flower
blossoms and other organic material all the time! So, in essence, mulching is natures’ idea. However, if mulch is applied too deeply or in
poorly drained areas, for example, the results can be disastrous!
Organic mulch is made of natural substances such as bark,
wood chips, leaves, pine needles, or grass clippings. They attract insects,
slugs, cutworms and the birds that eat them. They decompose over time and must
be replaced periodically. Inorganic
mulches, such as gravel, pebbles, black plastic and landscape fabrics, do not
attract pests and do not decompose. That
being said; both organic and inorganic mulches have distinct advantages and
disadvantages attributable to them.
Listed below is a quick over view of the benefits and
possible pitfalls of some of the most popular organic and inorganic mulches.
Advantages
Apply fresh mulch late in the spring and then again in the
winter to serve as an excellent insulator.
Organic mulches can improve the soil structure. As the mulch decays, the material adds
nutrients to the soil. Root growth is
improved, increasing the infiltration of water. The organic matter provides an
ideal environment for earthworms and other beneficial soil organisms.
Mulches dissipate erosive energy from rain, protecting the
structure of the soil surface and preventing soil-borne diseases from splashing
onto plants.
Mulches shield the soil from the effects of solar radiation
which conserves soil moisture, thus reducing the need for frequent watering.
In northern climates, like in West Virginia, a layer of
mulch applied in winter prevents tender perennials from being heaved out of the
ground by alternate freezing and thawing.
After the ground is frozen, spread a 2-3 inch layer of mulch over the perennials
to keep the soil cold. Mulching is not
required for hardier perennials, such as hardy iris.
Mulches reduce the growth of weeds, when the mulch material
itself is weed-free and plied deeply enough to prevent weed germination or to
smother exiting weeds. Mulches keep the
soil cooler in the summer and warmer in the winter, thus maintaining a more
even soil temperature.
Pine straw and oak leaves make great mulches, but they
acidify the soil over time, so use them only in and around plants that require
acidic soil, such as azaleas or blueberries.
Mulches prevent crusting of the soil surface, thus improving
the absorption and movement of water into the soil.
Mulching is beneficial in establishing new planting,
conserving moisture in the root ball of the new plants 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.
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.
Unlike plastic films, landscape cloth allows water and air
to move through it. They are effective
in controlling most weeds, although some grasses may grow up through the holes
in the fabric.
Conservation tillage, leaving the crop residue on top of the
soil, is now commonly practiced on farms.
These pieces of corn stalk, straw, etc. protect the soil against wind
and water erosion.
Research shows that leaving crop residue helps hold carbon
in the soil, thus reducing greenhouse gasses.
Strawberry growers in northern climates, like
Inorganic mulches such as clear plastic mulch can give warm
season crops a head start. The clear
plastic warms the soil underneath, acting as a mini-greenhouse.
Disadvantages
Don’t use too much mulch!
Piling mulch high around the base of trees could lead to rot even in
mature trees. Applying too much mulch,
more than two to three inches, can prevent water from penetrating.
Applying too much mulch can prevent the soil from warming up
properly in early spring, which can affect plant growth. If piled on top of emerging perennials, mulch
can cause the plants to rot.
When placed too close too woody plants, mulch provides a
safe haven for mice and voles to feed on the bark and roots.
If using freshly chipped mulch, i.e. from a city operation, realize
that as it decomposes it zaps nutrients from the soil, mainly nitrogen. Also, make sure you know the source of the chips. They may come from diseased trees or worse
yet, from walnut trees that contain a substance called “juglone” that is toxic
to a number of other plants.
Hay should never by used as mulch as it contains too many
weed seeds. Straw decomposes rapidly, so
you will have to replenish it to keep the weeds down.
Black plastic can prevent weed growth but it holds water in
the soil in poorly drained areas, causing the soil to remain too wet, which
could result in root disease. In dryer
areas of the garden, drip irrigation must be installed beneath the plastic or
holes punched into the plastic to allow water to pass through, also allowing
weeds to emerge.
Clear plastic does not suppress weed growth because light
penetrates the film and raises the soil temperature, resulting in an increased
growth of weeds in early spring. Plastic
is also impermeable, thus the soil does not get any oxygen.
Do not use gravel or stone around acid-loving plants since
they may add alkaline elements and minerals to the soil. These materials reflect solar radiation, thus
creating a very hot landscape environment in the summer. Gravel and stone can also be messy over time
as garden debris such as leaves and twigs settle on or inbetween the stones.
Aluminum –coated plastic and foil provide excellent weed
control. However, they decompose very
slowly, are very expensive and quite unattractive.
Ground-up rubber tires do not decompose. The use is relatively new and the
effectiveness is unknown.
Pine bark and wood chips in general attract termites and
other insects.
“Sour” mulch (caused by poor handling or storing of 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.
Spreading sour mulch can damage plants immediately. 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
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 remove.
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 needle.
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), some of which are toxic to humans.
It is a good idea to destroy them when small children have access to the
mulched area.
In summer, when determining which type of mulch, if any, to
apply to your garden, consider the conditions of your soil, the amount of
sunlight and shade, etc. For example,
will your garden area benefit from increased soil moisture or will excessive
moisture conditions promote pathogens or reduce plant vigor? Think before you apply any mulch. And remember; do not use too much mulch!!
To quote one of my favorite funny and informative Master
Gardeners, Paul James (the Gardener Guy), “I still think the biggest mistake
you can made when it comes to mulch is not to mulch.”
References:
http://www.hgtv.com/hgtv/gardening/article/0,,HGTV_3546_2165885,00.html
Mulching No-Nos – Paul James
http://hgic.clemson.edu/factsheets/HGIC1604.htm
www.wvu.edu/~exten/infores/pubs/homgardn.htm
http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-new/text/muching.html
Mulching Trees and
http://www.nrcs.usda.gov/feature/backyard/Mulching.html