HYG-1083-96
Dr. Mary Ann Rose1
Dr. Elton Smith
A mulch, by definition, is a layer of bark, sphagnum peat moss,
muck peat, compost, pine needles, gravel, plastic, or any similar
material uniformly spread on the surface of the soil under plants.
Among the objectives of applying mulch or mulching are to:
- prevent weed growth.
- conserve moisture in the soil.*
- cool soil surface and stabilize soil temperature.*
- reduce heaving (plant roots forced upward out of soil) of
small plants as a result of alternate freezing and thawing of
the soil in autumn, winter and spring.*
- add organic matter to soil, if mulch materials are organic
in nature.
- reduce soil erosion on slopes.
- keep fruits, vegetables and flowers cleaner.
- improve aesthetics of a landscape and add to property
values.
* Research by the Weyerhaeuser Co. (1969) indicated that two
inches of bark: 1) reduced moisture loss in summer by 21 percent, 2)
reduced soil temperature in summer, in the upper four inches of
soil, by 10 degrees F and, 3) delayed soil temperature in winter
from reaching the freezing temperature by two days compared to
unmulched soil.
Effects of Organic Mulches in Soil
Organic mulches, derived from plant material, will decompose in
time and enrich and improve the soil. This results in increased
aeration of silt or clay loam soils and added water-holding capacity
of sandy loam soils. Mulching improves and stabilizes soil structure
(arrangement of soil particles) by reducing the impact of rain,
weight (people walking on the soil) and cultivation of soils,
especially when wet. In short, compaction of the soil is reduced.
The pH (acidity or alkalinity) of soil can be changed depending
on the mulch selected. For example, most composts will be slightly
alkaline (pH greater than 7) and excellent for use in acidic soil
regions, while continuous use of oak leaves, pine needles, pine bark
and sphagnum peat moss will increase acidity. The breakdown products
of leaves, including oak leaves, will be alkaline, but continuous
use of oak, pine and sphagnum peat moss products will keep the soil
surface acidic (pH less than 7).
Organic mulches contain both major and minor mineral elements
essential for plant growth, but should not be considered substitutes
for fertilizer. In fact, materials such as straw, wood chips and
sawdust have high carbon to nitrogen ratios; and therefore, nitrogen
must be added to the soil at the time of mulching. The high carbon
to nitrogen ratio materials are easily decomposed by microorganisms.
The microorganisms require nitrogen to multiply and survive. As a
guide, 1 to 2 pounds of actual nitrogen per 1,000 square feet should
be applied. For example, if using a 10% nitrogen fertilizer, such as
a 10-6-4, applying 1 pound nitrogen to 1000 square feet would
require 10 pounds of the fertilizer product. If additional nitrogen
is not supplied, the nitrogen in the soil will be used by the
microorganisms in the root zone of desired landscape plants, and
nitrogen deficiency will likely occur.
Timing the Application of Mulches
Mulch is usually applied first in spring after the soil has
warmed and begun to dry from winter rains and snow. Depending on
soil texture (size of soil particles, e.g., clay [very small]; silt
[medium]; and sand [large]), temperature and other weather
conditions, this is usually mid to late spring. Mulching too early
can delay drying of soil and subsequent root growth which is
dependent upon sufficient aeration (oxygen content in soil) and
reasonably warm temperature in the root zone. In seasons of more
than ample rainfall and/or cool temperatures, mulching should be
delayed until late May or June.
A second application is often needed in autumn. For established
plants, timing of the mulch application may not be as important as
it is on new plantings, especially shallow rooted shrubs and
evergreens, herbaceous perennials, or strawberries. Heaving of small
plants, especially those newly planted, occurs following alternate
freezing and thawing of the soil. To reduce heaving, which breaks
roots and leads to winter injury, apply mulch following the first
indication of frost in the ground. Mulching slows the thawing cycle
and thus heaving. For established plants, water thoroughly in late
autumn if the soil is not already moist, then mulch.
Mulching Depth
The recommended mulching depth, depending on the material
selected, is 2 to 2.5 inches. At this depth, most mulches will
accommodate the primary objectives of weed control, soil moisture
conservation and temperature modification.
Mulching depths less than two inches may not satisfy the
principal objectives. However, mulch applied 3 to 6 inches or more,
still recommended by some, can lead to serious problems for
landscape plants. A mulch that is too thick may severely reduce or
eliminate drying and lead to water-logged soil, particularly during
wet seasons or in heavy clay loam soils. Extended periods of wet
soils in spring are most damaging to a number of perennials, azalea,
rhododendron, conifers in general, and taxus or yew in particular.
Excess mulch, particularly if applied right against the stem or
trunk of landscape plants, also leads to constantly wet bark and
conditions favorable for disease development.
Extra heavy mulch layers in autumn are often a haven for rodents
to nest, which may lead to girdling (destruction of the food
conducting vessel in the stem) of selected plants when wildlife food
sources are covered with snow. In autumn, pull the mulch back
approximately six inches or more from the stem or trunk of all woody
plants to avoid rodent feeding.
When reapplying mulch over an existing base, bring the total
depth up to 2 to 2.5 inches. Avoid applying two inches of mulch with
each application or soon a depth greater than three inches will
accumulate. For example, when using cypress bark, which is slow to
decompose, very little mulch will be needed in subsequent
applications to maintain desired depths.
Some Recommended Mulch Materials
Mulching products most frequently available in garden centers are
reviewed briefly below:
Inorganic or Fabricated Mulches
Aluminum foil. Use is limited primarily to vegetable
plants where research findings have indicated a significant
reduction in insect pests, such as aphids, and viruses carried by
insects.
Black polyethylene. Still used in vegetable and vine crop
production (cucumbers and melons), but no longer recommended for use
in the landscape unless the soil is very well drained. Since air
cannot penetrate polyethylene film, soils remain too wet and thus,
black polyethylene film has been replaced with landscape fabrics.
Landscape fabrics. Landscape fabrics, also known as
geotextiles or weed barriers, are relatively new to the marketplace.
The two types of fabrics available are woven and non-woven
polypropylene polymers (synthetic material). Polymers of non-woven
fabrics are spun-bound or meshed, while polymers of woven fabrics
are generally thicker in diameter and tightly woven. Most
polypropylene polymers are readily oxidized (combined with oxygen
and degrades) when exposed to ultraviolet light, and therefore, are
limited in use to an undercover for a more decorative product such
as shredded bark. Some fabrics, however, are surface coated with
carbon black. Since these fabrics offer a degree of resistance to
degradation from ultraviolet light, double mulching would not be
necessary.
Examples and sources of landscape fabrics include:
- DewittTM, black, woven, Dewitt Co., Sikeston, MO
- DuonTM, gray, spun-bonded, Blunks Wholesale
Supply Inc., Bridgeview, IL
- TyparTM, gray, spun-bonded, Remay, Inc., Old
Hickory, TN
- VisqueenTM, black, woven, Visqueen Film Products,
Richmond, VA
- TEITM, white, woven, Dupont, Baltimore, MD
- ExxonTM, white, spun-bonded, Landscape Supply
Inc., Roanoke,VA
Recycled rubber tires. Shredded recycled rubber tire mulch
is available in several colors. This mulch is used extensively in
parks, schools, highways and industrial sites.
Stone, chips, pebbles, gravel. Available in many sizes and
colors, these materials are utilized by landscape architects for
color and texture changes, particularly in plantings in blacktop
covered areas. All are fire resistant and have application near
public entrances to buildings, along sidewalks and in shopping
areas. They offer no value as breakdown products in the soil, which
is true of all inorganic mulches.
Organic Mulch Products
Animal Manures. Historically, manure has been available
from farms in bulk, but some are packaged for retail sales. Included
in retail packaging are manure from cow, horse, sheep, poultry and
zoological (zoo) animals. Manures are dried or dehydrated and sold
for soil amending, mulching, or fertilizer purposes. Weed seed are
sometimes introduced with manure.
Composts. This is an old term with new meanings as compost
technology and new products become available. Although usually not
packaged for retail use, grass clippings, leafmold and used mushroom
composts are available.
Composted municipal sludge. This material is now available
as a mulch with trade names such as EarthlifeTM, ComtilTM
and TechnaGroTM, among others. In the future we will see
composts containing municipal garbage, paper pulp, yard wastes and
many other by-products. The landfill legislation across the United
States has given rise to composting technology that will continue to
advance and bring new products.
Hulls, cobs, shells. This group of by-products, to a large
degree from the food industry, includes such mulches as cottonseed,
buckwheat, cocoa-bean, peanut or rice hulls, crushed corn cobs,
spent hops, tobacco stems and similar products. All have been used
extensively for mulch and are usually inexpensive. The disadvantage
is that availability is typically limited to a local area.
Sphagnum peat moss/muck peat. These terms are often
misunderstood. Sphagnum peat moss comes from mosses (sphagnum,
hypnum, etc.) and contains long fibers which resist decomposition.
It is available from Canada, Russia, Ireland and much of Europe.
Muck peat originates from well decomposed plant material that
once thrived in swamps such as cattails, reeds, sedges and other
water plants. While sphagnum peat moss is usually quite acidic, muck
peat is usually neutral to slightly alkaline, is well decomposed and
is dark brown or black with almost no fibers. Muck peat is available
from sources in many U.S. states including Ohio, Michigan and
Indiana. It is fine in texture, dries quickly and can be blown away
by the wind when used as a mulch. It, too, is often locally
available at a reasonable price but of somewhat limited value as a
mulch.
Pine needles. The needles of pine trees as well as
shredded cones make an excellent mulch particularly for evergreens
and plants that thrive in acidic soils. Pine needle mulch, formerly
available only in the southern states, is becoming available in
other regions.
Straw. Straw and hay are used for winter protection of
perennials, strawberries and small plants. If left as a permanent
mulch, additional nitrogen (one pound of nitrogen per 1,000 square
feet) is suggested, since this material decomposes readily. Weed
seed can be introduced from straw, hay and grain crop residues. Salt
hay, available along coastal states, is a very acceptable mulch.
Wood products. For simplicity, we will group all wood
products into a) shredded, chip or chunk bark, b) wood chips or
shavings and c) waste wood.
Shredded, chip, or chunk bark. This material is by far the
most popular landscape mulch due to its appearance, serviceability
and cost. Included are shredded hardwood and cypress bark, chipped
and chunk pine, and fir and eucalyptus bark.
Wood chips or shavings. They have become available through
utility companies and are usually not available in packaged form.
Since they consist, along with sawdust, of more wood than bark, they
decompose rapidly and should be supplemented with fertilizer at the
rate of one pound of actual nitrogen per 1,000 square feet of
mulched area.
Waste wood. Materials such as used pallets and other wood
pieces that normally would have been taken to a landfill are now
shredded and metal objects are removed. The resulting chips are
colored to match pine, hardwood, cypress or any other mulch product
on the market. This material also decomposes faster than bark and
requires the addition of nitrogen fertilizer.
AMERICAN LAWN CARE - 167 WASHINGTON ST
- PEMBROKE, MA 02359
PHONE 781-826-1118 FAX 781-826-1118 EMAIL
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