University of Vermont Extension
Department of Plant and Soil Science
Anytime News
Article
WOOD CHIPS AS MULCH
Dr. Leonard Perry, Extension
Professor
University of Vermont
If you garden, you may
often hear that arborist wood chips should not be used as mulch,
which actually
is not supported by studies. Wood chips
are one of the best mulches for trees and shrubs, but may not be the
best for
annuals and vegetables, according to Dr. Linda Chalker-Scott from
Washington
State University.
In a
1990 study comparing 15 different organic mulches, wood chips were
one of the
best for holding moisture, moderating soil temperatures, controlling
weeds, and
overall sustainability. Wood chips absorb more water than many other
mulches, water
which both cools the soil and is slowly released to plants.
Economically, wood chips work too, as in most
areas they can be obtained free from arborists or local recycle
centers. Obtaining
them from such local sources rather than in bags at chain stores,
trucked in
from distant locations, keeps the product out of landfills and
supports less
fuel wasted on trucking. They’re also
economical in that being slow to break down, wood chips will last
longer than most
other choices, so don’t need replenishing as often.
Dr.
Chalker-Scott points out that since wood chips contain materials of
various
sizes—bark, wood, and leaves—they are more resistant to compaction
than sawdust
and bark. This diverse selection of materials also supports a
diverse
selection of soil microorganisms. These,
in turn, are more resistant to environmental stresses and create a
healthier
plant environment.
One of
the main concerns, and reason often cited for not using wood chips,
is that
they will tie up nitrogen and cause plants to be hungry. When wood
chips are used on trees and shrubs,
many studies have shown just the opposite.
While there is likely a shallow zone near the surface under a layer
of
wood chips in which nitrogen is often lacking (organisms use up
nitrogen as
they break down organic matter), the deeper roots of shrubs and
trees should
have sufficient nutrients in good soils.
This shallow zone under wood chips, lacking in nitrogen, may help
reduce
weed seed germination. Fewer weeds means
more nutrients available for your plants.
Since this surface
zone, lacking nutrients, is the area of the shallow roots of annuals
and many
vegetables, wood chips are not best to use for these. Nor would
they be good on first year
perennials, or those with shallow roots such as yarrow. If
concerned about a lack of nitrogen, with
shrubs and trees, use a nutrient-rich compost layer on the soil
before applying
wood chips. This “mulch sandwich” is similar
to what one finds in a forest ecosystem, and is what the wood chips
will form
on their own over time.
Some gardeners apply
extra fertilizer to the soil surface, particularly nitrogen-rich
ones, prior to
applying wood chips. For organic
nitrogen sources, you could use blood meal (12 percent nitrogen),
fish meal (9
percent), cottonseed meal (6 percent) or soybean meal (also 6
percent) among
others. A 5-3-4 analysis organic fertilizer supplies nitrogen as
well other
nutrients.
Another
major concern is whether wood chips will bring in diseases. If this
is a worry, let them age for a year
or two before applying. The downside to
this is that some of the nutritional value will be lost. Studies
have shown that wood chips don’t
transmit disease organisms to roots of healthy trees. In healthy
soils, there are more good fungal
diseases that out-compete the bad ones on roots. In healthy plants,
weak plant diseases can’t
get established. These are often called “opportunistic” diseases,
as they take advantage of
opportunities for infection, such as wounding
of bark and damaged roots.
While
the studies noted were done on roots, tree tops weren’t mentioned.
Unfortunately, in my own experience, I unknowingly
imported some leaf diseases (needlecast in particular) on bark
spread under
susceptible spruce species. Control
required more spraying than was possible or desirable, so these
trees are now mostly dead. Other species not susceptible to this
disease
are fine. So my lesson learned is not to
use bark or other wood products like wood chips from unknown
sources, which may
have come from infected trees, around evergreens that may get leaf
diseases.
Yet, in
other areas and similar to some growers I know, I’ve used wood chips
on trees
and shrubs with no effect on their growth.
This mulch has not acidified soils, as is often claimed, nor has it
increased pests such as carpenter ants, nor has it killed the plants
through
leaching harmful (“allelopathic”) chemicals.
More on the use of
wood chips, and other horticultural myths based on a review of
scientific
studies, can be found on Dr. Chalker-Scott’s website (www.informedgardener.com).
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