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Using arborist wood chips as a landscape mulch. WSU Extension Fact Sheet FS160E.

Home Garden Series
Linda Chalker-Scott, Washington State University Puyallup Research
and Extension Center. FS160E
FS160E | Page 1 |
Using Arborist Wood Chips as Landscape Mulch
Landscape mulches are important components of
environmentally sustainable gardens and landscapes. Unlike
soil amendments, mulches are simply materials laid on top of
the soil rather than worked into it. Select the right mulch and
you reap the benefits of healthier soils and plants; choose the
wrong mulch and the only plants that thrive are the weeds.
This fact sheet teaches home gardeners how to use arborist
wood chip mulches in their landscapes.
Benefits of Arborist Wood Chips
In areas where trees are a dominant feature of the landscape,
arborist wood chips are one of the best mulch choices for trees
and shrubs. Studies have found wood chips to be one of the
best performers in terms of moisture retention, temperature
moderation, weed control, and sustainability. In many urban
areas, arborist wood chips are available free of charge (Figure
1), making them one of the most economically practical
Unlike uniformly textured sawdust and bark mulches (Figure
2a), arborist wood chips include bark, wood, and often leaves
(Figure 2b). The chemical and physical diversity of these
materials resists the compaction often found in sawdust and
bark mulches. Additionally, the materials vary in their size and
decomposition rate, creating a more diverse environment that
houses a diversity of microbes, insects and other organisms. A
biologically diverse soil community is more resistant to
environmental disturbance and will in turn support a diverse
and healthy plant population.
Figure 1. Arborist wood chips are often available free of charge.
Wood chips are considered to be slow decomposers, as their
tissues are rich in lignin, suberin, tannins, and other complex
natural compounds. Thus, wood chips supply nutrients slowly
to the system; at the same time they absorb significant amounts
of water that is slowly released to the soil. It is not surprising
that wood chips have been cited as superior mulches for
enhanced plant productivity. Wood chips have been especially
effective in helping establish trees and native plants in urban
and disturbed environments.
Arborist wood chips provide substantial weed control in
ornamental landscapes (Figure 3). The mechanism(s) by which
wood chips prevent weed growth are not fully understood, but
likely includes light reduction (preventing germination of some
seeds and reducing photosynthetic ability of buried leaves),
allelopathy (inhibiting seed germination), and reduced nitrogen
levels at the soil-mulch interface (reducing seedling survival).
Figure 2a. Bark mulch. Figure 2b. Arborist wood chips create a diverse environment for plants.
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Figure 3. Few weeds can survive in correctly applied wood chip mulches.
While there are imported wood mulches available for purchase at
nurseries and home improvement centers, they are not as cost-
effective as locally produced wood chips, which are often free. In
a society where using locally produced materials are increasingly
popular as a measure of sustainability, arborist wood chips are a
natural choice. Finally, the reuse of plant materials as mulches
keeps them out of the landfill – a benefit with both economic and
environmental attributes.
Frequently Asked Questions About
Arborist Wood Chips
Q: Will woody mulch acidify my soils?
A: No. In field situations it is difficult to significantly alter soil pH
without the addition of chemicals. Temporary changes in pH may
be found in the decomposing mulch layer itself, but these have
little effect on underlying soils. Significant changes in soil pH can
only occur after decades or centuries of mulch use.
Q: Don’t woody mulches, like cedar, leach allelopathic chemicals
that kill other plants?
A: No. Many living, growing woody plants contain allelopathic
chemicals, which can prevent seeds from germinating or kill
young seedlings. Most compounds have no effect upon
established plants. Cedars (Thuja spp.) have not been found to
have this ability. Even Juglans nigra (black walnut), the best
known allelopathic species, has not been shown to have negative
effects when wood chips are used as a mulch.
Q: Will mulches made from diseased trees infect healthy trees?
A: No. Most studies indicate that diseased mulch cannot
transmit pathogens to the roots of healthy trees. Under no
circumstances should wood mulch be worked into the soil as
an amendment: not only is this a poor planting practice, but
increases the likelihood of disease transfer. Using diseased
wood chips as a soil amendment puts them into close
proximity to roots where subsequent tissue injury or
environmental stress could lead to infection.
Fungal communities found in wood chip mulches are generally
decomposers, not pathogens. Under healthy soil conditions,
beneficial and harmless fungi (Figure 4) can outcompete
pathogens for space on plant roots that grow into mulch layers.
Furthermore, healthy plants are not susceptible to opportunistic
pathogens such as Armillaria and Phytophthora, which are
widespread, but inactive in well-managed soils.
Figure 4. Fungal hyphae in a wood chip mulch.
Q: Aren’t wood chips a fire hazard?
A: No. Coarse textured organic mulches, like wood chips, are
the least flammable of the organic mulches. Fine tex-tured
mulches are more likely to combust (Figure 5), and rubber
mulch is the most hazardous of all tested landscape mulches. If
organic mulches are kept moist, they are less likely to catch
fire. If you use flamers for weed control in areas near wood
chips, be sure to soak the mulched area first.
Q: Won’t wood chip mulches tie up nitrogen and cause
nutrient deficiencies in plants?
A: No. Many studies have demonstrated that over time woody
mulch materials will increase nutrient levels in soils and/or
associated plant foliage (Figure 6). However, there is a
nitrogen deficiency at the boundary between the mulch and
soil, which probably inhibits weed seed germination.
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Figure 5. Fine textured mulch like sawdust holds very little water and burns
Figure 6. Plants mulched with wood chips do not have nitrogen
Q: Will woody mulches attract termites, carpenter ants, and
other pests?
A: No. Many wood-based mulches are not attractive to pest
insects but are actually insect repellent. For instance, cedar
(Thuja) species produce thujone, which repels clothes moths,
cockroaches, termites, carpet beetles, Argentine ants, and
odorous house ants. In general, termites prefer higher nutrient
woody materials such as cardboard, rather than wood chips.
Action List for Using Arborist Wood
Chip Mulches in the Landscape
Begin mulch application before annual weeds are
established. Mulch is most effective in suppressing weeds
before weed seeds germinate. Therefore, bare soil should be
mulched as soon as practical, especially in the spring and fall
when weed seed germination is at its peak. If this is not
possible, the most effective, non-chemical way to remove
weeds prior to mulching is to mow them as close to the ground
as possible, followed immediately by mulching.
Prune or mow perennial weeds at the root crown. This is
best done in early spring when root resources are lowest;
generally just as leaf growth begins (Figure 7). Extensive
pulling of perennial weeds from unprotected soil is not
recommended, as this disturbance will destroy soil structure
and increase erosion, especially in sandy soils or in sloped
areas. It is better to keep unprotected soil undisturbed.
However, you can pull resprouting perennial weeds in
landscaped areas covered in mulch; the mulch layer prevents
erosion and facilitates pulling.
Figure 7. Mowing undesirable plants before mulching keeps soils
undisturbed and reduces regrowth.
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Remove particularly aggressive weeds from the site. Weeds
that easily go to seed or can reroot themselves after they’ve
been dislodged should be composted or disposed with green
waste materials.
Add a thin underlayer of compost. Before installing wood
chips for the first time, create a thin underlying layer of a more
nutrient-rich mulch (like compost, Figure 8) if there are
concerns about nutrient deficiencies. This “mulch sandwich”
approach is a logical one that mimics what you would see in
the mulch layer of a forest ecosystem. It’s not required,
though, and over time a wood chip mulch will develop this
same structure as the lower layers break down.
Figure 8. A layer of compost underneath coarse wood chips adds extra
Use fresh chips unless there are still concerns about disease.
Some of the nutrient value (particularly nitrogen, if the chips
contain leaves or needles) will be lost in the composting
process. Using fresh chips ensures that some of the foliar
nitrogen will feed the landscape rather than the compost pile.
Install chips to the desired depth. A successful wood chip
mulch must be deep enough to suppress weeds and promote
healthy soils and plants: research has demonstrated that weed
control is directly linked to mulch depth, as is enhanced plant
performance. A review of the research on coarse organic
mulches and weed control reveals that shallow mulch layers
will enhance, rather than prevent weed growth (Figure 9). All
plants, including weeds, respond positively to the benefits of
organic mulches, particularly the increase in soil water
retention. Wood chips maintained at a depth of 4 to 6 inches
will control weeds without detracting from appearances.
Keep mulch away from trunks of trees and shrubs. Piling
mulch against the trunks of shrubs and trees creates a dark,
moist, low oxygen environment to which
Figure 9. When mulch layers are too thin, weeds will grow rapidly.
Figure 10. Mulch volcanoes may enhance pest and disease problems.
above-ground tissues are not adapted. Fungal diseases require
a moist environment to grow and reproduce; piling mulch on
the trunk provides exactly the right conditions for fungi to
enter the plant.
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Likewise, opportunistic pests are more likely to invade a plant
whose bark is wet due to excessive mulching. Rather than
creating mulch volcanoes (Figure 10), instead, taper the mulch
down to nearly nothing as you approach the trunk. This donut-
shaped application will protect the soil environment as well as
the above-ground plant tissues.
Keep mulch away from building foundations. Although
wood chips do not attract termites or other pests, they and
other mulches can act as a bridge allowing pest insects to enter
houses and garages. Maintain a narrow strip of bare soil next
to the foundation to prevent infestations.
Reapply mulch as needed to maintain desired depth;
replacement rate will depend on decomposition rate. Once
mulch is applied, little management needs to be done other
than reapplication to maintain minimum depth. High traffic
areas are most likely to need replacement.
Further Reading
Bell, N., D.M. Sullivan, and T. Cook. 2009. Mulching woody
ornamentals with organic materials. Oregon State University
Chalker-Scott, L. 2007. Impact of mulches on landscape plants
and the environment – a review. Journal of Environmental
Horticulture 25(4): 239-249.
FS160E | Page 6 |
Copyright 2015 Washington State University
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FS160E | Page 7 |
... Plants that suffer from poor drainage, insufficient light, and other environmental factors that limit plant growth are not assisted by increased fertilizer use. A sustainable approach to landscaping ( Figure 6, which includes judicious use of woody mulch (Chalker-Scott, 2015,) reduces use of fertilizers (Hartin et al., 2014). ...
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