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Wood Chip Mulch: Should I Use it?


									miracle, myth…or marketing
                                                   Wood chip mulch:
                                                   Landscape boon or bane?

                                                   L    andscape mulches are increasingly
                                                        recognized as pivotal components of
                                                   environmentally sustainable gardens and
                                                                                                      An exhaustive review of the science
                                                                                                  behind landscape mulches is beyond
                                                                                                  the scope of this column (though I
                                                   green spaces. Select the right mulch and       have just completed such a review for
Linda Chalker-Scott, Ph.D.                         you reap the benefits of healthier soils and   upcoming publication in a scientific
MasterGardener WSU editor                          plants. Choose the wrong mulch and the         journal). Instead, I’m going to address
Extension Urban Horticulturist                     only plants that thrive are the weeds.         the documented benefits and drawbacks
and Associate Professor,                               Before selecting a landscape mulch         behind the use of arborist wood chips as a
Puyallup Research and Extension Center,            material, it’s important to reflect            landscape mulch.
Washington State University                        on the purpose of the landscape in
Puyallup, Washington
                                                   question. For instance, production             Perfect choice
                                                   agriculture generally requires short                In areas where trees are a dominant
                                                   term, intensive management of a crop,          feature of the landscape, arborist wood
                                                   while the philosophy behind landscape          chips represent one of the best mulch
                                                   horticulture is the long term, sustainable     choices for trees and shrubs. A 1990
                                                   management of a system. Therefore,             study evaluated the landscape mulch
                                                   those mulches that work best for crop          potential of 15 organic materials,
                                                   production (including vegetable gardens)       including grass clippings, leaves,
                                                   are often not the best choices for woody       composts, yard wastes, bark, and wood
                                                   ornamental landscapes, and vice versa.         chips. Wood chips were one of the
                                                                                                  best performers in terms of moisture
                                                   Direct benefits                                retention, temperature moderation, weed
                                                       The potential, direct benefits of any      control, and sustainability. In many
                                                   landscape mulch material can be grouped        urban areas, arborist wood chips are
           Interested in                           into four general categories:                  available for free, representing one of the
                                                       Soil benefits                              most economically practical choices.
       field trial research?
      Is your local Master Gardener program            •	improve structure                             Unlike the uniform nature of
         interested in conducting field trials         •	enhance gas transfer                     sawdust and bark mulches, wood chips
        on gardening products? Occasionally,           •	enhance water infiltration and           include bark, wood, and often leaves.
     companies will contact Dr. Chalker-Scott             retention                               The chemical and physical diversity
          with requests for product testing.
   Interested groups will receive basic training       •	prevent erosion and compaction           of these materials resists the tendency
    in designing and conducting experimental           •	moderate temperature                     towards compaction seen in sawdust and
       field trials; local programs will need to       Plant benefits                             bark. Additionally, the materials vary
        provide all materials and field space.         •	provide nutrients                        in their size and decomposition rate,
   If interested, please email Dr. Chalker-Scott
                at                   System benefits                            creating a more diverse environment that
                                                       •	suppress pathogens and pests             is subsequently colonized by a diverse
                                                       •	enhance beneficial organisms             soil biota. A biologically diverse soil
     M O R E I N F O R M AT I O N
                                                       •	increase biodiversity                    biota is more resistant to environmental
                                                       •	neutralize pollutants                    disturbance and will in turn support a
          Dr. Chalker-Scott’s extensive
      reference information is located                 Human benefits                             diverse and healthy plant population.
            at the following Web site:                 •	economic                                      Wood chips are considered to be
                                                       •	aesthetic                                slow decomposers, as their tissues are
   Scott/Wood%20chip%20references.html                 •	ease of application                      rich in lignin, suberin, tannins, and

SUMMER 2007                                             • MASTERGARDENER • PAGE 21
other decomposition-resistant, natural        them out of the landfill—a benefit with                      (e.g. Juglans nigra, black walnut). Cedars
compounds. Thus, wood chips supply            both economic and environmental                              (Thuja spp.) have not been found to have
nutrients slowly to the system; at the same   attributes.                                                  this ability.
time they absorb significant amounts                                                                            Concern: Mulches made from
of water that is slowly released to the       Drawbacks of wood chips—                                     chipping diseased trees can infect healthy
soil. It is not surprising that wood chips    mulch ado about nothing                                      trees.
have been cited as superior mulches for             There are a number of concerns                              Evidence: Most studies indicate
enhanced plant productivity. Wood chips       surrounding the use of arborist wood                         that diseased mulch cannot transmit
have been especially effective in helping     chips as a landscape mulch. I have                           pathogens to the roots of healthy trees.
establish trees and native plants in urban    constructed a quick summary here.                            Under no circumstances should wood
and disturbed environments.                   Relevant references can be found on the                      mulch be used as backfill. Not only is
     Arborist wood chips provide              Web site listed in “More Information.”                       this a poor installation practice, but a
incredible weed control in ornamental         Overall, the commonly expressed                              potential mechanism for disease transfer
landscapes. The mechanism(s) by which         concerns about woody mulches are not                         as well. Fungal communities found
wood chips prevent weed growth are            borne out in research trials.                                in wood chip mulches are generally
not fully understood, but probably                  Concern: Woody mulches will                            decomposers, not pathogens. Under
include light reduction (preventing           acidify soils.                                               healthy soil conditions, beneficial
germination of some seeds and reducing              Evidence: None. In field situations                    and harmless fungi can out-compete
photosynthetic ability of buried leaves),     it is difficult to significantly alter soil pH               pathogens for space on plant roots.
allelopathy (inhibiting seed germination),    without addition of chemicals. Transient                     Furthermore, healthy plants are not
and reduced nitrogen levels at the            changes in pH may be found in the                            susceptible to opportunistic pathogens
soil-mulch interface (reducing seedling       decomposing mulch layer itself, but these                    such as Armillaria and Phytophthora,
survival).                                    have little effect on underlying soils.                      which are often ubiquitous but inactive in
     While there are imported wood                  Concern: Woody mulches, such as                        well-managed soils.
mulches available for purchase at nurseries   cedar, leach allelopathic chemicals that                          Concern: Wood chips could be a fire
and home improvement centers, they are        kill other plants.                                           hazard, particularly when they are used on
not as cost-effective as locally produced           Evidence: Many plant materials                         landscapes around structures.
wood chips, which are often free. In a        contain allelopathic chemicals, which                             Evidence: Coarse textured organic
society where using locally produced          can prevent seeds from germinating or                        mulches, like wood chips, are the least
materials is increasingly popular as a        kill young seedlings. Most compounds                         flammable of the organic mulches. Fine
measure of sustainability, arborist wood      have no effect upon established plants.                      textured mulches are more likely to
chips are a natural choice. Finally, the      Only a few woody materials have been                         combust, and rubber mulch is the most
reuse of plant materials as mulches keeps     found to contain allelopathic chemicals                      hazardous of all tested landscape mulches.
                                                                                               Jim Black

                                                                                                                A bargain
                                                                                                              for gardeners
                                                                                                           Arborist wood chips are available
                                                                                                           locally in many communities. Most
                                                                                                           sources are free or inexpensive for
                                                                                                           wood chips made from recycled
                                                                                                           pallets and other discarded wood
                                                                                                           products. That this mulch is both
                                                                                                           cheap and scientifically proven to be
                                                                                                           superior makes it an ideal Master
                                                                                                           Gardener-recommended product.
                                                                                                               At left,Yakima County recyling
                                                                                                           and compost education program
                                                                                                           coordinator Mikal Heintz is shown
                                                                                                           with her wood chip operation located
                                                                                                           at the county landfill. Former head
                                                                                                           of the Yakima Area Arboretum,
                                                                                                           Mikal works with the community to
                                                                                                           promote composting and wood chip

PAGE 22 • MASTERGARDENER •                                                                              SUMMER 2007
     Concern: Wood chip mulches will          Arborist wood mulch, when used with
tie up nitrogen and cause deficiencies in     contrasting landscape textures, such
plants.                                       as the gravel, rock, and grass shown at
     Evidence: Actually, many studies         right, produces a low-maintenance but
                                              beautiful garden setting.
have demonstrated that woody
mulch materials increase nutrient
levels in soils and/or associated plant
foliage. My hypothesis is that a zone         is most effective in suppressing weeds
of nitrogen deficiency exists at the          when weeds are not yet present on site.
mulch/soil interface, inhibiting weed seed    Therefore, bare soil should be mulched as
germination while having no influence         soon as practical, especially in the spring
upon established plant roots below            and fall when weed seed germination is at
the soil surface. For this reason, it is      its peak. If this is not possible, the most
inadvisable to use high C:N mulches in        effective, non-chemical way to remove
annual beds or vegetable gardens where        weeds prior to mulching is to mow
the plants of interest do not have deep,      them as close to the ground as possible,
extensive root systems.                       followed immediately by mulching.
     Concern: Woody mulches will attract           Prune or mow perennial weeds at
termites, carpenter ants, and other pests.    the root crown in early spring when root
Evidence: Many wood-based mulches             resources are lowest (generally just as
are not attractive to pest insects but are    leaf growth begins). Extensive pulling of
actually insect repellent. For instance,      perennial weeds from unprotected soil
cedar (Thuja) species produce thujone,        is not recommended, as this disturbance
which repels clothes moths, cockroaches,      will increase erosion, especially in sandy
termites, carpet beetles, Argentine ants,     soils or in sloped areas. It is better to keep
and odorous house ants. In general,           unprotected soil undisturbed. However,
termites prefer higher nutrient woody         you can pull resprouting perennial
materials, such as cardboard, rather than     weeds covered in mulch; the mulch layer
wood chips.                                   prevents erosion and facilitates pulling.
                                                   Remove all noxious weed materials
Application                                   from site to prevent rerooting or seeding.
     Let wood chips age before using          Self explanatory!
them if there are concerns about disease.          Install chips to the desired depth. A
Personally, I have never done this; I         successful wood chip mulch must be deep
happen to love the smell of fresh wood        enough to suppress weeds and promote
chips and enjoy spreading them out over       healthy soils and plants: research has
the landscape. Additionally, some of the      demonstrated that weed control is directly
nutrient value (particularly nitrogen if      linked to mulch depth, as is enhanced
the chips contain leaves or needles) will     plant performance. A review of the               borers are more likely to invade a plant
be lost in the composting process. Using      research on coarse organic mulches and           whose bark is wet due to excessive
fresh chips ensures that some of the foliar   weed control reveals that shallow mulch          mulching. Rather than creating mulch
nitrogen will feed the landscape rather       layers will promote weed growth and/or           volcanoes, instead taper the mulch down
than the compost pile.                        require additional weed control measures.        to nearly nothing as you approach the
     Before installing wood chips, create a   I recommend 4-6 inches for ornamental            trunk. This donut-shaped application will
thin underlying layer of a more nutrient-     sites and 8-12 inches for restoration sites      protect the soil environment as well as the
rich mulch (like compost) if there are        and/or perennial weed problems.                  above-ground plant tissues.
concerns about nutrient deficiencies.              Keep mulch away from trunks of                  Replace mulch as needed to maintain
This “mulch sandwich” approach is             trees and shrubs. Piling mulch against           desired depth; replacement rate will
a logical one that mimics what you            the trunks of shrubs and trees creates a         depend on decomposition rate. Once
would see in the mulch layer of a forest      dark, moist, low oxygen environment              mulch is applied, little management needs
ecosystem. It’s not required, though, and     to which above-ground tissues are not            to be done other than reapplication to
over time a wood chip mulch will develop      adapted. Fungal diseases require a moist         maintain minimum depth. High traffic
this same structure as the lower layers       environment to grow and reproduce;               areas are most likely to need replacement.
break down.                                   piling mulch on the trunk provides                                     n
     Begin mulch application before           exactly the right conditions for fungi to
annual weeds are established. Mulch           enter the plant. Likewise, opportunistic

SUMMER 2007                                         • MASTERGARDENER • PAGE 23

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