University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article
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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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