Wood Chips: Pros and Cons

By Chuck Ingels, UC Farm Advisor

Landscapers and arborists often have large amounts of green waste that ends up in the landfill because it is more convenient than taking it to recycling facilities. But they would like to be able to give it away (they’d actually like to sell it!) at closer locations.

PROS. Wood chips are like gold – or should be – to many landscapers and backyard gardeners.

Wood chips offer the following benefits: suppress annual weeds, conserve soil moisture, cool the soil; enhance root activity in the upper layers of the soil, add organic matter and nutrients, and make a nice walking surface. In the Fair Oaks Horticulture Center demonstration orchard, weed control is very easy because we’ve put down a 6 to 9 inch layer. Also, we haven’t needed to irrigate yet this spring. Earthworms burrow extensively in the area of the soil/wood chip interface. The chips don’t tie up nitrogen in the soil unless they’re incorporated into the soil. Lastly, the thick layer helps to prevent soil compaction when walking or wheel-barrowing around the root zone.

CONS. The use of wood chips does have some drawbacks. There is little published information about problems, so much of the following are only my observations; please let me know of other problems if you know of any. Wood chips can conserve soil moisture to an excess in clay soils, keeping the root and crown area so wet that they rot from soil-borne diseases or simply starve for oxygen. In this case, or where frequent flood or sprinkler irrigation is used, mulch should definitely be moved away from the trunk of sensitive species to dry out the soil; if drip emitters are kept away from trunks, mulch can be placed right up to the trunk in the summer. The mulch also hides the soil, making it more difficult to know when to water. Also, wood chips break down and must be re-applied every year or two. Wood chips can also harbor insects that may attack plants, including slugs, earwigs, sowbugs, and ants.

After planting my home orchard, earwigs and sowbugs caused so much damage on the newly emerging shoots that I had to put tree tanglefoot on the trunks (I first used a barrier of masking tape to prevent injury to the trunk), which stopped all midnight snacking by these critters. Some unusual fungal fruiting bodies often appear on the chips, such as mushrooms, bird’s nest fungus, artillery fungus (!), and slime mold, which is also called "dog vomit" fungus for a reason. These are harmless to the soil, although mushrooms can be poisonous and slime mold is pretty gooshy on the feet. The Master Gardener Office has available copies of a brochure titled, "What is Growing in My Landscape Mulch?" 

WHERE TO OBTAIN WOOD CHIPS. In many areas of California, public utility agencies offer free wood chips.  Contact your local agencies for details regarding availability and delivery/pickup information. In Sacramento, SMUD information is available by calling 916-732-5854.   Some arborists are more than happy to deliver and may indicate this service in their telephone book or website listings. Be sure to ask for clean chips because some chippers shred rather than chip, and some arborists throw in logs. Coarse, woody chips last longest, while leaves and conifer needles break down quickly.