Advice from the Help Desk of the
UC Master Gardener Program of Contra Costa County
Generally, used appropriately, coffee grounds can be beneficial to your garden. And if you search the internet you can find lots of information on their use. As in all things “internet”, the question is “what is appropriate”.
The Master Gardener program attempts to provide the gardener with scientifically based advice on gardening. The use of coffee grounds is no exception. Recently, Washington State University's Dr. Linda Chalker-Scott (Extension Urban Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University) updated her earlier 2009 review of gardening use of coffee grounds with the publishing of her latest review of the scientific literature: “Use Of Coffee Grounds In Gardens And Landscapes” (http://cru.cahe.wsu.edu/CEPublications/FS207E/FS207E.pdf). This latest review has been peer-reviewed.
While it is strongly recommended that you read for yourself the short 6-page paper, below are the edited excerpts from her findings:
Action list for using coffee grounds in compost
- Use no more than 20 percent by volume of coffee grounds in a compost pile. A diverse feedstock will ensure a healthy diversity of microorganisms.
- Don't assume coffee grounds will make an acidic compost; pH levels will change over time.
- Be sure to allow coffee grounds to cool before adding them to your compost; heat can kill your beneficial microbes.
- Avoid adding coffee grounds to vermicomposting bins; they can injure or kill earthworms in these confined areas.
- Understand that disease suppression from nonpathogenic organisms found in decomposing coffee grounds has only been demonstrated under controlled conditions on a handful of crops, including bean, melon, spinach, and tomato. Their efficacy in gardens and landscapes is unknown.
Action list for using coffee grounds directly as a mulch
- Apply a thin layer (no more than half an inch) of coffee grounds. Cover with a thicker layer (four inches) of coarse organic mulch like wood chips. This will protect the coffee grounds from compaction.
- Don't apply thick layers of coffee grounds as a standalone mulch. Because they are finely textured and easily compacted, coffee grounds can interfere with moisture and air movement in soils.
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BLOG editor's addendum: Most of the comments on Dr. Chalker-Scott's latest review center around “avoid adding coffee grounds to vermicomposting bins…” with more than a few anecdotal comments that it is common practice to use coffee grounds in compost bins. I think there is some clarification needed between compost bins and vermicompost bins… Her response so far is that she is reporting what is in the scientific literature. Vermicomposters should take special notice of this finding and manage their composting accordingly. You might search for further updates and discussion on Dr. Chalker-Scott's Facebook page “The Garden Professors”
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Help Desk of the UC Master Gardeners of Contra Costa County
Note: The UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523. We can also be reached via telephone: (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog (//ucanr.edu/blogs/CCMGBlog/).