Coffee Grounds— Will They Perk up Plants?
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myths, miracles…or marketing Coffee grounds— will they perk up plants? ere I am, a Washington native an organic mulch advocate, I investigated living in Seattle, and I don’t drink the science behind using coffee grounds coffeeH (Earl Grey tea is my brew of in the garden and landscape. choice). Nevertheless, I do most of my There’s actually a lot of research out writing in a coffee shop where coffee and there on reusing coffee wastes—but much Linda Chalker-Scott, Ph.D. coffee byproducts surround me—most of it relates to what’s generated through MasterGardener WSU editor notably the bags of used grounds that are coffee production and coffee bean Extension Urban Horticulturist free for the taking. Many people use them harvesting. These wastes include coffee and Associate Professor, as part of their compost pile, but increas- pulp and peel (from the coffee fruit or Puyallup Research and Extension Center, ing numbers of people are using them “cherry”), coffee hulls and husks (covering Washington State University straight up as mulch, claiming they repel the coffee seed or bean), and coffee Puyallup, Washington cats, kill slugs, prevent weeds, aerate and effluent (the waste water used in several of www.puyallup.wsu.edu/~Linda%20Chalker- acidify the soil, provide nitrogen, attract the stages of coffee manufacture). Scott earthworms … the list goes on. Since I’m Since most of us don’t have access to these coffee byproducts, and because they are physically and chemically different from the beans, I only reviewed articles that examined used coffee grounds. Research From coffee shops to the instant coffee industry, spent coffee grounds can be generated in massive quantities. Enterprising researchers have investigated using coffee dregs for: • growth media for lab insects and earthworms; • sileage and herbal remedies for livestock, including cattle, buffalo, sheep, pigs, and chickens; • biofuel and biogas production; • composite building materials; • controlling mosquito larvae (albeit with variable success); • treating wastewater; • a natural antioxidant for treatment of diabetes and cardiovascular disease; and • compost and mulch materials for gardens and landscapes. Before discussing these last studies, let’s consider the chemistry of your used coffee grounds. WINTER 2009 www.MasterGardenerOnline.com • MASTERGARDENER • PAGE 3 Chemical composition Disease suppression Italian ryegrass (Lolium multiflorum) Not everything contained in a coffee As they decompose, coffee grounds were all inhibited by coffee grounds, as bean makes it into your morning coffee. appear to suppress some common fungal was that of ornamentals including inch Nitrogen-rich proteins needed for rots and wilts, including Fusarium, plant (Tradescantia albiflora), geranium, seed germination and growth comprise Pythium, and Sclerotinia species. In these and asparagus fern. One investigator over 10% of coffee grounds. In fact, studies, coffee grounds were part of a speculated that toxic substances released the carbon-to-nitrogen ratio of coffee compost mix, in one case comprising from decomposing coffee grounds were grounds can be as low as 11:1, an ideal as little as 0.5 percent of the material. responsible for their inhibitory effect. ratio for plant and soil nutrition. Since Researchers suggest that the bacterial This effect also reduces weeds, and coffee is extracted in water, most of the and fungal species normally found on perhaps in a landscape dominated by large hydrophobic compounds, including decomposing coffee grounds, such as shrubs and trees, only germinating seeds oils, lipids, triglycerides, and fatty acids non-pathogenic Pseudomonas, Fusarium, and seedlings would be injured. But as remain in the grounds, as do insoluble and Trichoderma spp. and pin molds there has been no experimental research carbohydrates like cellulose and various (Mucorales), prevent pathogenic fungi on coffee grounds and woody plants, this indigestible sugars. Structural lignin, from establishing. A similar biocontrol is only speculation on my part. protective phenolics, and the wonderful effect was noted on bacterial pathogens aroma-producing essential oils are also left including E. coli and Staphylococcus spp., Recommendations over from the brewing process. It’s this which were reduced on ripening cheeses Given the incomplete and conflicting last group of chemicals that are reported covered with coffee grounds. scientific data so far, the scientific jury to have antioxidant and antimicrobial Currently, disease suppression is still out reviewing the evidence. properties. from coffee grounds has only been Nevertheless, I do feel comfortable in demonstrated under controlled making some general observations and Decomposition conditions on a handful of vegetable recommendations for those of you who Over the course of several months, crops, including bean, cucumber, would like to try coffee grounds as part of specialized bacteria and fungi break down spinach, and tomato. Their efficacy in your compost pile or as a mulch: the various chemical components of gardens and landscapes is unknown, as coffee grounds. Some larger consumers, is any protective activity on other plant In compost including earthworms, are also able to use materials such as trees or shrubs. • Percentages of 10 to 20 percent of total this food source. The fact that earthworms compost volume have been reported pull coffee grounds deep into the soil may Effects on plant growth as optimal for compost quality and account for noted improvements in soil Given their antimicrobial activity, effectiveness, while over 30 percent can structure such as increased aggregation. it’s not surprising that attempts to be detrimental. Humic substances, which are important cultivate mushrooms in coffee grounds • Only small amounts of coffee grounds chemical and structural soil components, have been variable and species-specific. are required for effective disease are produced through coffee ground Likewise, their effects on plant growth are suppression. Therefore, I recommend degradation. Carbon-to-nitrogen ratios unpredictable. using no more than 20% by volume change as well, generally starting out a Coffee ground composts and of coffee grounds in a compost pile. A bit higher than ideal (e.g. 25-26) and mulches have enhanced sugar beet seed diverse feedstock will ensure a diversity decreasing to 21, 13, 11, or even 9.4 in a germination and improved growth and of microorganisms. year’s time. yield of cabbage and soybeans. It’s been • Don’t assume coffee grounds will Less straightforward are the changes an effective replacement for peat moss make an acidic compost; pH levels will in pH that occur during decomposition. in producing anthuriums. Increases in undoubtedly change over time. A commonly held assumption states that soil nitrogen as well as general mulching coffee grounds are acidic, but this does benefits, such as moderating soil For mulching not hold true experimentally. While two temperature and increasing soil water, are • Coffee grounds are finely textured and studies on coffee ground composting proposed mechanisms for these increases. easily compacted. reported mildly acidic pHs of 4.6 and • Compacted coffee grounds create a 5.26, others have measured neutral (7.7) Not all get a jolt barrier to moisture and air movement, to somewhat alkaline (8.4) pH levels. Not all plants get a jolt from coffee especially when applied in thick layers. One researcher found that the pH of soil grounds. Seed germination of alfalfa • Therefore, I recommend against treated with coffee compost increased (Medicago sativa) and white and red using pure coffee grounds as a mulch; after 14 to 21 days of incubation, clovers (Trifolium repens and T. pratense) instead, try using a thin layer (no more gradually decreasing thereafter. Obviously was inhibited by water leached through than half an inch) of coffee grounds the pH of decomposing coffee grounds is coffee grounds. Growth of crops such and cover with a thicker (four inches) not stable and one shouldn’t assume that as Chinese mustard (Brassica juncea), layer of coarse organic mulch like it will always, or ever, be acidic. komatsuna (Brassica campestris) and wood chips. n PAGE 4 • MASTERGARDENER • www.MasterGardenerOnline.com WINTER 2009.