Are Spent Coffee Grounds Good for the Garden?
Everyone writes about spent coffee ground uses in the garden. Which is heartening - it means that home gardeners are really considering how to view ‘waste’ as a resource! Not all sources are reliable, so let’s talk take a moment to look at the science and research behind coffee grounds: the good and the bad.
At Garden of Oz, we’re all about seeing where we can find second, third, or fourth uses in quality byproducts like spent brewer’s grain, food waste, yard debris, animal bones, fish parts, and of course: coffee grounds.
The good news is, spent coffee grounds are a beautiful, nutrient rich resource that can add enormous value to your home garden when used correctly.
Let’s start with some well-intentioned farm remedies that don’t hold up to scientific research.
The Myths
1) Coffee Grounds Keep Slugs At Bay
There’s currently no published evidence proving that the use or application of coffee grounds can kill or repel garden pests. (1)
Folks that have seen otherwise in their garden may have a different opinion, and if you’ve noticed a benefit - keep on keeping on! And maybe lobby a scientist to conduct a study to ‘prove it’.
2) Coffee Grounds Make Soil Too Acidic
Coffee drinkers know (and love) the acidic brew, but there’s no studies that prove that coffee grounds make soil too acidic. (1)
In fact, studies have shown that soil pH treated with coffee grounds can range from mildly acidic to somewhat alkaline - a very broad range indeed. (1)
Another key consideration: pH changes associated with coffee grounds will only be in the immediate vicinity of the coffee grounds, NOT across the entire soil profile. (1)
3) You Can Grow Seeds in Straight Coffee Grounds.
Nope. Don’t do this. Studies from the ‘90s to early 2010s all show that growing seeds in straight coffee grounds is a bad idea. You would see reduced seed germination AND dramatically reduced plant growth if you tried this one. (1)
4) Worms Just Love Coffee Grounds
This one has some mixed evidence, but research points to no for folks with worm bins (earthworms in the garden are a different story). A 2011 study confirmed that coffee grounds in vermicomposting bins or closed environments, can kill or injure worms. (1)
This one isn’t worth the risk to your wiggly decomposer pals in bins. We do not recommend giving your worms in vermicomposting bins or closed systems spent coffee grounds as a substrate or food source. There are plenty of other healthy food sources that divert waste for them to monch on.
The Benefits
1) Nutrients
Coffee grounds used as a thin mulch layer can help supply nutrients and other plant boosting benefits to soil and plants (1). Apply a thin layer of 1/2 inch of coffee grounds in your garden beds to see the benefits. Just be careful to not apply thick layers of spent coffee grounds since they can become too compacted and prevent moisture and air from entering the soil.
2) Disease Control
Coffee grounds contain and attract beneficial bacterial and fungal species as it decomposes that help prevent pathogenic (bad) fungi from taking hold of your plants and their roots. (1)
3) Useful for Thinly Mulching Gardens
Spent coffee grounds have the ability to moderate soil temperature (a great feauture in extreme and changing climates) while also increasing soil water.
4) Prevent Leaching of Pesticides and Heavy Metals
Research in the 2010s found that spent coffee grounds can bind pesticide residues (!) and toxic heavy metals like cadmium and stop their movement into and throughout the soil. This is great news for gardens previously suffering from pesticide and heavy metal exposure.
5) Earthworms Dig It
Unlike in vermicompos bins or closed systems, earthworms out in nature or in your garden bed love coffee grounds. Bioconsumers like earthworms in your garden use coffee grounds as food, and pull the grounds deep into the soil. This helps increase aeration and improve soil structure. Both are incredibly helpful for root development and nutrient dispersal.
6) Potting Mix Additive
When sufficiently broken down, spent coffee grounds help the porosity of potting soil mixes and provide a beneficial, slow-releasing nitrogen boost to potting mixes and soil over time.
Try adding coffee grounds to your garden today!
With so many key benefits for your home garden, consider setting aside some coffee grounds after you enjoy your morning cup to add to your garden and see the results for yourself!
Want to learn even more? Below are some exceptional studies looking at the impact of spent coffee grounds in your garden.
Adams, P.B., J.A. Lewis and G.C. Papavizas. 1968a. Survival of root-infecting fungi in soil. IX. Mechanism of control of Fusarium root rot of bean with spent coffee grounds. Phytopathology 58(12):1603-1608.
Ballesteros, L.F., J.A. Teixeira and S. Mussatto. 2014. Chemical, functional, and structural properties of spent coffee grounds and coffee silverskin. Food and Bioprocess Technology 7(12):3493-3503.
Bouchenafa-Saïb, N., A. Mekarzia, B. Bouzid, O. Mohammedi, A. Khelifa, K. Benrachedi and N. Belhaneche. 2014. Removal of malathion from polluted water by adsorption onto chemically activated carbons produced from coffee grounds. Desalination and Water Treatment 52(25/27):4920- 4927
Hamanaka, Y., K. Toyota and K. Hayashi-Ikeda. 2005. Screening of fungal strains responsible for strong fungistasis against Fusarium oxysporum f. sp. radicis-lycopersici in a coffee compost-amended soil. Japanese Journal of Soil Science and Plant Nutrition 76(6):817-824.
Kim, M.S., H.G. Min, N.M. Koo, J.S. Park, S.H. Lee, G.I. Bak and J.G. Kim. 2014. The effectiveness of spent coffee grounds and its biochar on the amelioration of heavy metalscontaminated water and soil using chemical and biological assessments. Journal of Environmental Management 146:124- 130.
Kitou, M. and S. Okuno. 1999. Effect of mulching with coffee residue on the growth of soybeans and growth of following crops fertilized with coffee residue: study on agricultural utilization of coffee residue (part 2). Japanese Journal of Soil Science and Plant Nutrition 70(5):495-503.
Liu, K. and G.W. Price. 2011. Evaluation of three composting systems for the management of spent coffee grounds. Bioresource Technology 102(17):7966-7974.
