Purdue Extension Service

Vanderburgh County, Indiana

Coffee in the Garden

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, February 26, 2006

With so many trendy coffee houses these days, there is a lot of interest in recycling used coffee grounds to be used as garden amendment, or even as a pest control product. There have been a few companies studying the use of coffee grounds as a soil or compost amendment, and there are even a few companies marketing it as such. So here's my cup of counsel, based on some information from Purdue.

Coffee grounds are a low-level source of nitrogen, having a fertilizer value of around 2.0-0.3-0.2, as well as a minor source of calcium and magnesium. Post-brewed coffee grounds are reported to be slightly to highly acidic, depending on the source, but no more so than peat moss. So, one could apply them to the soil for acid-loving plants, such as rhododendrons, azaleas and blueberries, etc. They might even help keep your bigleaf hydrangeas blue.

Gardeners could spread them out over a larger garden area to minimize the pH effect. It's difficult to make a specific recommendation for an application rate, but it's always better to err on the lighter side, since the pH can be variable. A rate of 10 pounds (dry weight) per 1000 square feet would be conservative. Avoid using fresh grounds or grounds with coffee flavor and color still present.

Composting is also an excellent method to recycle the grounds, which have a carbon-to-nitrogen ratio of around 20:1. Use the grounds as you would green, leafy material, mixing with some dry, brown plant materials in the compost. The Environmental Protection Agency suggests adding no more than 25 percent volume coffee grounds. Worm ranchers report that coffee grounds are an excellent food source for the little critters. Again, be sure to mix the grounds with dry brown materials, even in the worm bin.

Some companies indicate that a shallow layer of coffee grounds can be used as mulch around landscape plants; however, I am hesitant to recommend this. Because of the fine grind that is typically used for brewing, the grounds are likely to pack down tightly, decreasing aeration as well as posing the risk of fungal growth. Likewise, I would avoid using coffee grounds with potted houseplants, not only because of the potential for fungal growth but also potential buildup of soluble salts.

Washington State Master Gardeners found that fruit flies were attracted to coffee grounds, especially in situations such as enclosed compost bins, where moisture content was high. This is yet another reason to avoid using the grounds with houseplants. In more open aerated systems, where the grounds are able to dry down, fruit flies and other pests are less likely to be a nuisance.

I've seen some references on the internet for using coffee grounds to control slugs. Some of the comments mention that the coffee grounds repel the slugs; some say that the caffeine is toxic to them; and others say the rough grind of the beans tears the slugs' little bellies and kills them. Unfortunately, there is no reliable university research to back up these claims that coffee grounds make an effective pesticide. As with other home remedies, I would not consider using coffee grounds as a pest control product.

For more information on composting, soil amendments, and pest control, contact the Purdue Extension Service at (812) 435-5287.