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Yard & Garden: Are coffee grounds good for gardening?
Yard & Garden
By B. Rosie Lerner
Consumer Horticulturist, Purdue Extension

Do Johnny jump ups jive with java? How about a little coffee on your cucumbers? With so many trendy coffee houses these days, there is a lot of interest in recycling used coffee grounds to divert them from the landfill. And being a plant product, a frequent question is whether coffee grounds are useful for gardening.

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 hereís my cup of counsel.

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. Or, you could spread them out over a larger garden area to minimize the pH effect. Itís difficult to make a specific recommendation for an application rate, but itís 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.

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 composters 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.

This farm news was published in the March 1, 2006 issue of Farm World.