B. Rosie Lerner
Extension Consumer Horticulturist
Grounds for Gardening
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 here's 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. 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.
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.