Dear Recyclebank,

I tried adding used coffee grounds to my house plants and had a mold issue. What did I do wrong? Also, can I use the actual coffee as fertilizer?

-Dawn R., Madison, WI

Coffee grounds are an excellent addition to compost piles and outdoor gardens, but they can be tricky when it comes to indoor container gardening. When mold appears on grounds in a compost pile, it is eventually consumed by bacteria and turned into compost — no big deal. In an indoor setting, mold is unsightly and can pose hazards to human health. Fortunately, there are a few strategies you can take to successfully add grounds to your indoor plants.

• Reduce the amount and/or frequency of watering. A common reason for mold appearing on potted soil is excessive moisture. You may want to let the soil dry out a bit before adding grounds, especially if the grounds are already damp.
• If applying to the surface of the soil, layer the grounds thinly. Grounds can be compacted, retaining moisture and preventing aeration. This can result in an ideal environment for mold growth.
• Try mixing the grounds in with the top layer of soil. The variable particle size helps prevent compaction, thus improving drainage.

As for using the coffee itself as fertilizer, there seems to be little scientific evidence confirming its benefits but plenty of anecdotal evidence in the positive. Coffee is rather acidic, which can be beneficial to plants that thrive in acidic soil. However, it’s probably still a good idea to dilute any coffee you’re adding.

SOURCES
Oregon State University Extension Service
University of Illinois Extension