Coffee grounds are a low-level source of nitrogen and a minor source of calcium and magnesium, so the grounds, especially when composted, can be an effective fertilizer for acidic outdoor plants. While coffee grounds can be applied directly to your garden, as a top dressing to nitrogen-loving plants,  the nitrogen will be more available to plants after the grounds decompose in your compost pile.  Although coffee is highly acidic, most of the acidity is removed during the brewing process and most coffee grounds have near neutral acidity (around 6.8pH). 

For indoor plants, however, direct application of the grounds is not recommended. The fine grind typically used for brewing causes the grounds to  pack down tightly, decreasing aeration, and providing a medium for fungal growth and a buildup of soluble salts.

A word of caution: fruit flies are attracted to coffee grounds, especially in enclosed compost heaps, where moisture is high--yet another reason to avoid using coffee grounds directly on indoor plants or composting with them indoors.

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service