- There are a few nutrients in coffee grounds that make them good candidates for flower fertilizer. They are a low-level source of nitrogen, about 2 percent according to Oregon State University. They also contain magnesium and calcium. Although coffee itself is acidic, most of the acid percolates out of the grounds with the hot water used in preparation. Most coffee grounds have a close-to-neutral pH of 6.5 to 6.8, but some varieties can be more acidic.
- Since the acidity of coffee grounds is variable, horticulturist B. Rosie Lerner from Purdue University recommends adding them directly to the soil for acid loving flowers and fruits like rhododendrons, azaleas and blueberries. They may even help keep bigleaf hydrangeas blue, she says. Horticulturists at Oregon State University recommend adding nitrogen fertilizer to the soil along with coffee grounds since it can take microorganisms some time to break down the grounds. Coffee grounds do improve the structure of soil, however, and can be added directly to the soil as an amendment if they are mixed in well.
- Perhaps one of the most salient properties of coffee grounds is their ability to generate heat in a compost pile. According to Oregon State University, they can maintain temperatures of around 140 to 160 degrees Fahrenheit for up to two weeks. High temperatures in compost piles are important for prohibiting the growth of harmful bacteria. Add some dry brown material in your compost along with the grounds and be sure grounds equal no more than 25 percent by volume. Compost is an excellent soil amendment for healthy flowers.
- Coffee grounds as fertilizer for indoor flowering plants is not recommended. Salts in the grounds can accumulate in soils without excellent drainage and harm plants. Coffee grounds kept moist by frequent watering are prone to fungal growth. These fungi can attract unwanted pests like fruit flies and fungus gnats.
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