If you frequent a favorite caffeine purveyor regularly, especially during the summer growing season, you may notice bags of coffee grounds walking out the door along with tasty beverages in the hands of eager gardeners. This is because of a relatively recent craze of adding the by-product of coffee filtration, the grounds, to gardens. Although the purported benefits of this fertilization practice are varied and conflicting, researchers are beginning to discover a few favorable outcomes you may depend on when using coffee grounds to fertilize plants.
Coffee Grounds Composition
What is hidden in coffee grounds that make them a black gold for gardeners in the know? The main component is nitrogen. Linda Chalker-Scott, Master Gardener with Washington State University, suggests that grounds may be composed of as much as 10 percent nitrogen, which is important for seeds to germinate and grow.
The biggest myth recently debunked by researchers is that coffee grounds are inherently acidic, and spreading grounds over a hydrangea or other acid-loving plant bed promotes bloom color change. In fact, experts at Oregon State University suggest that post-brewed coffee is basically pH neutral, ranging from 6.5 to 6.8 on the scale. Another myth is that every plant benefits from composted coffee grounds. Geraniums, Chinese mustard, and Italian ryegrass have been adversely affected by coffee ground compost in testing trials, reports Chalker-Scott.
Coffee grounds become more rich in nutrient value as they decompose. When composted grounds are tilled into a bed, earthworms consume the grounds and incorporate them further into the soil, creating a better overall texture. Another exciting benefit is the disease fighting bacteria found in the grounds. Various fungal infections, including Fusarium wilt, Pythium rot, and Sclerotinia blight appear to be deterred, at least for some plants, by the presence of coffee grounds in the soil, according to Chalker-Scott.
Composting Coffee Grounds
Be sure the coffee grounds you use are well composted. Mix them into equal parts grass clippings and leaves, turning weekly for up to six months for full benefits. Apply composted grounds at a rate of no more than 20 to 25 percent volume of the planting bed for best results. Amounts over 30 percent are likely to damage or inhibit seed growth.
You may choose to mulch with fresh coffee grounds, but do not use more than a half inch as a layer over existing beds. Since the grounds are fine and may compact, stifling water flow, be sure to mix the grounds lightly with the top soil. Mulch with up to four inches of an organic compost high in carbon, such as composted leaves, over the top of the grounds to provide balance.