Recycling Coffee Grounds in the Garden

Stephanie Rukowicz

Growing up, my parents always dumped out their used coffee grounds, filter and all, around the base of a rhododendron in our yard. Thus began my experience using coffee grounds in the garden.

Now in the Master Gardener training program, I wondered what exactly are the benefits of using these spent grounds in the garden?

Recently popularized further by Starbucks' program, Grounds for Your Garden, where customers can pick up 5 lb. bags of used coffee grounds for free from participating stores, Sunset Magazine commissioned a study by a lab out in Bellevue, Washington to find out what is in these coffee grounds.

For the full report, visit http://www.sunset.com/garden/earth-friendly/starbucks-coffee-compost-test-00400000016986/. To summarize their findings, the grounds contain nitrogen, potassium, phosphorus, magnesium, copper, calcium, and other trace minerals. Out of these elements, not all are available for immediate plant uptake. The potassium and magnesium are immediately available, as are 50% of the copper and calcium. The other elements, such as nitrogen, are considered slow release, as microorganisms need to work to break them down into an available form for plants.

Ways to use them around your garden include:
1) Composting them. Be sure to remember the grounds are considered a "green", even though they are brown. If using in your vermicompost, depending on the size of your bin, limit to about 1 cup of grounds per week.

One cup of coffee grounds a week makes for happy worms.

2) Amending your soil. Hand till or Rototill them into your soil at a 6-8" depth to improve the quality of your soil in the short and long term, improving the drainage and texture as well as improving availabilities of phosphorus, potassium, magnesium and copper.

3) Side dressing or mulching your plants. Long known as a good practice for acid loving plants, yet the lab study found that the grounds only have a pH of 6.2--not as acidic as commonly thought. Add less than a 1" thick layer at a time, as the grounds can cake and prevent soil and oxygen from getting to plants' roots. To prevent this caking, you can lightly work into the soil surface.

 Side dressing with coffee grounds around blueberry bushes. The paper filters are easier to separate from the grounds after they are dry.

4) Making some "tea." Like you would make a compost tea, you can also make a coffee ground tea. Add two cups of used grounds to a 5 gallon bucket of water, let steep for a few hours or overnight. Use to water or foliar feed garden or container plants.

5) Controlling pests? There are many posts around the web that attest using coffee grounds in the garden help control pest problems by:
 - Creating a slug barrier: sprinkle around base of slug-prone plants.
 - Deter flea beetles: sprinkle around base of beetle-prone plants.
 - Controlling aphids: sprinkle around base of plants to prevent infection; use "tea" on infected plants.
 - Controlling ants: sprinkle where ants are for them to carry home; use "tea" on ant hills.