I have always wondered about those free coffee grounds for your garden offered at most Starbucks. I never took them up on the offer, and pretty much decided that it was a good way for them to get rid of untold pounds of grounds and not feel guilty about caffeinating the landfills. Anyway, I have my own grounds to contend with, and that's what got me wondering again. So off I went to internet land and lo and behold, a plethora of posts and articles singing the praises of coffee grounds in the garden, as well as a few warnings and reality checks.
Kit Smith, an El Dorado County Master Gardener, warns that adding unlimited coffee grounds to the compost pile is not a good practice. She explains that coffee grounds include the pulp, hulls and effluent of the coffee bean. Additionally, coffee grounds, though a good source of nitrogen, are acidic, and excess acid prevents the compost heap from heating up enough to decompose. She recommends that grounds make up no more than 15 to 20% of the total compost volume.
Because they are acidic, coffee grounds make good acid mulch. Of course, too much of anything is just too much, so apply coffee grounds in limited amounts. Kit recommends a layer no thicker than half an inch. Working coffee grounds into the soil will improve its tilth, but do this sparingly unless you have acid-loving plants, like camellias and azaleas.
Sharon Lovejoy, the author of Trowel & Error, extols “Our ancestors had it right. Waste not, want not—And that includes coffee grounds.” She recommends them as pure gold for your garden, compost pile, and best of all, your worm bin. She goes on to say that, “From my point of view, the invention of the garbage disposal was one of the worst moments in household and garden history. For every pound of “garbage” washed down the drain, we waste at least 8 gallons of precious water and compostable vegetable matter that could be put to good use in our gardens.
The best article I came across was from Sunset Magazine – they actually sent a batch of Starbucks grounds to a soil lab – Soil and Plant Laboratory Inc., in Bellevue, WA. The findings were encouraging – amending soil with up to 35% grounds will improve soil structure over the short and long term. They should be tilled into the soil to a depth of 6 to 8 inches and will improve the availability of phosphorus, potassium, magnesium and copper – negating the need for additional sources of these nutrients. In addition, each cubic yard of coffee grounds provides 10 pounds of nitrogen (0.09% available). The grounds will provide nitrogen in a slow release fashion for use by plants over the long term. It is an excellent soil amendment and is recommended to be used at a rate of 25 to 35% by volume to improve soil structure. The slightly acid properties of the grounds are also a welcome addition, especially here in the west where soil tends more towards the alkaline.
Don't let the opportunity for freebie coffee grounds pass you by. Get courageous and ask your local coffee shop or restaurant to save some of their leftovers for you. Or, after you finish brewing your morning pot of coffee, take that treasure trove of nutrients and compounds out to the compost bin where they can release these compounds as they decompose and make a healthy amendment for the soil in your vegetable garden.
Put one-third coffee grounds, one-third grass clippings and one-third dried leaves into a compost bin. Mix the coffee grounds and carbon-rich matter together thoroughly with a pitchfork.
Allow the compost to develop a soil-like appearance and an earthy aroma before using it. It may take three months or longer for compost to fully break down, depending on the materials used.
Spread a 1-inch layer of moist coffee grounds on the soil in your vegetable garden. Add a nitrogen fertilizer to the soil according to the package directions. The nitrogen fertilizer speeds the decomposition of the coffee grounds and gives your vegetable plants more nutrients Mix the coffee grounds and the fertilizer into the soil with a pitchfork or shovel. Don't leave the coffee grounds on the surface of the soil, exposed to the air and causing them to dry out; dried-out coffee grounds repel water.
Though there was not much hard research on this, many gardeners enthusiastically insist that coffee grounds repel unwanted pests, such as snails and slugs, in your vegetable garden. Personally, I would take a wait-and-see approach to this.
Finally, if you run out of room for coffee grounds in your compost bin, store the remainder in a plastic trash bin until you can use them. The coffee grounds never expire or go bad. Better yet, give them away to gardeners in need. Tea drinking gardeners can compost their bags and leaves, but they cannot be used directly in the soil, so a gift of grounds would certainly be a welcome one.
References
Oregon State University: Coffee Grounds and Composting
University of Arizona Cooperative Extension: Using Coffee Grounds in the GardenCity of Davis: Backyard Composting Guide
Environmental Protection Agency: Composting
How to Compost Coffee Grounds