Coffee Revs Up Your Morning and Compost

Photo: Pixabay

A recent post on the Penn State Extension Master Gardener Facebook page reminded me of this post.

Stopping by the Master Gardener compost pile I noticed a large plastic bag which turned out to be full of coffee grounds. Why are these here? This prompted me to see what’s up with coffee grounds and compost.

Composting is the process where natural materials are broken down by microorganisms to form a nutrient rich, soil-like material call humus or compost. Compost uses two different materials:  Green matter, such as vegetable scraps, grass clippings and, yes, coffee grounds, and brown matter like dead leaves, straw, shredded paper and non-diseased plants. Composting is an easy way to reduce waste and create a valuable gardening supply.

Coffee grounds are the granules that are left after brewing coffee. These are considered green matter in the compost pile. Checking Penn State Extension sources as well as Extension Services in other states revealed that coffee grounds are a good addition to compost as part of the green matter. They can be added to vermicomposting or worm composting. This is a natural method using worms to help compost food and organic scraps. It can be done year-round, indoors and outdoors. 

What’s in Coffee Grounds?

The Brooklyn Feed Website reports “Starbucks commissioned a study in 1995 to better understand the make-up of the organic matter we call coffee grounds.” See the chart for the information provided.

Applying coffee grinds directly to your garden: Coffee grounds can be applied directly as a top dressing/mulch to acid loving plants like blueberries, hydrangeas, and azaleas. Adding brown material such as leaves and dried grass to the mulch will help keep a balanced soil pH.

I am quoting Linda Chalker-Smith, Univ. of Washington, on her bottom line to this topic:

"Recommendations

Given the incomplete and conflicting scientific data so far, the scientific jury is still out reviewing the evidence. Nevertheless, I do feel comfortable in making some general observations and recommendations for those of you who would like to try coffee grounds as part of your compost pile or as a mulch:

In compost

• Percentages of 10 to 20 percent of total compost volume have been reported as optimal for compost quality and effectiveness, while over 30 percent can be detrimental.

• Only small amounts of coffee grounds are required for effective disease suppression. Therefore, I recommend using no more than 20% by volume of coffee grounds in a compost pile. A diverse feedstock will ensure a diversity of microorganisms.

• Don’t assume coffee grounds will make an acidic compost; pH levels will undoubtedly change over time. For mulching

• Coffee grounds are finely textured and easily compacted.

• Compacted coffee grounds create a barrier to moisture and air movement, especially when applied in thick layers.

• Therefore, I recommend against using pure coffee grounds as a mulch; instead, try using a thin layer (no more than half an inch) of coffee grounds and cover with a thicker (four inches) layer of coarse organic mulch like wood chips."

More information about composting for the home garden is available at:

Penn State Extension: Home Composting Guide

Washington State University: Myth of Coffee Grounds (pdf)

Bucks County, PA: Coffee Grounds for the Gardener

~ Carol Kagan, Master Gardener

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