Mulching Soil

Using Coffee Grounds as Compost: Are They Good for Plants?

Cup of coffee with coffee grounds.

Photo: Shanegenziuk, Wikimedia Commons

By Ruby Collins

Most people rely on a cup of coffee to get them going in the morning, and over the years, we have heard about other benefits that coffee has for the human body. For example, it contains antioxidants that fight off disease and boost our performance, but do you know what else you could do with coffee? You can use the grounds to mulch your plants, of course. 

The idea behind using coffee grounds as a mulch mixed with bark or manure is to increase the healthy activity of microorganisms in the soil, the ones that regulate the soil temperature and water activity. 

Used coffee beans reduced into grounds contribute nitrogen to aid in photosynthesis. Although it was formerly believed that coffee grounds were acidic and therefore possibly harmful to plants, that has been proven to be untrue. Instead, they offer a balanced pH level, making them safe to use with bark as a mulch and with animal manure as compost. Here’s how:

Natural Gases and Compounds

Prokaryotic life (bacteria and other organisms) in the soil uses nitrogen in the coffee grounds by converting it from a “gas” into a “fixed” compound through ammonification. This process occurs when the bacteria in the environment excrete the “fixed” nitrogen back into the soil in the form of inorganic matter, which plants can then use for their growth and production. 

Coffee grounds can be mixed with bark mulch or manure and layered onto the planting area. As there is only about 2% nitrogen in coffee grounds, they need to be combined with animal manure or bark mulch for their fertilizing effect to be truly useful . Still, it is essential to note that adding coffee grounds to your compost isn’t an immediate fix. The effects it has on the soil are long term and can take quite a while to be felt. 

Some of the many components in coffee grounds are: 

  • 0.06% Phosphorus which facilitates the use of energy from the decomposition of nitrogen, the conversion of starch and sugars, aids synthesis and root growth.
  • There is 11.7 g of potassium per kg of coffee ground. The purpose of potassium is to regulate the intake of carbon dioxide.
  • 1.9 g of magnesium per kg of coffee grounds help absorb heavy metals in the soil and activate enzyme systems to prevent stunted plant growth. 

The belief that coffee grounds can act as an insect or bug repellent is yet to be confirmed. You might be tempted to revive a dying plant with this fertilizer booster if you are an avid gardener. Still, coffee grounds should never be used on their own for mulching: some gardeners have noticed a reverse effect from pure coffee grounds that was disastrous for their plants. They must always be mixed with other amendments. Adding a cup of grounds once a week to bark mulch should give you sufficient food to attract earthworms that will help take the nutrients more deeply into the soil. Again, there is no confirmation that this will increase worm or insect activity in your soil. 

Make sure not to overdose your plants with leftover coffee grounds. As with most things in gardening, coffee grounds need to be used in moderation. 

Larry Hodgson is one of Canada’s best-known garden communicators. After studies at the University of Toronto and Laval University where he obtained his B.A. in modern languages in 1978, he succeeded in combining his language skills with his passion for gardening in a novel career as a garden writer and lecturer. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He is a regular contributor to and horticultural consultant for Fleurs, Plantes, Jardins garden magazine and has written for many other garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening, Rebecca’s Garden and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 50 other titles in English and French. He can be seen in Quebec on French-language television and was notably a regular collaborator for 7 years on the TV shows Fleurs et Jardins and Salut Bonjour Weekend. He is the President of the Garden Writers Association Foundation and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. An avid proponent of garden tourism, he has lead garden tours throughout Canada and to the gardens of over 30 countries over the last 30 years. He presently resides in Quebec City, Quebec.

2 comments on “Using Coffee Grounds as Compost: Are They Good for Plants?

  1. I worked in a Coffee processing plant for 37 years & have used chaff, green beans(not a True bean),roasted beans & ground coffee beans that have not been brewed, as well as the tiny amount of coffee & tea I used at home.
    The only problem I had was it turned to slick mud when it rained & hard when it was real dry out.
    As a organic gardener I never used bag fertilizers, so all the break down came from the soil & it never hurt my plants in anyway. I tilled in truck loads of dry leaves in the Fall, & some blood mill in early Spring on about 1/2 acre garden.
    I had great fun showing photos to unbeliever for many years, the 1500 pound bags of coffee waste looked like gaint eggs in the field. The gaint bags were called “super sacks” & you can buy them on line.

  2. I am not convinced by the hype. We collect coffee grounds from the big cafeteria type kitchens at work only because we are expected to, and it certainly is not bad for the compost. Besides, it is better than disposing of it. (During the summer, we collect several buckets of it three days weekly. In my former home, I put (well weathered) coffee grounds around the houseplants as mulch, just because it had a nice fine texture that ‘looked’ better than potting soil. (Although most of us do not notice potting soil of houseplants. I later covered the grounds in big pots with shredded paper from a document shredder.) In the future, I would like to keep some of the coffee grounds separate from the rest of the compost, so that I can use it as a top dressing for my planter box downtown.

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