According to popular belief, coffee grounds are practically a miracle product. They keep slugs and cats at bay, stimulate plant growth, prevent fungus, and enrich the soil. And if you’re coffee drinker, they are essentially free! However, if you look at actual studies, you’ll soon see while coffee grounds are not without some use in the garden, they are not the miracle product they are claimed to be. Here’s a short portrait of the real situation.
Claims and Counterclaims
Coffee Grounds Repel Pests
Many gardeners claim that coffee grounds repel harmful insects and other garden pests, including flea beetles, lily beetles, aphids, slugs and even cats, but this idea would appear to be mostly wishful thinking. Every time a serious study is done on the issue, the result is always negative. For example, slugs will readily cross a layer of coffee grounds in order to reach the plant of interest.
Despite all the studies (and there are many), you’ll still find people who claim great success when using coffee grounds to repel pests. I’ll always remember one woman who insisted she had great luck keeping the scarlet lily beetle (Lilioceris lilii) off her lilies (Lilium spp.) just by using coffee grounds as a mulch. When I challenged her about it, she admitted she had to do a “bit of hand picking”, dropping the bright red pests into soapy water, say 5 or 6 beetles per day… but by mid-summer, their numbers were down to 1 or 2 per day, which she considered proof of the efficacy of coffee grounds. Well, if you consider that success, you’re not as picky as I am! I used to get about the same results when I hand picked lily beetles and I didn’t use coffee grounds or any other supposed repellant. Hand-picking simply does lead to a lower pest population, period!
Coffee Grounds Prevent Soil Diseases
Yes, coffee grounds can help prevent soil diseases… in a laboratory. Mixed into a growing mix, they have been found to control such harmful fungi as Pythium (which causes root rot) and Fusarium and Sclerotinia (vascular diseases). However, they also were found to repress beneficial fungi. Plus so far, tests in the garden have not been so promising, giving results that are either inconclusive or not statistically significant. Also, they have only been tested on a small range of plants, mostly vegetables and cereals like tomatoes and cucumbers. They seem to give somewhat positive results under some circumstances, but not under others. More studies may well show what those circumstances are.
Coffee Grounds Can Be Used as Fertilizer
Coffee grounds are rich in minerals, especially nitrogen, a mineral that plants need to grow well, so can certainly be added to the compost bin… but they are no miracle product: they’re about as rich in minerals than many other table scraps. So by all means add them to your compost. And do note that coffee grounds, despite their dark brown color, are considered a green material rather than a brown one. (Home compost makers will know what I mean.)
Coffee Grounds Acidify Soil
The popular belief is that the coffee grounds are very acidic and that you should restrict their use to acid-loving plants… but legitimate studies arrive at a different conclusion. True enough, during the period of decomposition, the pH level of coffee grounds can be very acidic… but it doesn’t stay that way. It tends to move to more acceptable levels towards the end of the process, sometimes even becoming slightly alkaline. That shouldn’t be a surprise to anyone who has looked into composting: most organic materials through an “acid” phase during decomposition only to become closer to neutral at the end. For most soils, therefore, coffee grounds’ effect on pH will essentially be zero, but they could possibly help to make a very acidic soil slightly more alkaline.
Coffee Grounds Stimulate Plant Growth
The claim that coffee grounds stimulate plant growth is a very complex one to analyze because coffee grounds seem to stimulate some plants while inhibiting others. When mixed into soil, they seem to stimulate the growth of cabbage and soybeans, for example, but those plants may the exceptions rather than the rule. In fact, grounds tend to inhibit the growth of many plants, sometimes markedly so. Poor results have been shown on tomatoes, mustards, clovers, and geraniums, among others. One theory is that toxic compounds released by coffee grounds as they decompose may be at fault. There is even talk of using them as a herbicide! However, once fully decomposed, any toxic products ought to be eliminated, and it has been shown that coffee grounds can safely be used in home compost bin.
Experts suggest using coffee grounds in moderation if they are applied directly to the garden as mulch. This is partly because they tend to form a crust over the soil that prevents the free flow of air and water, but also because of their fluctuating pH and their inhibitory effect on the growth of some plants. Ideally therefore they should be mixed with other mulches (forestry compost, shredded leaves, etc.) when used for mulching, never making up more that 20% of the mulch or, if used alone, only as a very thin layer.
Coffee grounds can safely be used in home composting where they are, as mentioned above, considered a green material. Nevertheless, due to their inhibitory effects on beneficial fungi needed for decomposition (I didn’t get into that above, but it is a known complication), they should not exceed 20% of materials added to the compost bin.
Coffee grounds, in short, are like many products: they have both benefits and disadvantages. By all means recycle them in your compost bin, but they are certainly not a miracle product.