Coffee is so steeped in modern culture that humans generate about 15 million tons of spent coffee grounds each year. As such, people have been trying to figure out what to do with those grounds for years.
One of the most common uses I read about all the time is that you can toss your used grounds right into your garden. As with much garden advice, there is a lot of nuance to this one: Dumping spent coffee beans into the garden willy-nilly is not a good idea, but there may be some niche applications where it makes sense.
What's actually in coffee grounds?
Let’s start with what’s in coffee grounds. Coffee itself is a bean, and all beans are nitrogen fixers by nature, meaning that they produce available nitrogen for the soil. Aside from the protein in the bean, there’s also oil, lipids, triglycerides, fatty acids, cellulose, and sugars. On top of that, the brewing process can add lignin (an organic polymer), phenolics (an aromatic) and essential oils, none of which are problematic in the garden and can actually be antioxidant. There are three organic bodies that can break down and utilize all of those compounds in the garden: fungi, bacteria in the soil, and earthworms. So far, so good, right?
Debunking common gardening myths about uses for coffee grounds
Myth 1: Coffee grounds will acidify soil
So, to bust the first myth around coffee and your garden, adding it to the soil around blueberries, hydrangeas, and azaleas won’t create an acidic environment, which those kinds of plants enjoy.
Coffee beans create a steeped drink that is acidic, which would be bad for your garden, since you want to keep your soil as close to pH neutral as possible. However, the grounds for coffee actually are pH neutral, or close to it. This is, in part due to the fact that by the time coffee becomes grounds, much of the nitrogen is gone, creating an almost 1:1 ratio of carbon to nitrogen. Instead, use a fertilizer specifically for these plants, which will help acidify the soil.
Myth 2: Coffee grounds should be used as an can be antimicrobial
Another common piece of gardening advice around coffee is that coffee grounds may help prevent pathogens in your garden, like fungi and viruses. There is a nugget of truth to this, but it is a double-edged sword.
The evidence shows that coffee grounds can provide a modicum of antibacterial and antimicrobial benefit in the soil, but that these results are variable, which makes sense, since there are so many kinds of coffee and much depends on the brewing and roasting process. Remember, though, that microbes are not all bad—and in fact, good microbes like mycorrhizae (fungal structures) are what keep your garden thriving. Coffee grounds can’t discriminate, so if they’re getting rid of microbes, they’re getting rid of all of them.
Myth 3: Coffee grounds make good mulch
Finally, on the notion that coffee grounds make great mulch. Dr. Linda Chalker-Scott is quoted in a piece out of the University of Arizona Cooperative Extension as saying that grounds are so finely ground that when left on the soil in bulk, they can prevent airflow and penetration of other elements, which is detrimental to the garden.
Coffee has some additional downsides, including the caffeine, which can stunt plants growth. Plus, even though it has potassium, phosphorus, calcium, magnesium, and very minor amounts of iron, copper, manganese, and zinc, the available amount of each of these is so small as to be negligible in the garden. As usual with gardening myths, while there are some small truths in each myth, the reality is that there are products that will accomplish the goal much more efficiently. In this case, garden fertilizer.
However, coffee does show great promise as a slug and snail killer
The one thing that coffee grounds can do, apparently, is help with slug and snail control. In a piece from Oregon State University Extension Office, a soil scientist named Linda Brewer reports that research has proven a very mild steeped tea of grounds to water poured in the garden can dramatically reduce slug and snail populations. More exciting, it is more effective than commercial products like Sluggo.
She recommends you prepare a solution of one part water to two parts strong brewed coffee, and then drench your soil with it. You can also use it as a foliar spray if you’ve got slugs feeding on your vegetables, like cabbage. In that case, use nine parts water to one part brewed coffee and spray it on. At these low concentrations, the acidity is mitigated, so it shouldn't be a problem, but you should always try out a solution like this on a small area, on a cloudy day, before applying it to the whole garden.