I am occasionally asked about using coffee grounds to help fertilize certain plants. They’re okay, but they’re really just a soil amendment. If you’ve been using them and like the results, I’m not here to change your mind. But I would like to clear up misconceptions about their use as a “fertilizer.” And I’ll also stress that “the dose is the difference” … using too much can lead to problems.
Coffee grounds are about 2% nitrogen, so while they’re great for greening things up, they’re not helpful in making plants pop with blooms.
And here’s the biggest misconception: coffee grounds are not highly acidic. They are actually closer to neutral, between 6.5 and 6.8 pH. Most acid is transferred to the beverage
I also advise working the coffee grounds into the soil - left to dry out on top, they will crust up and create a mostly impenetrable layer that won’t allow water to percolate through. (Did you see what I did there? Coffee and percolate?) By blending coffee grounds into the soil, they really act as a soil amendment, improving the tilth or structure. Just sprinkle them around the drip line of plant you want to fertilize, and use a steel-tine or hand rake to scratch them into the soil. If you use the grounds on indoor plants, the crusting up may not be quite as problematic as in outdoor use.
You can also add them to a compost heap. Despite their color, they’re actually a “green” nitrogen-rich organic material. Just be sure to balance them with a good portion of “browns” – carbon-rich materials such as dried leaves, woody pruning, or newspaper. Your compost heap’s tiny munchers and gnawers will process and mix them effectively, so using coffee grounds is widely considered safe and beneficial.
By the way, many vermicomposters (worm ranchers) say their worms love coffee grounds, so small quantities could also be regularly added to that bin if you have one. Paper coffee filters can go in too, but shredded a bit.
Anecdotal evidence also indicates that coffee grounds work as a natural insect repellent, but there’s really not a lot of empirical research on that. Plus, that’s “repellant” - don’t think of it as a natural insect killer. And it’s thought to only repel certain insects, namely slugs, snails, ants, pill bugs and sow bugs. Many say spreading used coffee grounds around plants that are vulnerable to slug damage repels the pests. There are two theories why: (a) coffee grounds’ texture is abrasive, and soft-bodied slugs prefer not to cross them. Or (b) caffeine is harmful to slugs so they tend to avoid it. In any case, to run them off, you’ll need to use 2-5 pounds of grounds per 100 square feet. I know … that sounds like a lot – and it is – but remember that coffee shops give away bags of coffee grounds every day.
One final note: Don’t throw away coffee grounds if they don’t look pristine. Coffee tends to develop a green or blue-green fungus that looks like mold, but don’t worry - that’s good. The green fungus (Trichoderma species) is really beneficial, and blue-green versions are also reported to be moderately beneficial. In any case, moldy coffee is fine to use in the garden and compost piles.
PHOTO: AgNet West