I get the question occasionally about using coffee grinds to help fertilize certain plants. And while I do recommend it, it’s not really more than adding an amendment to the soil. If you’ve been doing it, and you like the results I’m not here to change your mind, but I would like to clear up a couple of misconceptions about its use as a “fertilizer.” I also remind folks that “The Dose is the Difference” because using too much can lead to other problems.
Coffee grounds are about 2% nitrogen by volume, so while it’s great at greening things up it’s 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 than most people think, between 6.5 to 6.8 pH. The acid is leached into the cup of coffee.
I also advise working the coffee grounds into the soil, because if they are left to dry out on top, they will crust up and create a sort of impenetrable layer that water won’t percolate through. (Did you see what I did there? Coffee & Percolate?!) By working coffee grounds with the soil, it really ends of acting more as a soil amendment, improving the tilth and/or structure with its tiny little bit of fertilize. Just sprinkle it around the drip line of plant you want to fertilize it with, and use a steel tine rake or hand-rake to scratch it together with the soil. If you use them on indoor plants, the crusting up aspect isn’t quite as problematic as outdoor usage.
You can also add them to a compost heap. Despite their color, for the purposes of composting they’re a ‘green’, or nitrogen-rich organic material. Make sure to balance them with enough ‘browns’ – carbon-rich materials such as dried leaves, woody prunings or newspaper. Your compost heap’s tiny munchers and gnawers will process and mix them effectively, so using coffee grounds in this way is widely accepted to be safe and beneficial.
By the way, many vermicomposters (Worm Ranchers, if you will) say that their worms love coffee grounds, so small quantities could also regularly be added to a worm bin if you have one. Paper coffee filters can go in too, but shredded a bit.
Then there is anecdotal evidence that coffee grounds work as a natural insect repellent. In other words, there’s really not a lot of empirical research, rather word of mouth advice. Again, I’m saying “repellant”, so don’t think of it as a natural insect killer. And it only repels certain insects, namely slugs, snails, ants, pill bugs and sow bugs. An oft-repeated nugget of advice is to spread used coffee grounds around plants that are vulnerable to slug damage. There are two theories why: either the texture of the grounds is abrasive, and soft-bodied slugs prefer not to cross them, or the caffeine is harmful to slugs so they tend to avoid it. To run pests off effectively, use anywhere from 2 – 5 lbs. of coffee grounds per 100 square feet for insect control purposes. Now, if that sounds like a lot, and it is, remember that coffee shops give away bags of coffee grounds every day.
One final warning, so you don’t errantly throw coffee grounds away if they don’t look pristine, so to speak, is that coffee tends to develop a green or blue-green fungus that looks like mold. Don’t worry – that’s good. The green fungus is really beneficial (Trichoderma species) while the blue-green one is reported to be moderately beneficial. In any case, moldy coffee is great to use directly in the garden and compost piles.