I was having my husband save coffee grounds to toss in the composter. He went off to stamp out forest fires, and I forgot about the grounds, although they were covered, and when I finally saw and uncovered them they were covered with fungus gnats.
So I don’t think coffee grounds are a good idea for fungus gnat eradication on indoor plants. Here are a couple of alternatives, or addendums if you have coffee grounds on the plants already, I think they’re probably otherwise good for the plants-
- Buy a jar of mosquito bits (bacillus thuringiensis), put a fairly heavy layer on top of the plant, water it in, and keep the soil moist so the bacilli don’t desiccate and die out too soon. The bacilli eat the fungus gnat larvae. It will take a week or so, but the gnats will disappear. This is what I use.
- Buy a bag of fine sand (my Ace Hardware has bags of it called ashtray sand) and put a thick layer on top of your soil. It’s supposed to disable the gnats from burrowing into the soil to lay eggs.
- Find a source for predetaory nematodes, like arbico.com. I had an entymologist friend give me a batch she made up herself, and I’ve never seen gnats disappear so quickly, Arbico is a great company, by the way.
