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Earlier today, I noticed a lot of small tiny flies emerging from the Chinese Evergreens I purchased last weekend.

At the time, I just had my freshly ground and used morning coffee beans... so I tried dumping the grounds in a layer on top. This seems to have solved the flies problem. (And I do like the smell of coffee!)

Should I just leave it there? Should I try to remove the coffee grounds? Is this actually a good sustainable pesticide and/or fertilizer?

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The black flies in the soil are probably fungus gnats, Coffee grounds aren't known to be a control, but sand and diatomaceous earth on the top of the soil are known controls, so perhaps the coffee grounds may take care of the problem. It would be interesting to know if that's the case. This site indicates that there may be problems with uncomposted grounds as a mulch, while this site discusses the science behind it. In general, you're probably fine using them as a thin mulch.

If the coffee grounds don't take care of your fungus gnats, my answer to this question contains a list of controls that you can use for them. If you're planning on planting the evergreens into the ground, that in itself will kill the gnats.

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  • Maybe the acidity in coffee grounds helps? – ina Nov 22 '20 at 2:40
  • As noted in my second link, coffee grounds have been shown to NOT be consistently acidic: "While two studies on coffee ground composting reported mildly acidic pHs of 4.6 and 5.26, others have measured neutral (7.7) to somewhat alkaline (8.4) pH levels. One researcher found that the pH of soil treated with coffee compost increased after 14 to 21 days of incubation, gradually decreasing thereafter." I haven't seen any info that coffee grounds can control the gnats, but would love to see if this is anecdotally true. – Jurp Nov 22 '20 at 14:20

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