Adding coffee grounds to indoor basil plant

I’ve found conflicting information online if this will harm the basil plant. I want to use coffee grounds like mulch on top of the soil. I had gnats recently and had to starve my plant to dry out the soil so the gnats would die off. I’m now watering the bottom of the plant and hoping the grounds give me an added layer of protection. In any case, has anyone used coffee grounds with their basil plants? Since it’s a small pot, the coffee ground will be much more concentrated in one area than when I sprinkle them outside.

I’ve used coffee grounds sprinkled in beds and in compost (complete with unbleached filters). I haven’t used it as a mulch on house plants.

Accordingly I’m making this up. How about spreading the coffee grounds on a sheet pan and baking dry in a very low oven? If you’re bottom watering that should really slow down leaching of acids and tannins (are there tannins in coffee?) into the soil.

I can help you with gnats if the gnats are really fruit flies. They make me nuts. Lots of research on this end. In addition to getting fruit and veg off the counters the instant it starts to get soft, I learned that they breed in sink traps. I heat white distilled vinegar and poor that in the drains (six sinks, three showers, a bidet, a clothes washer, and a pump-up sump tank in the basement). I do this at the same time as cleaning the coffee maker and counter kettle, so less waste. This has knocked down the fruit fly problem here to one or two insects over the summer, which I presume come in with groceries.

I remember from classes when I was a master gardener about baking soil for house plants before potting up.

I used to bring basil and peppers in for the winter in my previous house - doesn’t work for us here so I’m out of date and have a CRS issue. grin


Good idea. I’m in an acidic coffee phase right now (I prefer acidic during summer), so I will dry out the grounds and try it.

These were gnats. Fruit flies come in with farmers market produce every now and then. We had a fruit fly issue a couple months ago that drove me insane. I wish I knew about the vinegar solution then.

I always have trouble with insects in house plants. This is the first time I went 3 weeks before any issue, so I can’t blame the soil this time.

Let us know if you come up with a good gnat solution. Hasn’t been an issue here but you never know.

Just dry out the soil 2 inches from the top and wait for their babies to hatch and die. It will stress the plant some but it will recover. I switched from bottom to top watering because it was easier, but then the gnats came.

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From what I read, you can add ground coffee after the growing season in late autumn. This is like adding to composter, which I do regularly. As for plants, adding coffee changes the PH value of the soil, so I bet it depends on plants’ preference with acid or neutral soil.

Will like to see how your basil likes that.

Basil like neutral soil, maybe slightly acidic. Coffee grounds might be too acidic. if you use them use only a light covering on top of the soil, and bottom water if at all possible for a week or two until the gnats are gone.

I can’t give much advice since, while I have a green thumb, basil and cilantro are my kryptonite. Happens almost every year lately. Just can’t get them to grow, while my other stuff, dozens of types of veggies and flowers, are growing insanely well.

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I didn’t know you were a master gardener! How about that!

I don’t know about coffee in potted plants.

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1995 to about 2005 in Virginia. I remember most of it, and still have some good references. Have my well worn Felco’s and a big assortment of digging tools.

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After the gnats died, I went back to watering top and bottom with no issues. Haven’t had any more gnats since then.

the gnats and fruit flies drives me Loco Loco. I literally bleed from scratching. Allergist suggest I use local honey, so son is raising bees for the past two years but this is the first time he has honeys but I am allergic to bees as well!!! ER twice for their bites them and yellow jacket!!!
Another problem was the horse flies. We bought a HORSE PAL . It help with those big flies. It was sturdy and we had moved it from one part of the garden to another.
however, the last tornado a month ago blew it down but it still fine, jsut some torn canvas .
We place vinegar in bowls and cover them plastic wrap with tiny holes all over the kitchen counter!!!
Now, I know why my son is baking some sands and soil!!!


Just made a comment on another thread about my win over gnats/fruit flies. Will c/p here:

This comment will get lost in the sea of replies, but here’s my two cents: Never had one method work exclusively. The following three methods combined have eradicated the stinkers from my home completely. It’s not overnight, but drastic differences are noticed in a week or so.

+1 for the vinegar dish soap method. Fruitflies are also attracted to alcohol. Random science bit: Fruit flies prefer to lay their eggs on rotten food that can contain ethanol in as high as 7% concentration. (That’s 14 proof to you bar hoppers.) And just like people, the insects differ in their ability to hold their drinks. :slight_smile:
So, when I have some leftover dregs of makgeoli (a homemade korean rice wine), or wine, I do that in lieu of vinegar and attract a ton more.

Sticky paper. But, I’m not a fan of the sticky yellow cutouts. There are clear ones sold for the window. I find most flying insects congregate there if all the food is away. It’s gross but strangely satisfying when we replace those sticky sheets.

If you have houseplants, they could be breeding in the dirt. There’s a product called “mosquito dunks” that can be found online or at any hardware store- sold as donuts or pellets. The main ingredient is BTI which produces a bacteria toxic to these at larval stage; it’ll kill the fruit flies/gnats/fungal gnats/mosquitos at larval stage . This is human, pet, & even aquarium safe. With these BTI pellets/donuts, I soak overnight in a gallon jug and make a “tea.” I use this when watering my house plants and dump the excess in the drain (completely safe) to get any bugs trying to colonize in the damp pipes.

Hope this helps~


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 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.