I am new to the world of gardening/planting. I had a huge bag of potting mix that I bought from amazon last summer and used to plant some herbs. I used half of it back then and kept the rest in the bag. The bag had holes in it for breathing so I thought it would be safe to keep it out on the balcony over the winter (in a secluded area away from snow and rain) since I don't have a place for it in my apartment. Long story short, I was just checking the bag to get ready to plant new herbs for this season and I was shocked to see that the bag was full of tiny flying insects.

I read online that it might be fungus gnats, but I didn't see anything online about them being in the bag itself. I was wondering if there is anything I could do about this issue or if I should throw away the bag and buy a new one? I would appreciate any input regarding this matter!

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  • 1
    Fungus gnats are usually only a problem indoors in houseplants that are kept too wet. If your winters are cold and the potting soil has been outdoors, not sure what the insects are - don't suppose there's any chance of a photograph of these insects? Which particular type of potting soil is it?
    – Bamboo
    Mar 23, 2017 at 0:03
  • Bamboo I would think this should be an answer. Definitely a picture of insects because if I were an insect with nice warm soil with lots of food that is exactly where I would go. Don't think it will be a big problem but even fruit flies inside make me go crazy. I do get to practice my speed and timing trying to smack them in the air!! Maybe it is just an insect that will fly away and never be a bother again...unless it has already made babies, they do that quickly. Grins.!
    – stormy
    Mar 23, 2017 at 16:38
  • @Bamboo I added a picture to the main post.
    – adelejjeh
    Mar 23, 2017 at 16:47
  • @Bamboo They are very tiny insects, not larger than the size of an ant. They seem to be black with light-colored wings, kind of like tiny flies. The type of potting mix is: "Miracle Go Organic Choice Potting Mix". I live in the midwest, so it is very humid during summer/fall. I wanted to mention that during summer/fall I've opened the bag once or twice to find a white residue on top of the soil. A quick internet search lead me to deduce that it was fungus. I removed a lot of it back then and left the bag open in the sun to get the moisture out; I'm not sure if this is relevant.
    – adelejjeh
    Mar 23, 2017 at 16:47
  • @stormy it seems like it made babies since there are a lot of them roaming around the bag! I guess you can say they kind of look like fruit flies. At least same size/shape.
    – adelejjeh
    Mar 23, 2017 at 16:48

2 Answers 2


Well, welcoming a certain degree of "bonus wildlife" is in the spirit of the organic approach your choice of this mix implies. A healthy soil is a living soil. It's astounding what quantity and variety of microscopic critters live in soil, doing good things or at least no harm.

Since these little insects apparently thrive in your bag of soil, that suggests though does not guarantee that they eat dead plants (detritivores). Compost = dead plants. Some insects eat both live and dead plants, but lots stick to one or the other only. I'd cut a bit of almost any plant, nice juicy green leaves, and maybe put the stems in a little glass of water. Nestle the experiment into the potting mix. Leave the bag open, mimicking the air that a potted plant would have. Then look in a day or two and see whether the insects congregated on or obviously damaged the leaves. If not, I'd use it as is. I certainly wouldn't throw out the bag.

If you really, really want the insects gone and suspect there are eggs, another option is to put a bunch in a big pan and bake it at about 250 degrees until it's hot through. Warning: This can really smell up a kitchen!

On good rumor (I'm not taking on Miracle Gro with an absolute assertion!), they make at least some of their organic potting material by composting ground up ponderosa and lodgepole trees from the Front Range in Colorado, or did a couple years ago. If that's what you happen to have and forest conifers are the origin of the insects, the chance the insects also favor tiny tomatoes seems low.

  • So I ended up using the soil to plant my herbs. The insects are all over my basil but they don't seem to be harming it. I do see a few brown dots on the leaves here and there though, I don't know if that's a "disease" or if it is from the heat of the sun. I'm starting a new post with a question about some of my plants so I'll move this issue to that thread. Thanks for your answer though! (And sorry I saw it late!)
    – adelejjeh
    Jul 12, 2017 at 14:44

I would not recommend tolerating fungus gnats in soil that you intend to use indoors, unless you like the idea of cohabiting with gnats, as they will stick around as long as you let them. If you want to use that soil, drench it with BTI when you bring it to field capacity. After you've used it to pot whatever you're going to plant, spray the plant with a pyrethrum spray. Then cover the topsoil with sand or vermiculite.

May sound like a lot, but a fungus gnat infestation indoors is hell.

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