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Wednesday, December 9, 2015

Every Potting Soil Contains Fungus Gnats

Q. It seems no matter what brand, no matter where I buy my potting soil I have to bake it before I can use it.  If I don't, I get hundreds of tiny fly that hatch and swarm.  I put out water to catch and drown them. I spray insect oil on top of the soil several times a day. I'm so afraid I'll kill the plants.

A. Yes, fungus gnats are a big problem in potting soils and they can create damage to plant roots. They feed off of both decaying plant parts and soft, succulent living roots as well. Potting soil does not smell very good if you have to put it in your oven at 150° for 15 minutes to kill these nuisance critters.

The larvae can cause damage to new, tender roots of plants. If they are extremely happy in their environment they can multiply very rapidly and cause some severe damage. Besides that, they are pesky and a nuisance inside the house.
This is Garden Gourmet potting soil and I have never seen fungus gnats in this product. However, bagged potting soil that is not been heat treated will carry fungus gnats. This product sells for $5 for one cubic foot.

If potting soil is sterilized with a heat treatment by the manufacturer it should kill all of the fungus gnats. If the soil is introduced into an environment where there are fungus gnats present then it will get reinfested again.

Here is what the University of California says about them

They can be controlled with some organic pest control products; nematodes that go after these larvae and a bacterium which does the same thing. You should be able to find some of these products in your local nursery or garden center. I have never used them so I am reporting only what has been reported on the internet using these methods for fungus gnat control. However, cooking them does work.

Where to get beneficial nematodes
Where to get beneficial bacteria  It says for mosquito control but it is the same product used for fungus gnats and will be included on the label.

Another method is to sterilize this potting soil yourself by placing it moistened into a clear plastic bag in full sunlight and let it bake. Temperatures need to get up to about 160 F for at least 30 minutes for good control. Keep in mind that if you introduce this back in the environment where a fungus gnats are present and they will reinfest this sterilized potting soil. Another option is to apply pyrethrin to the soil and water it in.


  1. Where can I buy garden gourmet potting soil?

    1. Garden gourmet line of Bagged soil mixesIs only available in Las Vegas from viragrow www.viragrow.com
      You can look at them here http://www.viragrow.com/bagged-soil-amendments

  2. This fungus gnat problem reappeared in the Review Journal's March 17-18, 2018, issue.

    I hesitate to bake potting soil in my house oven because I've been warned that, as the soil heats up, it can smell pretty bad! If I ever attempt this baking method, I'd probably trying "roasting" it in a gas grill (lid down) outside where nasty smells aren't as much of a problem.

    I have had a serious fungus gnat problem in the past from commercial potting soil. The problem was solved by using Gnatrol, which contains a specific bacillus thuringiensis bacteria that kills most gnat larvae. One needs to apply regular doses in order to eradicate the larvae. Gnatrol is not expensive when you consider the dosage is 1-2 teaspoons per gallon of water. So one small package of Gnatrol goes a very long way. It can be found online, on Amazon, Ebay, etc.

    I've also learned to water indoor plants sparingly. If pots are left standing in saucer water, you're asking for a fungus gnat problem.

    1. Thanks for bringing up these comments. The idea that potting soils stink when they’re put in the oven to sterilize or sanitize them is true. For this reason I avoided the recommendation to put them in an oven. Instead, I usually tell people to put it in a sealed, clear plastic bag and solarize the moist potting soil in direct sunlight for two or three days. This raises the temperature of the potting soil high enough that it kills all of the insects that might be present in the potting soil.

      I also used to recommend the use of that particular Bt for controlling them in their juvenile stages until that particular form of BT was hard to find. You can still find posts of mine on this blog that recommend it. We ran into a problem this spring because of the prolonged cool wet weather. We could not generate enough heat to solarize our potting soil to make sure that all the fungus gnats were dead by putting it in a clear plastic bag and “cooking it” in sunlight. We needed to use that potting soil right away. I told the woman who was starting our transplants to use her oven at home but I warned her about the smell problems. She could control the temperature of her oven down to about 180° F. She took the potting soil home and put it in the oven on sheets and baked it at somewhere close to 180° F for 30 min. The next day she reported to me she had no problems with smell issues at that temperature. It worked for us and no smell as long as the temperatures are low enough. For insect control, 140 -150° F is usually good enough if you can keep it at this temperature for at least 30 minutes. The temperature has to be more than this if you are looking at disease control. That temperature should be about 180° F for 30 minutes. But you are right, if you don’t control the temperature you can have some real smell issues as well as burn up all the organics in the potting soil.