Fresh Bag of Potting Soil with Gnats Flying out of it.

Asked December 31, 2018, 2:57 PM EST

I just opened a “fresh” bag of organic potting soil, only to discover gnats flying out of it. I am told I can bake it to get rid of them.

#1. Is this true?
#2. At what temperature and for how long?
#3. Will it destroy the soil, and must I compensate it in any way?
#4. Will it destroy any baking dish I use in terms of future food use.
#5. Can you recommend any non-Miracle Gro/Monsanto product that destroys gnats, both adult and larva that is not hazardous to me or my plants?
#6. If this method works, can I also bake previously used soil that may have gnats in it, and, after baking, reuse? Do I need to add something to supplement it?

Thank you so much.

New York County New York

1 Response


First, what is your potting mixture contain?

the easiest way to get rid of fungal gnats is to make sure the soil is completely dried out. If gnats still remain, catch them with sticky traps, available from nurseries or online. If adult gnats are completely gone, they will produce more.

Getting back to the mixture: Soilless potting mixes are relatively free of living organisms, but mixes made with soil or compost are not. Some gardeners talk about “sterilizing” their potting mixes by baking them in the oven to rid the soil of harmful organisms, limiting the hazards of damping-off and other diseases. What I hope they mean is that they “pasteurize” their mixes. Heating homemade potting mixes to sterilizing temperatures wipes out all living things, beneficial and detrimental, leaving a clean slate for possible invasion of pathogens and causing nutritional problems such as ammonia toxicity. Pasteurization, which occurs at lower temperatures, kills only a fraction of the organisms. The best way to pasteurize your soil is to put it in a baking pan with a potato embedded in the soil. Bake it at 350°F for about 45 minutes. When the potato is cooked, the potting mix is ready.

Pasteurizing potting mix is not necessary. I rely, instead, on healthy container-gardening practices such as timely watering, good air circulation, and adequate light to avoid disease problems. Beneficial microorganisms in compost and garden soil also help fend off pests.

Hope this helps.