Baking Potting Soil to Rid Fungus Gnats

Q. I find it impossible to purchase a bag of potting or garden soil that does not contain gnats. This has been a problem with different brands, from local nurseries and home stores. How does one avoid this problem?

I don't care how expensive it is, nearly every potting soil has problems with fungus gnats.When they are in large numbers and houseplant soil they can damage young tender roots of growing plants.

A. Fungus gnats (small flies coming from potting soil) are a very common contaminant in potting soils because many are not heat treated to kill them. Raw components are simply mixed together and not composted when it is finished. You will find this problem in nearly all bags of potting soils from expensive mixes to less expensive mixes.

            The best way is to raise the temperature of the potting soil to at least 140F for 30 minutes and kill them. Put the potting soil in a clear plastic back, moisten it slightly if it is dry, and let it “cook” in the sun for a few days near a south facing wall in full sun. Turn it over and try to bake both sides as thoroughly as possible.

             Another method is to put the potting soil in the oven and set the temperatures as low as possible, making sure it hits at least 140 thoroughly for 30 minutes. You don’t want the temperature much above 180F because it can harm the organic content of the soil.

If you keep the temperature of the soil below 200° F, there will be very little smell associated with baking it.