Gnats in Potting Soil

By Ellen Brown


Has anyone ever had problems with gnats in their potting soil? I recently opened a bag of potting soil and used it to sow some Spinach seeds in a pot indoors, and now little tiny gnats are all over the place. I tried spraying the top of the soil with insecticidal soap, but that didn't kill them. I spend a lot of time squishing them with my finger. Help.

Hardiness Zone: 7a

Kathy from Canton, NC



The little gnats you're seeing are probably fungus gnats. They seem to suddenly (and mysteriously) appear out of nowhere soon after watering plants or after opening a fresh bag of potting soil. Adult fungus gnats like to lay their eggs on moist soil. Soon after they are laid, the eggs hatch into larvae, which feed on fungus growing in the damp soil. After a short period of feeding, the larvae pupate and hatch into adult fungus gnats and the cycle starts all over again. In the case of gnats showing up in potting soil, I suspect that the eggs get laid in the soil before it's packaged at the plant. The moisture, the nutrient-rich soil and a lack of air circulation combine to create the perfect conditions for growing fungus. It's either that, or we all have adult fungus gnats in our homes that fly about undetected until we open a fresh bag of potting soil. Seeing as how gnats have such a short life span in the first place, it's hard to imagine that they would wait around for us to open up a new bag of potting soil.


In any event, the good news is that without food (fungus) to feed on, the gnats cannot develop into mature adults. One way to help halt (or at least disrupt) the fungus gnat reproductive cycle is to let the soil dry out a bit between watering. It also helps to make sure the top layer of your potting soil is exposed to plenty of sunshine and fresh air.

I once had a huge crop of fungus gnats hatch inside a terrarium I planted. When you first plant a terrarium it's easy to add too much moisture. Sometimes it takes a few days to figure out the right amount of moisture needed to create a self-sustaining environment for the plants. Although I didn't see any visible signs of fungus growing on the fresh potting soil, I soon had dozens of fungus gnats flying around the inside of my terrarium and getting stuck to the water droplets running down the sides of the glass. The whole project looked terrible and I was understandably horrified and mystified as to where the gnats came from. I ended up taking the terrarium outside to let the gnats escape. For several weeks after that it seemed like they kept right on hatching anyway. I guess they ran out of fungus, because eventually their reproduction cycled stopped and they all died.



About The Author: Ellen Brown is our Green Living and Gardening Expert. Click here to ask Ellen a question! Ellen Brown is an environmental writer and photographer and the owner of Sustainable Media, an environmental media company that specializes in helping businesses and organizations promote eco-friendly products and services. Contact her on the web at


Gnats in Potting Soil

I know you hate those awful bugs. Since you have already planted Spinach there is not much you can do. If you did not use all the bag of soil put the rest of the soil in the oven for an hour at 300 degrees. That will get rid of the gnats remaining in the soil. (10/12/2006)

By Ella

Gnats in Potting Soil

Try this, put a few drops of dish soap in a small dish with about a cup of water. Mix so that it suds, then pour on top of the soil. The soap kills the gnats and should not harm the plant if used lightly. (10/13/2006)

By Jess

Gnats in Potting Soil

I recently learned from a Master Gardener that all you have to do is to put a thin layer of clean sand on the top of your soil so the gnats can't get to the soil easily. Water drains through the sand, but the surface will dry out, inhibiting the gnats (who like the damp surface of the soil). Given the life cycle of the gnats this makes sense. Hope this helps.


By Chris


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