Ask Dr. Phipps: Oh Gnats!

By Phipps Master Gardeners

Have a question about your perennials, houseplants or turf grass? Worried about pests in the garden, hydrangeas that won't bloom, or tomatoes that died on the vine? Dr. Phipps can help! Ask Dr. Phipps is a free service provided by Phipps Master Gardeners. Contact us with your questions and you may be featured in an upcoming blog post!

Q: I need to get fresh potting soil to repot my indoor plants to feed them and keep them healthy. They really don't need a bigger pot, but I've had bad experiences of buying bagged potting soil that is infested with gnats that infest my plants and house and are near impossible to get rid of. I don’t like to use herbicides or pesticides, especially in my house. So my plants need repotted, but I'm paranoid to buy new potting soil because I don't want to deal with a new gnat infestation problem. Can you recommend any bagged indoor potting soil that you've had good luck with that doesn't have a gnat infestation?

By the way, how often do you change potting soil for your indoor plants?

A: I've been experimenting with different potting soils myself. You can sterilize the potting soil (make sure it is moist!) by heating it in the microwave. Be sure to let it cool off before using. It will smell a bit after doing so, but will kill any pests that may be living in the soil. The soil that you have been buying is probably holding too much water, which is how you are getting and keeping the gnats. To help with your current problem, the Terro company makes great traps that can be purchased at Home Depot. 

Foxfarm produces the soil that I am currently using. I have also had success with Dr. Earth brand bagged soil. I have not used the Dr. Earth potting soil, but you could add it to your experiments!
 

I don't actually change my indoor plants' potting soil, I only pot them up into larger pots when needed. If you are worried about the amount of nutrients available to your plants, you can add a topdressing of worm castings. You can usually find bags of worm castings for sale at the East End Food Co-Op, and local nurseries or garden supply stores may have some for sale as well. Here is some information about worm castings from the University of California.

If you are having a current problem with gnats, you can also add a topping of sand about 1" deep to keep the gnats from laying more eggs in your damp soil. Before trying any other means of control, be sure to read this information about fungus gnats so that you understand how and why they become a problem.   

Hope all of this information helps!  Good luck finding potting soil that suits you!