Fungus gnats seem harmless enough.  That is until your garden starts are thoroughly infested.  The carnage begins to look like a gardener’s version of a battlefield.

This spring, we experienced this worst-case scenario.  Seedlings imploded overnight with their root systems completely wiped out.  And after attempting multiple solutions with no success, we were afraid we would have to scrap our entire crop of seedlings and start over.  Losing a month of our precious growing season here in the far north is not a good solution, so we kept experimenting until we finally remembered something that had worked years ago for us.  And it worked again!

How It Started

Life was very busy this spring and it seemed easier to just buy some potting soil rather than make our own (as we usually do).  This was mistake # 1.

Our second mistake was that we purchased Black Gold potting soil.  We had experienced issues with this brand in the past and sadly, we didn’t learn from our mistake.  They were pretty much the only organic potting soil we could easily put our hands-on, so we went with it.

Third mistake…we didn’t sterilize our potting soil.  Once again, we were short on time.  Normally, we make our own unsterilized potting soil and have not had issues so far.  With homegrown goods, we prefer to not sterilize.  As long as it is healthy soil, there should be plenty of good microbes that will be beneficial and hold the bad guys in check.  But when purchasing commercial potting soil, you have no idea what you are getting.  So it’s better to be safe and sterilize.  For sterilizing, I’d suggest a heat treatment since there are no chemicals involved.  Here’s an article from the University of Colorado Extension on how to do this.

What We Tried

First, we tried an old standby—diatomaceous earth.  This remedy helped to knock back the little gremlins for a time, but sadly, the infestation was too serious for it to bring an end to the onslaught.  In the past, it has worked reasonably well if used early on.

Our next try was a solution of hydrogen peroxide, which was supposed to kill the fungus gnat larvae.  We used a 1:3 ratio (1 part H2O2 to 3 parts water).  Perhaps if we had caught the infestation earlier this would have helped over a period of weeks, but we didn’t notice much difference.

Then we tried Neem Oil.  This is one of the most common natural treatments suggested for fungus flies.  But strangely enough, our hardy crop of winged visitors acted like the Neem Oil was candy.  After spending a week or two emptying a $40 container on our starts and the soil, their population seemed to explode.  Whether the Neem Oil aided them or not, I can’t say.  But it certainly didn’t seem to hinder their procreation.

Finally, for good measure, we tried a solution of insecticidal soap.  Once again, perhaps this would have helped if we had used it early on and been able to apply it over several weeks, but for our acute situation, we didn’t notice any effect except more and more plants dying.

The Sand Method Gets Rid Of Fungus Gnats!

At this point, we thought we were going to have to scrap our entire crop of seedlings and start over, losing more than a month of time, and over 5 trays of seedlings.  This would have been a big hit in our northern climate.

Then I remembered a method I had used years ago with success and then forgotten about.  It involved using horticultural or builder's sand.  Not the fine sand that you'd find on a beach or in a children’s sandbox.  These are larger particles.

In a nutshell, you create a good layer of sand on top of the soil and below the bottom of the pots (if there are holes in them).  While this doesn’t chemically kill the larvae, it breaks the cycle of the flies/gnats burrowing into the ground and laying their eggs.  And it creates conditions that make it extremely difficult for the fungus gnats to survive.

Here’s how to do it:

The Results

Simply amazing!  We noticed a dramatic difference within a day or two.  Any unsanded plants were quickly inundated with the fungus gnats, as they couldn’t get into their old hosts.  So remember to sand everything, and I mean, absolutely every bit of soil in reach. We also sanded all our house plants.

Within a week, our worst-ever infestation of fungus gnats was over.  With a bang!

Don’t you love it when there’s a super simple and completely non-toxic solution that doesn’t harm the plant and works oh so well?!  That’s what we’ve found, and we wanted to pass it on to you.

Please do yourself a big favor and practice an once of prevention by avoiding the 3 mistakes we made (at the beginning of this post).  Don't buy Black Gold organic potting soil, make your own soil, and if you do buy soil, be sure to sterilize it. And if you find yourself in a hand-to-hand battle with the entomological world, whip out your secret sandy weapon and save yourself a lot of grief!

Happy gardening!