Flying Dive Bombers–aka Fungus Gnats

It has been decades since I’ve had fungus gnats in any serious way.  Yes, anyone who has brought home a plant potted in peat (and thank goodness they don’t do that much anymore–has anyone else noticed that most house plants seem to be potted in coir now?) has most likely had an issue with fungus gnats at one time or another.


They look like regular gnats, except that they are abnormally attracted to your plants, and every time you water, you find the gnats crawling out of your potting soil.  Ick.


fungus gnat example




[Fungus gnat example–photo coutesy of Wikipedia creative commons]

It used to be an easy thing to solve–let the plants dry to the point of wilting between watering and the gnats would die off and disappear.  Not so much anymore and I’ll explain why in a moment.


First, I’ll say that I am NOT an over-water-er.  Anyone  who has well over 100 house plants can’t be.  I’d never have time for anything else.

Next, I’ll tell you that once I noticed the infestation, the first thing I did was deliberately let the plants get even drier than they would normally.  I have plants dropping leaves all over the place from drought stress.

Has any of this helped?  Not a bit.  Even my 9 day vacation out of state, with no one watering the plants, didn’t solve the problem.  It was then that I realized no matter what I did,  it was my soil that was the problem and I was going to have to bring in a soil drench (more about that fiasco on Monday).

How I came to get myself in these straits is I used a potting soil I normally would never have used.  I purchased it for a client that was having issues with watering and then they managed to resolve their issues so I just kept the bag for myself on the theory that there’s no such thing as “too much potting soil.”

That may be the case, but in this case, there’s certainly the wrong kind of potting soil.  I’m so glad I didn’t give this to my client!

As I said, I began to suspect the soil when, even after a 9 day drought, I was still over-run with the nasty little creatures.

A visit to my basement potting shed confirmed my worst fears.

I pulled open the bag of potting soil–which was completely dry, by the way.  Had I wanted to plant anything in it, I would have had to re-wet it.  And the number of fungus gnats crawling all over the soil and the bag were alarming.  There were dozens, if not hundreds!  In a bag of completely dry soil!

How can this be?  This is one of those soils with the polymers to prevent over-watering or under-watering.  It’s not something I would ever normally use myself because it’s not organic, and, quite frankly, after 40 years of growing house plants, I’ve learned how to water properly.

Worse yet, I have another un-opened bag yet to use.  Fortunately, it’s outside, in my garage.  With any luck, the little devils have frozen to death.

Let this be a caution to you though: those “moisture control” soils sometimes bless you with all sorts of unanticipated consequences!

6 thoughts on “Flying Dive Bombers–aka Fungus Gnats

  1. Laurrie February 1, 2013 / 8:47 am

    I brought in a big Rex begonia from the unheated porch this winter and put it in my bedroom, only to find the **%# gnats everywhere. And I keep the begonia very dry between waterings. I found that placing Bounce dryer sheets on top of the soil got rid of them. Gone now. That was easy.

    • gardendaze February 1, 2013 / 9:04 am

      Hi Donna,
      I always have too–that’s why it’s been decades in between my infestations. The only reason I used this nasty stuff is that I had bought it for a client. I’ve never used it before and I will surely never use it again and I’ll never recommend it. If I’d had any sense I just would have returned it when the client didn’t need it. I thought I was saving time. Penny wise and pound foolish, as they say.

      Thanks for reading.


  2. gardendaze February 1, 2013 / 9:01 am

    Wow, Laurrie, great tip. I’ve heard that covering the soil with something–I’ve heard saran or other plastic wraps but that seemed counter-intuitive since that would hold in moisture–, and I’ve also heard foil would work. I also know that a layer of sand on the soil seems to help because the nasty little gnats won’t burrow down.

    You’ll see how I solved the problem with the bag of soil next week–after reading tips from the “so-called” experts and using a soil drench for going on 6 weeks now. It’s getting better but I’m not there yet!

    I’ll have to get out the dryer sheets.


  3. Sue February 1, 2013 / 1:25 pm

    We had to get rid of every plant in our office a year or so ago because of fungus gnat infestations. I have them in the house from using leftover bagged soil from the outdoor containers but so far they haven’t presented too much of a problem. Next winter when I repot things to take in I’ll use something organic. I’ll have to give the dryer sheets a try.

  4. gardendaze February 1, 2013 / 1:40 pm

    I had a similar office situation decades ago–I think I post about it on Monday. That’s really been my only other experience with these pesky critters. A colleague tried pouring straight Sevin into the soil as a drench (without consulting me, of course) and even that didn’t help! We’re lucky we all weren’t poisoned!

    At that time, I managed to get rid of them with a BT soil drench. But this nasty moisture control soil is really working against me here. And of course the suggestions from the organic types that I just “repot” the plants would be fine if I didn’t have over 100 of them. That’s just not practical, especially when some of them are in 18″ and 20″ pots.

    I definitely like the Bounce idea. Again, it probably won’t work with all 100 pots, but it’s going to work for the most badly infested ones for sure!

    Thanks for reading.


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