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–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!