Fungus Gnat Controls–Or Why You Want To Stay Away From “Moisture Control” Soils

On Friday I talked about the problem I was having with fungus gnats–not because I don’t know how to water house plants properly, but rather because one of these “new-fangled” moisture control soils wasn’t allowing the soil to ever dry properly so I could completely get rid of the pesky critters.   And I’ve discovered that it’s not just me having a problem with this “moisture control” soils.  A quick “Google” search reveals hundreds of entries entreating folks never to buy the stuff (some of them naming brand names) and relating just exactly the same sort of experiences that I’ve had–bags infested with fungus gnats.

Take my advice–stay away from the stuff.  It’s really bad news!

To recap a bit, fungus gnats are those little gnat-like flying creatures that burrow into houseplant soil.  The problem isn’t really the gnats themselves, unless you have a severe infestation (and right now, I pretty much do.  They’re dive bombing plates of food and drinks, and hovering around in the shower and when I wash my face.  It’s just yucky.)  But otherwise, they pretty much leave people alone.

The real harm is what they do to the plants.  Why they’re always hanging out around plants is because they burrow into the soil and lay eggs.  The eggs hatch into larva and the larva feed on roots.  With a severe enough infestation, they can compromise the health of the plants–never mind get annoying.

Fortunately there’s a lovely organic remedy that consists of a BT soil drench.  I remembered it from years ago when this happened in an office setting and one of my office mates was drenching the plants with Sevin™ in an effort to kill the critters.  Fortunately I found something called Knock Out Gnats™ before she killed us with the Sevin™ (and before anyone tries it–the Sevin™ had no effect on the gnats).

There’s also a product called Gnatrol™ which is a similar product by a different manufacturer.

These products, which must saturate the soil, do nothing to control the annoying flying adults.  They instead kill the larva which is the stage of the pest that does the most damage anyway.

According to most conventional wisdom, 3 treatments will take care of the problem.  I’ve done 4 treatments so far and nada.  I really believe that the soil is wreaking havoc.

But winter is no time to repot and if the plants are already stressed from insect infestation and now insecticide treatment (organic though it may be) I really don’t want to further stress them by repotting.  So I’ll continue to treat, to let dry as much as possible and to hope for the best.

Interestingly enough, when I ordered my most recent container of Knock out Gnats™ the instructions were for 100 gallon quantities.  Now, I’ve got a lot of house plants, but even I don’t mix up quantities like that–I might use 6 gallons in a given watering!  It took a lot of math to convert those instructions for home use.

I went online to see if anyone else had saved me the trouble, but no–only one other soul, looking for the same thing.  As she said, she wasn’t trying to treat a ranch, just a few plants.  Someone’s got to get with the program on these instructions!

4 thoughts on “Fungus Gnat Controls–Or Why You Want To Stay Away From “Moisture Control” Soils

  1. John February 7, 2013 / 6:59 am

    Try giving Beneficial Nematodes a try to control the fungus gnat larvae. A single application should give noticable improvement. many suppliers,and while not the cheapest it is better than numerous applications of the alternatives which (as you are finding with BT) don’t always work anyway.

    • gardendaze February 7, 2013 / 8:19 am

      Thanks so much for reading and commenting. The Organic Gardening article lead me to believe that the nematodes weren’t worth a try, paticularly with the moisture control soil issues I’m having. It said that they often die off and do poorly in wet conditions–exactly my dilemma. But of course, I have little to lose by trying them at this point, now do I?

      If they do work, I’ll post a follow-up. Thanks again!


  2. John February 8, 2013 / 5:47 pm

    Good luck Karla,
    They actually like a moist soil as they need the water to survive. Drying out the soil totally will stress the plant anyway as it has become used to the moisture, and will need a gradual adjustment to less watering.
    Nematodes do die off, but that is when there are no more larvae left to infest and breed inside of. When you get a pack to try, split it in two and keep half to apply approx 2-3 weeks later. Use yellow sticky traps (and/or other options) to catch the adults as the adults will continue to breed if you don’t control them also.
    Some of the organic articles either arent aware of nematodes, or haven’t tried them, and seem to always be looking for a cheap wonder cure from the cupboard that (in my opinion) often are more hassle and can be very repetitive for bad infestations. Rather spend the $20 on nematodes – the more in a pack the better
    The nematodes are used by professional nurseries etc, so are not a snake oil product. They just aren’t widely known.
    Good luck again.

    • gardendaze February 8, 2013 / 5:54 pm

      Thanks again!

      I will certainly agree that most pest experts know very little about plants. At this point, my poor plants are so stressed, the nematodes couldn’t do anymore harm. And $20 is surely a reasonable cost to save plants, some of which I’ve had 35 years!


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