Potting Soil, Fungus Gnats and Tiny Spiders

Whenever I lecture on house plants, one of the things that I am sure to talk about is the various moisture control soils on the market. It can be difficult to find container soil without the polymers for moisture control that are almost always added.

If you are gardening in containers that will be used strictly outdoors, it’s fine to use a moisture control soil (unless you want to adhere to strictly organic principles–in which case, as you might surmise, anything called “polymers ” would not be organic).

But there are several reasons not to use these soils indoors, no matter how good an idea it seems at the time.

The most important reason is fungus gnats, which seem to be getting more prevalent all the time. Outdoors, they’re not a problem because “good ” bugs like earwigs and ground beetles and those cute little roly-poly bugs (technically called woodlice or pillbugs) eat their eggs and larva and prevent them from getting out of hand.

But indoors? Well, there are no natural predators so they get out of control. And the types of remedies like repotting the infested plant quickly becomes impractical if your whole collection is infested.

There is an organic remedy that consists of a soil drench made from bt, but having had to do that once, I can tell you that it’s also impractical, takes months to kill the eggs and larva and will kill some of the plants too. So it’s not a perfect remedy.

Fungus gnats on windowsill, the victim of a helpful spider

So here’s where allowing tiny spiders to do their work is useful. Yes, it’s a little gross to clean up after them. But really, is it any worse than having fungus gnats flying all over your house? Because all of these would be flying in my house if the spiders weren’t killing them.

Worse yet, they could be infesting my plants, and I would have to do that nasty, stinky soil drench where I would have to let pails of water sit for 24 hours to dechlorinate, then I would have to mix up the bt drench and water with it. Yuck. The spiders are so much more helpful.

And yes, allowing the plants to dry as much as possible between watering helps too. But in some instances, when plants like to be kept evenly moist, that’s not possible.

So small spiders it is–and I am grateful.

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