Gardening Indoors 5: Gnatty Problem

Houseplants are a great addition to the decorating scheme.

They're baaaaaaack. (Violin going "screeech screeech screeech")

Those naughty gnats have returned. Fortunately, not in the numbers they had before. I have a plan; I am prepared.

After we had our microgreens going for a month or two, we noticed a proliferation of tiny black fly-things swooping up from the soil every time we cut microgreens. I'd seen these things before, hovering about the houseplants, but never in large numbers, so I wasn't ever concerned about them. However, they came up almost in clouds out of the microgreens.

I thought I knew what they were, but still I pulled out my organic gardening reference book, and did some online searches.

Yep, I was right. Fungus gnats.

Most of the info I found indicated that fungus gnats weren't a problem, although some said that they could nibble on plant roots, etc. But none of the information I found caused me concern. The fungus gnats were an annoyance, nothing more (so I thought, trying to avoid taking any action).

However, some of the microgreens started falling over, or not germinating at all. Damping off disease. (See my post from 2/4/19) The gnats could be transmitting damping off disease while hopping from one container to another. On top of that the weekly Horticulture Newsletter from Kansas State University Extension said that fungus gnats do cause damage. They won't really do harm to my mature houseplants, partly because they're so tiny and the plants are big. Because of the conditions the fungus gnats require, they don't proliferate to extremes in the larger pots, either.

But they are an issue for tiny, vulnerable seedlings, like my microgreens, and now my seedling transplants (see last post). And it was in the microgreens where they proliferated most.

Because... fungus gnats need moist soil conducive to fungal growth (which they eat as larvae and maybe as adults). One way to keep them under control in regular size houseplants is to always let at least the top two inches of soil dry out between waterings. Easy enough. You can't do that with microgreens, though, which have only an inch of soil to begin with. If I tried letting it dry out completely I'd end up with a bunch of withered seedlings -- damping off or no damping off, fungus gnats or no fungus gnats.

Drastic measures were needed. So I harvested all the microgreens and removed the soil-filled trays, planting no more microgreens. I purchased Mosquito Bits (like Bacon Bits that are not made of actual bacon, they do not contain actual mosquitoes). The Mosquito Bits contain a biologic toxin Bt, but not the same variation of Bt that I spray on my cabbages against the various butterfly and moth larvae that eat them. MB contains Bacillus thuringiensis var. israelensis, which not only kills mosquito larvae but also fungus gnat larvae.

Other products with Bt-i are Gnatrol and Knock-Out Gnats. The Gnatrol I found only in large quantities. It took some searching to find it available in anything less than a 16-pound bucket for more than $400. I had trouble finding Knock-Out Gnats. The Mosquito Bits were $19 for 30 ounces, which should last a little while. You might even be able to find them at a hardware store.

So I put the MB on the soil in my larger potted plants and watered to release the Bt. I didn't want to put it on the microgreens soil because they are only a couple of inches tall when you harvest, and so close to the soil. Bt is pretty safe as insecticides go, but I'd rather keep the greens clean. So we had no microgreens for two weeks. The gnats disappeared. The microgreen operation started back up. We started seeing a gnat here and there, so I reapplied the Mosquito Bits, and I keep a spray bottle of Safer Insecticidal Soap in the plant room. Whenever we see a gnat doing an aerial display we grab the bottle of soap and squirt, praying we actually hit it. The soap only works on contact when it's wet. Once it dries, no luck.

The gnats disappear quickly, so we can only guess where to spray. We coat the leaves in the area we saw the gnat to increase our odds. We don't spray the microgreens or the baby cabbages because insecticidal soap can damage seedlings.

The tactic is working, but requires diligence and repeat applications.

Other products also help to prevent fungus gnats. One product is essentially ground up glass that you spread in a half-inch or so layer over the entire soil surface creating a barrier to the gnats so they can't lay eggs or eat roots. Pretty much anything that creates such a barrier would work. My first reaction to the glass was "no way." But I would maybe consider it for a larger potted plant that won't be repotted often. However, it's not cheap. So I'll stick with my current program.

Fungus gnats. There are worse pests to have and shouldn't be an issue with ordinary houseplants. But if you've got microgreens or are starting transplants, don't ignore them. Don't be like me.