Fungus Gnat Remedies That Don't Work

"Just let the soil dry out between waterings"

This one does help, and it is the most consistently recommended cultural control: a dry top 1–3 inches is less attractive to egg-laying females, and eggs laid there fail to hatch. But it did not end an active infestation for me. Kansas State notes that larvae survived best in growing media at 52% moisture, which is far from soaking wet, and that letting plants get too dry isn't workable when they need water to grow. It is also no good for seedlings and cuttings that need steady moisture, or for living soil. And larvae can be found right down at the drainage holes, where the mix stays wet longest.

"Add a layer of sand on top"

Kansas State University reports that not even a 3.1 mm (about 1/8 inch) layer of sand stopped adults from emerging or females from laying eggs. The layer has small gaps and cracks that let larvae pupate underneath and let females lay eggs through it. In my pots it made no difference.

"Use diatomaceous earth"

Studies by Raymond Cloyd's group at Kansas State University tested diatomaceous earth (DE) both mixed into growing media and spread on the surface. Neither had an effect on fungus gnats, and over time the DE layer cracked, which gave females places to lay eggs. Kansas State does not recommend it. It is still all over gardening forums.

"Sticky traps will solve it"

Yellow sticky traps are excellent monitoring tools, but they only catch adult gnats, and on their own they won't significantly reduce a population. A female can lay 100–200 eggs in her short life, and plenty get laid before she ever hits a trap. You will catch adults indefinitely while the larval population continues to grow in the soil. Keep using them, because they tell you whether your treatment is working. They just can't fix the problem on their own.

"Mosquito Dunks/Bits in the watering can"

Mosquito Dunks and Bits do contain BTI (Bacillus thuringiensis subspecies israelensis), which is the correct active ingredient. Dunks are made for standing water like bird baths and rain barrels and are built to release BTI slowly, over 30 days or more, so soaking one in a watering can is a slow way to get BTI into soil. Bits are now labeled for fungus gnats: the label says to soak 4 tablespoons in a gallon of water for 30 minutes, skim off the granules, drench, and repeat weekly for three weeks. That's the same weekly schedule as the concentrates. In my pots, Dunks and Bits kept things from getting worse but never ended an established infestation. A liquid concentrate was what finally did it.

They are also a pain to use. Every watering means measuring out granules, waiting half an hour, and fishing soggy corn cob bits out of the can, three weeks in a row. Dunks have to be broken up and left to soak, and you still don't know how much BTI ended up in the water. A concentrate is a measured teaspoon stirred into a gallon. When a treatment is this fiddly, people skip a week, and skipping a week is how the next generation gets through.

"Coconut coir resists fungus gnats"

Coir has been promoted as a medium that holds gnats back, but research has shown that fungus gnats survive and reproduce just as well in coir-based media as in peat-based media. Coir has many legitimate benefits as a growing medium, but resistance to fungus gnats is not among them.

"Apple cider vinegar traps"

Vinegar traps are the standard trap for fruit flies (Drosophila), which are drawn to vinegar and other fermented liquids. If your vinegar trap is full of flies, check whether they are fruit flies rather than (or as well as) fungus gnats. Either way, a vinegar trap does nothing about larvae in the soil. The standard monitoring tool for fungus gnats is yellow sticky cards placed horizontally at the soil surface.

"Hydrogen peroxide kills everything"

The usual home recipe is 1 part 3% hydrogen peroxide to 4 parts water. Peroxide is a strong oxidizer that kills a lot of organisms, which is the problem: it doesn't pick out gnat larvae, so it is a poor fit for living soil or for pots where you've added beneficial nematodes. I couldn't find a university trial showing it controls fungus gnats, and extension advice is to try gentler controls first and to keep it off seedlings. It also does nothing about the eggs laid after you pour it. For me it was a one-off hit, not a fix.