Hydrogen peroxide drench for fungus gnat larvae

Why this is optional, not a default step

A diluted hydrogen peroxide drench is a widely repeated home suggestion, and the basic mechanism — oxygen release disrupting soft-bodied larvae on contact — is plausible. But it isn't uniformly recommended across university extension sources the way Bti is: some describe it as a reasonable short-term option, others don't mention it as a first-line treatment at all, and none of it is a substitute for fixing the underlying moisture problem. Treat it as an optional add-on you choose deliberately, not a required step.

If you decide to use it

The dilution commonly described is 1 part regular 3% pharmacy hydrogen peroxide to 4 parts water (roughly 1 cup peroxide to 4 cups water). Higher-strength salon or food-grade peroxide is not the same product and is not a simple ratio conversion — don't substitute it.

Water the pot with the mix until the soil is evenly moist and a little drains, then let it drain fully and empty the saucer. This drench is a full watering, not an extra one — time it for whenever that pot was due for water anyway, and let the top of the mix dry out again before the next round, however long that takes for that specific plant.

Cautions

Spot-test on one plant first, especially delicate ferns and seedlings. Peroxide is a short-term contact treatment — it doesn't stay active in the mix the way Bti does, and repeated use may also affect beneficial soil microbes, which is part of why it isn't the default recommendation here.

Succulents, cacti, and other drought-adapted plants are already vulnerable to rot from excess moisture — a repeated liquid drench schedule is a poor fit for them regardless of what's in the water. If gnats are breeding in a succulent's mix, fixing drainage and watering frequency matters more than any drench.