Fungus Gnats. Those tiny little flies that zoom out of your plants and get all up in your face time and time again. They look like fruit flies, and unfortunately, they are par for the course of owning indoor plants.
They live in the soil of your plant where humidity and moisture are in abundance. So not overwatering becomes more important than ever (especially in Winter) but it won’t solve a fungus gnat problem long term.
Fungus Gnats are the most common houseplant pest, and although the adult fly won’t cause any damage to you or your plant, man is they annoying, and they can spread like wildfire if you don’t keep them in check!
What can become a problem is the Fungus Gnats young larvae that are buried in the soil and can feed on the fine root hairs of your plant, causing slow growth and even yellow leaves. Their optimal conditions for the gnats to lay their eggs in moist soil which is why you hear us bang on about allowing your pot soil to dry out completely a couple of inches between watering. This way the larvae don’t have enough fungi, algae or decaying matter to feed on.
There are loads of natural treatments for Fungus Gnats such as leaving trays of vinegar around, cinnamon sprinkled on the soil and pot rim, spraying with neem or white oil or yellow sticky traps, all of which we have listed in this handy article here. And while these are quite successful at killing the adult flies, the problem will not go away unless you treat both the larvae and the adult flies.
And cos we are all about keeping things au naturale here at Green Assembly we only want to promote methods that are safe to have in your home or office. Enter diatomaceous earth. Specifically, food-grade diatomaceous earth (DE) is mineralised fossil dust that is both natural and non-toxic to the environment (as opposed to pool grade which is not pure enough to use around food or pets).
How does it work?
DE has microscopic shards of silica (a hard, unreactive, colourless compound that occurs as the mineral quartz) that will rip those tiny Fungus Gnats larvae to shreds as they crawl through it (plus any other insect that tries to walk through it!).
Plus, it will also kill other common houseplant pests like mealy bugs, spider mites or aphids!
How do you use it on indoor plants?
Literally, sprinkle a layer of the powder over the topsoil of your pot plant. Ideally, add when the soil’s surface is dry to the touch—but before your plant is in dire need of watering (soaking up water will make it ineffective).
The best bit is that it is 100% safe to use in the home around food and pets (as long as you get the food-grade version). But we do recommend using gloves and a mask (or a scarf or similar tied around your mouth and nose) to prevent the tiny particles from getting into your lungs cos like any fine dust it may cause you to sneeze or cough.
If your fungus gnat problem is kinda out of control, we also recommend doubling your treatment method with diluted neem oil (from any good nursery) the next time your plants need watering so that it soaks right the way through the soil.
If you have been trying these methods for months but still seem to be losing a grip on them (and your sanity!), then you can go the hardcore method and have a go with some common fly spay. Spray any flies that you can see buzzing and spray the top layer of soil every few days. We don’t love this method cos God knows what is in those cans of death, but desperate times call for desperate measures!