- Don’t overwater –Excessive watering can lead to the growth of algae in saucers and on the surface of the soil which fungus gnats will feed on. This is particularly important overwinter, when the irrigation requirement from indoor plants is much lower than other seasons. Overwatering can also lead to weakness or rot in plant roots, making them “soft targets” for fungus gnat larvae.
As they say in the classics, prevention is always better than a cure, here are some tips to avoid a full-on “Gnat Attack”.
Most house guests are welcome, especially those that love and admire our indoor plants as much as we do, but there is one group of gate crashers that every gardener dreads. Fungus gnats (Bradysia sp.) are often first noticed in our houses or greenhouses in their adult form – small (about 3mm long) black to dark grey mosquito-like flies hovering in small groups around our beloved indoor plants. And while the adults can be an annoyance, it is the tiny (less than 5mm), black-headed, soil-dwelling larvae that can do some serious damage to the roots and stems of a huge range of indoor plants, chomping through the base of cuttings, soft stems and roots with a voracious appetite.
Fungus Gnats are often introduced into our homes in newly-purchased plants or even potting mixes and organic matter, so it’s a good idea to inspect plants before you bring them home. Give the potting mix around the base and stem of the plant a gentle turn in a couple of spots keeping an eye out for any of the tell-tale clear, shiny larvae (maggot sized). If larvae are spotted, it’s best to give these plants a miss.
If you haven’t noticed the larvae in the soil, or the adults buzzing around but you think there may be gnats in your plants, before you undertake unnecessary treatment, there’s an easy gnat test that anyone can do – cut a slice of raw potato and leave on the surface of those pots potentially harbouring these houseguests. Leave overnight and check for the presence of the larvae on the potato the next morning – gnats are attracted to starch and cannot resist this tasty treat!
- Soil Drench – Apply a Neem-Oil based soil drench to the affected pots at a rate of 30ml per 10L of water, using about 1L of the mixture per 25cm pot. Repeat weekly for three weeks.
- Gnat-loving Nematodes– Just as we dislike the fungus gnats, there is a particular strain of entomopathogenic (insect killing) nematode that just adore them and will make short work of an infestation at your place. Available at some nurseries commercially, these nematodes are applied as a soil drench, and a standard sachet will treat 24 x 25cm pots. Two treatments, a week apart, is all that is generally required to nip your gnat problem in the bud, and these nematodes won’t harm pets, plants, people or beneficial bugs and bacteria.
Fungus gnats love rich, damp soils and given the right conditions, fungus gnats can complete their lifecycle from egg to adult in about three weeks, with female adults capable of laying between 100 – 300 eggs at once. Because of this, it is possible that your pot plant may indeed be playing host to overlapping generations of Fungus Gnats at any given time, meaning swift control is required to prevent serious infestations. Their lifecycles can be easily interrupted and broken by allowing at least the top 10cm – 15cm of potting mix to dry out between waterings.
- Sticky Traps – While store bought yellow sticky traps will work to attract and trap the adult gnats, why not fight gnats with nature, and pop a couple of carnivorous Butterworts (Pinguicula sp.) or Sundews (Drosera sp.) around the house. These gorgeous plants are the living equivalent of fly paper, and use their sticky leaves to lure, trap and digest insects, including fungus gnats.
- Mulch your pots – A layer of inorganic mulch like aquarium stones or small pea gravel across the surface of the pots is an effective way to both discourage infestation and break the lifecycle of existing fungus gnats. This mulch layer prevents the females being able to lay eggs in the soil, and the adults from emerging once mature.
Fungus Gnats are the bane of many gardeners with indoor plants, especially over the cooler winter months. Jane gives us some tips on how to avoid a gnat attack at your place, and what to do if you already have these unwelcome house guests
Jane shares her favourite tips and techniques for getting rid of an unwelcome house guest, fungus gnats
Therefore, keeping new or infected plants in quarantine can be very effective at reducing the spread of fungus gnats to other potted plants. That way, you can focus your treatment on just one plant instead of them all! For severe infestations, some folks may choose to completely remove the infected plant from the house.
Houseplants bring a dazzling, outdoorsy, peaceful energy to any home! They also help to cleanse your space by purifying the air around them. However, while you may be eager to create a jungle-esque vibe inside, you probably aren’t looking for a full immersion experience – with bugs flying all over your house! Unfortunately, fungus gnats are a fairly common problem with house plants. The good news is – it’s easy to get rid of fungus gnats, once you know the tricks!
After potting your houseplant, consider adding a layer of horticultural sand (not play sand!) to the top of the soil. You can water the plant through the sand, and meanwhile it will deter fungus gnats from laying eggs in the pot. In addition to sand, there are other soil-topping products like this one (made from crushed recycled glass) that are specially made to eliminate fungus gnats from your potted plants.
When shopping for neem oil, I always suggest choosing a high quality cold-pressed pure neem oil over pre-mixed products that contain a lot of other additives. In order to fully mix neem oil with water for an even and effective application, the neem oil will need to be emulsified first – because oil and water don’t easily mix. Check out our article all about properly mixing and using neem oil in the garden, or, for houseplants!
In a similar method to hydrogen peroxide or Bti, a neem oil drench can also be applied to the infected soil to kill fungus gnats. Do not use straight concentrated neem oil though! Create and apply a dilute neem oil solution by following the manufacturer’s instructions on the neem oil product you select. Watering with a dilute neem oil solution can help to kill fungus gnats, and also repel them in the future.
To apply Bti, follow the instructions for a “soil drench” on the product that you purchase. It is usually recommended to water the plant with the solution, since only spraying the surface of the soil may not penetrate deep enough to kill all of the fungus gnat larvae. Repeat as needed, following the instructions. One popular option is this Gnatrol brand. Get it? Natural gnat control…
Another good indicator of moisture control is the drainage tray or trough at the bottom of the pot. This should be dry within a day of watering the houseplant, and not have standing water. If it does, you may be overwatering! A quick-filling drainage tray could also be a sign that the plant’s soil has poor water retention, is root-bound, or that water may be running around the sides of the root ball and soil – rather than seeping through. To read more about houseplant care tips for watering, soil, fertilizing and more, check out this article – “Houseplant Care 101: The Ultimate Guide to Happy & Healthy Indoor Plants”.
Fungus gnats reproduce by laying eggs in the top couple inches of damp soil. The eggs hatch into larvae, the larvae feed on organic matter within the soil for 2 weeks, and then they pupate. A couple of days later, the adult flies emerge and start buzzing around. You’ll commonly see fungus gnats hanging out on the soil surface, edge of the pot, or maybe around the drainage holes of the pot. They most often only fly in short bursts, and otherwise crawl around.
Fungus gnats are drawn to light, as well as the color yellow. Use yellow sticky traps near, hanging from, or inside the potted plant to catch adult flies. These sticky traps on stakes are designed especially for potted plants! We also hang these larger ones to catch gnats and other flying pests in our greenhouse.
Read along to learn 5 easy and non-toxic ways to get rid of fungus gnats in your houseplants, plus a few tips on how to prevent them in the first place!