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How to tackle those unwanted house plant flies

Image by Paul Gilbey

For those in the know, plants are hard work. Not that we hate them, quite the opposite (obviously!), but for a hobby that’s supposed to bring joy there sure is a lot of uphill struggles. Overwatering, under watering, mixing the wrong soil, not enough light, too little light - the battles to keep your little green pet alive are seemingly endless, nonetheless worth it.

But for our money, the worst aspect of caring for houseplants is the sometimes-unavoidable problem of little black flies appearing in your home. It’s not a rare thing either, with 72% of plant enthusiasts we surveyed admitting that at one time in the last 5 years they’ve had issues with flies making themselves at home. You’ll find them flying around your plants, or if you look closer at the soil, rummaging around over the compost. 

These little insects are sciarid flies/fungus gnats. For houseplants, they’re not actually an issue - they don’t eat the foliage, or cause any damage really, but they are a pest to have in the home, especially if you have a big indoor jungle, as their numbers can quickly increase. They tend to lay their larvae in the soil of your plants, which is where the problems can appear. Healthy house plants usually tolerate very small root damage, but seedlings and weaker plants need extra care, and the larvae can be quite troublesome. 

Where do they come from, and why?

If you’ve noticed flies in your home, the first step to take is always establishing where they’re coming from. Plants can provide a perfect breeding grown for flies, but so do stale water in sinks, uncovered alcohol bottles, and recycling bins, so before you blame your little Ficus Elastica, it’s always a good idea to get to the root of the issue with a little detective work.

Check the foliage/flowers, planter rims and soil for any insect activity. Usually if there is an issue you’ll notice black spots on the plant, as well as larvae in the soil (1-6cm on top, as files luckily don’t burrow all the way to the bottom of the top to lay eggs).

Fungus gnats tend to be relatively small (1/16”) and have thin, long bodies. They look a bit like a mosquito, but their movement isn’t as fast, they make no noise, and obviously don’t suck your blood. Most commonly, these flies tend to sneak in riding the leaves of newly bought plants, in a bag of soil, or sometimes they come through an open window.

Change up your watering style

Like the classic Pothos, pests like gnats are at home in wet, damp soil. After all, they need that moisture to help the eggs and larvae gain valuable nutrients so they can grow. If you notice that your soil is grown zero of your gnat infestations, then ask yourself ‘does this plant need to be watered as much as I’m doing it’?

The finger test is a classic method of gauging whether needs more water and is as simple as sticking your finger in the soil about 2 inches deep and seeing if it’s still moist. If it is, restructure your watering habits and try to give your plants a bit more time to take in the water. And hey, even if you don’t have fly issues, it’s a great rule to follow anyway as the number one cause of houseplant deaths is overwatering.

Short back and sides

Dead and dying foliage is gold to Fungus gnats (or should that be a 3-course meal)? As foliage starts to decompose, it releases nutrients and becomes easier to breakdown for insects. To avoid this issue, you need to trim your plants regularly. It can be hard cutting away something that you’ve paid for, but dead is dead.

Remove dying foliage, especially close to the soil, as the decaying waste is like a pheromone to flies. In turn, it’ll also help your plant flourish, as it provides more space for them to regrow.

Drainage not pain-age

Most plant pots for home use come with a tray underneath to catch excess water leakage from when you provide your plants with their weekly drink. But it’s always good to remember that the tray always needs cleaning, as water can get caught down there, resulting in a prime breeding grown for insects and mould. Even if the tray doesn’t look damp, over time it can collect moisture that results in mould or fungus.

Once again, it’s important to not water too much, and allow the water to evaporate and escape. If you’re finding that your plant just can’t seem to dry out, then you can always move them to a dryer, less humid room, or install a small humidifier to capture the excess water in the air. 

Gravel and sand - the perfect barrier

Anyone who has dealt with gnats’ eggs knows that they lay their eggs on the surface of the soil, or a few centimetres deep. Because of this, there’s a rather simple and genius way to stop the issue - cover the soil. Don’t worry, we’re not going to suggest clingfilm or sometime that ruins the aesthetics of your indoor jungle. Instead, you can cover your soil with gravel used for aquariums, or coarse sand.

Obviously, it’s a case of making sure the layer is tight and well packed, as you’re basically building a line of defence over your soil. If you have larvae in the soil already, it will die in a day or two. One of the main positives of this approach is that the stones and gravel add a to the look of the plant. 

If all else fails - clean

This isn’t the last stage of trying to fix the problem, nor is it a scorched earth approach, but if you’re looking for the quickest way to get to a resolution, or you feel the issue is out of hand, then you can go wrong with ‘starting again’.

The method involves removing the plant from its pot, and gently cleaning the roots of all its soil, making sure not to damage the roots as you do so. Remember, it’s the soil that’s the problem, and not the plant, so the only thing you should be disposing of is the infested soil. You’ll want to throw it out asap, and don’t look to compose or recycle it - unless you want an even bigger infestation.

Now that you’ve removed the soil, disinfect the pot with soap and water and the repot your plant with fresh soil (preferably not from the same bag as the last batch. A tried and tested technique that requires you to put a bit of work in, it almost always ensures that the problem goes immediately away.

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image by paul gilbey

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