If flies are taking your houseplants by storm right now, here’s what’s worsening the problem – and how to keep them at bay

My biggest houseplant bugbear right now? Flies around my houseplants. Lots of them.

While I find myself wondering how to keep flies off my houseplants all year round, I’ve seen more of the critters than usual lately. As it turns out, there are two reasons for the explosion in numbers this December – and they're easily missed.

Here’s what's attracting so many flies to your houseplants this month – and a few expert-approved ways to keep them at bay.

1. The heating

I tend to see flies as a summer problem – that’s when I usually find myself looking for ways to get rid of fungus gnats – but this month, I’ve spotted more around my plants than usual.

Temperature-wise, we're having quite a mild December. It isn't freezing outside, but it’s still chilly enough to turn the heating on. That, sadly, could be the root of the problem.

‘These houseplant flies are known as fungus gnats, and they explode in numbers at this time of year because the cold outdoor weather forces us to turn up the central heating,’ explains Andy Little, houseplant buyer at British Garden Centres.

‘Warm homes with sealed windows trap the flies inside, so this, plus central heating, speeds up gnat lifecycles indoors.’

Those lifecycles are a force to be reckoned with when the heating’s on, but I just discovered a clever way of making things trickier for them on an Instagram reel by garden presenter Gardening with Ish. You’ll just need some horticultural grit, like this BetterGrow Coarse Horticultural Grit, which is £6.99 at Amazon.

‘Fungus gnats and little flies burrow in the soil and breed, and then emerge,’ Ish says in the video. ‘However, give them something they can’t dig through or emerge through, like horticultural grit, and they’ll stay in there and won’t cause you any bother.’

2. Overwatering

Overwatering your plants is never a good thing, but soggy soil and central heating? They’re a diabolical pair – and together, they're an absolute magnet for flies around your houseplants.

‘If your houseplant soil is too wet, this results in perfect conditions for the gnats to breed,’ warns Andy. ‘Overwatered soil attracts females that lay hundreds of eggs in the damp top layer, where larvae munch on fungi and roots.

‘Add in nutrient-packed compost that breaks down fast, poor pot drainage trapping moisture, and fallen leaves on the surface, and flies will thrive.’

Andy recommends letting the top inch or two of soil dry out completely between waterings. If you haven’t already added horticultural grit to the soil, it’s worth putting a layer of gravel or sand over the surface to prevent female flies from laying eggs.

Ish also suggests popping some fly papers around your plants to catch any straddlers. You can buy 30 fruit fly traps for £4.99 from Amazon.

So, central heating and overwatering are the two leading drivers of flies around your houseplants this December. Keep some horticultural grit handy!