Flies on indoor herb plants

[MrsCordial]

Hello. This is my first year growing herbs on the windowsill. I have basil and coriander. They've done really well and we're eating our own body weight in herbs every day, but we seem to have tiny flies breeding on the plants. They're driving my husband mad to the point where he's asked me to put all the plants outside today! His theory is that the nippy start will kill or discourage the flies.

Does anyone know why this has happened and how I can kill the flies and prevent it happening again next year? Thanks.

[WendyC]

Are they on the plants or in the compost? If they are a nuisance to you I think they are compost gnats or sciarid flies. They like damp compost so you could try letting the compost dry out more between watering. I have had some success with plastic milk bottle tops filled with almond oil placed on the compost. The flies obligingly drown themselves in the sweet oil. I find the peat free composts are worse for flies so this is one time I use a peat based one.


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[MrsCordial]

Thanks Wendy. I'm pretty sure they're in the compost, so what you say makes sense. Likely I do overwater, so I'll hold off for a bit. I'm bound to have almond oil somewhere so I'll try that too.

[teakdesk]

I agree with Wendy; try and water from the bottom of the pot only and cover the soil with a fine gravel.

I keep one of those battery fly swatters to hand!!!

[WilliamD]

They're called fungus gnats and you can beat them. Good advice above and from the RHS here.

[snakeshack]

I grow sundews and butterworts on the windowsill, the flies end up as free fertilizer


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[Two_Sheds]

Likely I do overwater

Yep, that's why you've got them.
Your basil isn't going to live much longer so I wouldn't bother repotting them, but if it was earlier in the year I'd say repot them in clean compost, washing the roots well to get rid of any eggs/larvae.

Then, don't water the soil surface, water by putting the plant pot in a saucer of water for a couple of minutes. The soil surface should remain pretty dry