Compost infested with flies

[Fettyheather]

This may get the award for daftest question!

I started planting seeds in pots today using john innes compost from an opened bag stored in the garage from last year. There are lots of tiny black flies in it.

I'm not sure whether it will be ok to use still? Any advice gratefully received, I'm a novice.

I had planned to plantsome for window sills but obviously not with this compost, its really put me off!

They are probably fungus gnats. The pesky things lay their eggs in damp compost, and the eggs lie dormant in cold weather, then hatch out when it is warm. Sorry to break it to you but they are a nightmare, as the larvae eat the roots of seedlings and the whole life cycle keeps repeating itself.

I know all this because I made exactly the same mistake as you and had to throw the seedlings away. I'd recommend you throw the lot away double quick and buy some new seed compost, which in theory should be sterile. I dumped the infested compost at the back of our garden, under trees and bushes that tend to have gnats there in warm weather anyway. The cold nights will kill most off, and the birds should eat the rest.

Thanks for that urban. I did wonder but am loathe to waste anything.
I waa given a selection of seeds for my birthday so would like to give them the best chance of taking.

I do have another unopened bag but from reading around that they sometimes get in through the tiny holes, I will have to check that bag carefully too I suppose!

If not guess I'll be off to the garden centre!

Thank you again.

I'm the same, it seems silly to throw away unused compost, but honestly the hassle of a bad infestation and ruined seedlings is not worth it. Also, if you're in the UK the nights are still very cold, so I guess you probably have the pots/trays indoors to germinate? If so, bear in mind that one gnat can lay up to 200 eggs and the life cycle is very fast - you could have a plague indoors in a week or two! I learned the hard way.

The larvae will be hatching out into flies. In theory, if you can stop them laying into the compost, you’d have no problem, but that’s difficult to achieve. Household fly killer will kill the flies, but you’ll have to reapply when more emerge. Then you could use it on outside pots, adding slow release compost if it’s seed compost.

I bet you’ll make sure compost bags are very firmly closed in the future!

Really, really common. I had no idea these flies ate the roots of the plants though! I thought they were just a bloody nuisance.

You dont have to waste the compost, stick it all in strong black refuse bags and leave it until next year to sterilise it. The black plastic will raise the temp in the summer sun enough to kill pests off.

Also, if you do find you have fungus gnats in your indoor pots you can spray hydrogen peroxide on the surface soil which can kill the eggs. It was still a battle for me but I didn't lose the seedlings.

for indoors, i.e. small quantities you can microwave the compost to get it hot quickly to kill things.
it does smell though.
yellow sticky paper helps catck the adult flies and breaks the lifecycle of the gnats. take a couple of weeks.

It's the larvae of the flies that eat roots. If you have lots of flies walking up the outside of the pot, it means the larvae in your pot have turned into flies. ie when you spot the flies the damage has already been done. But killing the flies will stop them spreading to your other pots.

They're looking for humus rich soil with lots of soft vegetable matter for their larvae to eat. So spreading gravel or sand over the tops of your pots goes a long way to deterring them.

I've read that sprinkling cinnamon powder on the surface of compost helps to prevent fungal/bacterial growth, and apparently the gnats don't like the smell.

Also, pouring boiling water over compost can sterilise it and kill eggs/larvae, but it's a bit of a faff and makes quite a smell, so only practical for small quantities of compost.

Thanks for all your comments, really helpful! Today I have sealed the ingested compost bag in a black bin liner as suggested and will see what it's like next year!

I haven't checked the other bag (stored unopened). Given the weather I decided to hold off for a while!

Thanks again, so glad I asked before taking pots inside, sounds like a nightmare averted!

Infested not ingested, that would be grim!