soil New soil infested with fungus gnats :(

So I made some 5-1-1 about a month ago. I had read to make it early and to let it cook for a while before planting in it. So it has been sitting inside 10g Home Depot bags since.  I knew these bags were going to cause issues, but they were the only bags I had at the time.  My local Walmart doesn't sell the famed Wally bags for some reason.  Just some ugly expensive animal print bags.  These Home Depot bags are not made of fabric, but vinyl.  I knew they wouldnt drain well, but was hoping they might drain at the seams sufficiently.
 
So I manage to find some Wally bags after all and decide to move my 5-1-1 over to them.  When I go to dig them out the soil was nice and dry until I got to the bottom 6-8".  Then it was sopping wet.
 
The issue is, even though there was nothing planted in them, they are now infested with fungus gnats!  And I mean badly infested.  There is nothing else in my garden with them except for a few buzzing around my prop trays inside.  I am pretty sure it is due to the poor water drainage in these bags.  I would just throw away this soil and start fresh, but this is $100 worth of soil!
 
I started to move it over to the Wally bags, and made sure to empty the top 4-6" of the Depot bags into the bottom of the Wally bags in a hope to bury the eggs down far enough that they can not surface and die.  I have no idea if this will actually work though.
 
Any advice?
 

     Here's what I would do. Buy a tarp and spread the soil out on it to dry in the sun. Mix it every day or so so sunlight has a chance to really irradiate the little f**kers and give them a hard time. Once the soil is dry, the number of survivors should be knocked down to the point where they won't pose too much of a problem to the plants. 
     I have never had a problem with fungus gnats outdoors. Only with very young seedlings and with certain lab experiments that involve plant tissue and high humidity. My thinking is that you'll probably just need to cut down the population enough to ensure that they don't murder your plants immediately. Once they are established to the point where they can survive dry soil between waterings, your fungus gnat problems ought to be over.
 
edit: You could also buy a couple of mosquito dunks and crumble them up really fine and mix that into the soil before potting it up. I sprinkled that on the surface of my seedlings' soil this year and it has worked fantastically!

Hybrid Mode 01 said:

     Here's what I would do. Buy a tarp and spread the soil out on it to dry in the sun. Mix it every day or so so sunlight has a chance to really irradiate the little f**kers and give them a hard time. Once the soil is dry, the number of survivors should be knocked down to the point where they won't pose too much of a problem to the plants. 
     I have never had a problem with fungus gnats outdoors. Only with very young seedlings and with certain lab experiments that involve plant tissue and high humidity. My thinking is that you'll probably just need to cut down the population enough to ensure that they don't murder your plants immediately. Once they are established to the point where they can survive dry soil between waterings, your fungus gnat problems ought to be over.
 
edit: You could also buy a couple of mosquito dunks and crumble them up really fine and mix that into the soil before potting it up. I sprinkled that on the surface of my seedlings' soil this year and it has worked fantastically!

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+ 1  when your plants are outside and bigger those damn pests can't hurt them .  i would do exactly as hybrid mode stated .     

Hybrid Mode 01 said:

     Here's what I would do. Buy a tarp and spread the soil out on it to dry in the sun. Mix it every day or so so sunlight has a chance to really irradiate the little f**kers and give them a hard time. Once the soil is dry, the number of survivors should be knocked down to the point where they won't pose too much of a problem to the plants. 
     I have never had a problem with fungus gnats outdoors. Only with very young seedlings and with certain lab experiments that involve plant tissue and high humidity. My thinking is that you'll probably just need to cut down the population enough to ensure that they don't murder your plants immediately. Once they are established to the point where they can survive dry soil between waterings, your fungus gnat problems ought to be over.
 
edit: You could also buy a couple of mosquito dunks and crumble them up really fine and mix that into the soil before potting it up. I sprinkled that on the surface of my seedlings' soil this year and it has worked fantastically!

Click to expand...

 
Do you think some diatomaceous earth would work instead of the mosquito dunks?  Just a thought.

Scuba_Steve said:

 
Do you think some diatomaceous earth would work instead of the mosquito dunks?  Just a thought.

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     No, for a couple of reasons. Diatomaceous earth only works on insects with an exoskeleton. So unless you want to sit by your plants and try to dust mature gnats as they fly by, it won't have an effect. Second, it doesn't work once it gets wet. So even if it could kill larvae, your soil would have to be bone dry throughout.
 
 
edit: Maybe a thick layer of DE on top of the soil would prevent mature gnats from laying eggs in the soil. But I think another material like sand or perlite would be much cheaper and probably more effective. But in your case, I think the main problem is the larvae that are already in your soil. Once they are gone, I think the chances of adult gnats laying eggs in a large pot are pretty slim. Just make sure your soil surfaces stay dry as much as possible, and allow your soil to dry out between waterings.

Anything with Bt in it inhibits the eggs from forming into larva.  Mosquito Dunks are excellent sources of the Bt.
 
I found just Bt liquid in a hydroponics store to use, but it was $$$$.  Go with the Dunks.  Put about 1/4 to 1/2 of one dunk in a gallon of water.  Before planting, wet the soil with the Bt liquid.  This should help with those gnats.

Alright, I just picked up a pack of mosquito dunks at the store.  I will apply it tomorrow and then again at planting just to be sure.
 
Thanks everyone!