I just started three new girls and already have gnats. I am thinking of doing a hydrogen peroxide drench to put a stop to this, but I dont know if this is safe for seedlings? If so how much should I dilute.
I would also like some advice on sterilizing the bag of sunshine #4 that I’ll be using when i up pot. Would hydrogen peroxide work for this or should I hit the medium with boiling water. I have microbes on the way to replace what will be lost due to sterilization.
I wouldn’t recommend peroxide on seedlings… sticky traps and drier soil should put a major dent in the population.
Gnats are a symptom of overwatering.
Sterilizing is insane if you’re growing in soil.
What is your growing method?
I would be very careful about adding it to your soil. Most of the time fly strips and letting your plants dry out. Peroxide in your soil Will kill good bacteria and bad bacteria. I use molasses. It makes the soil crusty. It hard for them to escape. They won’t be able to molt. And they will die. They don’t like it windy. Fans help. But those fakers will craw and hide in crack’s. Dr.Earth has final stop. It burns there ass up. Neem oil on top of your soil and in drain pans will stop them.
I have used all of these products. Anything with oil can and will burn your plants. Make sure the lights are off for a minute if you spray your plants. The soil and drain pans are nothing to worry about. Neem oil with dawn dish soap is my go to on everything. Even spray the floor around your plants.That will stop some. I don’t guarantee it. Drying out is the best thing you can do. I’ve seen some people put rice on top of their soil, sand and they have another product for them. I can’t remember the name but if you Google it looks like dog food.
Perhaps you mean Mosquito Bits:
I use them, you can slightly bury them where the larvae live, sticky traps and watering from the bottom also helps … 
Crumble up a Mosquito Dunk & water it in. Should help.
I would just ease up on the watering. If it’s drying out to fast use a dome or a clear solo cup to help hold in the moisture.
Thanks for all the replies.
I guess h2o2 drench and boiling water is out.
Gnats have been a constant problem throughout my past three grows(all three of them), I’ve tried a few things that didn’t really solve the problem, mosquito bits, DE, neem oil spray and a layer of sand on top of medium.
I don’t water until my pots are light as a feather but to be honest they never really seem to dry up, when I water again the gnats come right back. I’ve run a fan directed at the fabric pots that I use, rotating them 180 degrees every 12hrs trying to get them to dry a little faster. On my second grow I mixed the Sunshine #4 50/50 with perlite so they wouldn’t hold so much water. Still had soil that was to moist and the damn gnats.
This is the first time that I have had the gnats while in the solo cups at seedling stage, so I thought I’d get a head start by killing them off in the medium I’ll be using before I transplant them into it.
I do have Dr Earth Spinosad on hand that’s supposed to be safe for the beneficial microbes maybe this will work as a preventative drench?
Drench is again overwatering, keeping the problem going. You don’t need any of that nonsense.
Simply only water from the bottom. Sit your pots in trays and fill them up when they’re completely dry (maximum once a day, ideally maybe once a week), since they’re fabric pots the gnats can’t penetrate from below but the water will still be absorbed, this way you can let the top layer of soil dry out without killing the plant.
Also install a fan sucking the humid air out of the tent (24/7 or when lights out only), leaving an intake vent open to let dry air in.
Humidity inside tent when lights out is probably through the roof?
why? its really common in agriculture to pasteurize your substrate, either by high temperature composting or with steam. this applies equally to soil and coir. for me personally i struggle with gnats coming in from fresh coir. i pasteurize in a cooler for an hour @ 170f
Spraying the soil with Sulfur is great if you want to kill fungus gnats. My wife cleaned up most of her fungus gnat problem with on top spray of the soil. I’ve been messing with Sulfur after reading @JoeCrowe and his research on Sulfur, PM and related. I tried ruining a few things as my Sulfur test. Not so easy. I did a 180 on Sulfur and now will be used for preventative maintenance in veg.
Diatomaceous earth top dress and keep dryer soil I water seedlings with spray bottle n just wet where roots are and a little bit out from em but small root system can’t drink the water up so it’s easy to overwater. And as said above overwatering is what gnats thrive on.
I tried most of the stuff mentioned in this thread when I battled gnats for a few years and I could never eradicate them. They love organic soil with dry amendments, tried putting mosquitoe dunks in my water for at least a year, tried DE stuffs terrible to use, gets everywhere and then is useless as soon as it gets moist. Tried Einstein oil as neem oil is illegal in Canada, there’s probably other methods I tried that I’ve forgotten.
Didn’t eradicate them until I sprinkled aquabac on top of my pots. I use blumats and put organic dry amendments right under my drip lines for top dressings. That stuff is like crack for fungus gnats so I just sprinkle aquabac on top of the top dress every time and it works great. If I stop using it or I don’t put any on my vegging plants that I don’t use blumats with I will get some flyers. So now I just sprinkle on top of soil or on top of top dressings and no more gnats.
Been working for a couple years
for a printed catalog - they carry it all – Abrico Organics.com - it’s specific to all insects that affect marijuan plants !!! good reference
Just to be clear, I’m only planning to drench the medium prior to planting in it.
I’m guessing that when bottom watering, that every so often I would need to top water ie: flush the medium to control salts build up.
I’m not using a tent this grow, I’ll be growing in my Rv. So open air with AC to control temp and humidity. Humidity in Rv is usually high 30’s to low 40’s.
3 minutes in microwave at full power works great with coco,dont let the missus catch you though,earache!!!
our local garden centre gives free gnat larvae with every bag of compost you buy!
kill.kill.kill.
I spray my soil surface with a light neem solution and bottom water when I see fungus gnats. *and use yellow stickytraps
Thermo composting is one thing, but sterilizing all the soil you’re gonna use is something else. @Biglizard you’re not giving much info to go by…
We all wanna give advice but first we gotta know exactly how you’re growing, the more info and details you share the better advice we can give.
Since you’re talking about salts building up I suppose you’re using synthetic fertilizers in soil, which is again, unnecessary.
It’s a very different method from growing in no-till organic living soil which I’m doing, so I’m giving advice from that point of view, others will give advice from their experience growing with synthetic ferts, others give advice from hydro point of view, so it’s important to be very clear and detailed about what you have going on if you want any of our replies to be helpful. 
