How to Prevent Fungus Gnats from Damaging Your Transplants

Here are the steps that I am taking this year to prevent, and to actively kill, fungus gnat larvae and the damage that they can do to my seedlings in the greenhouse. Hoping to avoid last year's disaster! As promised, here are the yellow sticky traps that I mentioned: amzn.to/3P3S7sG
Other resources, if you need them:
• Clip-On Fan: amzn.to/3Il0MTP
• Heat mat: amzn.to/430rFWE
• 72-cell tray: amzn.to/4c26NlW (I like Bootstrap Farmer for their durability)
• Mesh trays: amzn.to/3T1uQcf
• Bottom trays: amzn.to/3P5MqdE
• Humidity dome (I do the cheaper option, here): amzn.to/49zqhgk
(As an Amazon Associate, I may earn from qualifying purchases; there is no extra cost to you)

[@C3Voyage]

My experience. Bug eggs are in the mix you buy. Sanitize your seed-starting mix. Bake or microwave. Sanitize pots/containers/trays with a bleach solution of 1 parts bleach to 5 parts water. Leave till dry and rinse. That's the key--everything (bugs/disease) killed first. The rest of what you mention is good insurance/info as well especially the sticky. Not sure I'd add HP. The sticky actually prevents next gen by removing the breeding population. It alone will work post sanitized. I've been through all this over decades.

[@WellGroundedGardens]

That’s a great tip! I make my own mix and it’s one half home-grown worm castings, so I KNOW I’ve got the gnat eggs in it…but I love the germination boost that I get from the castings.

[@C3Voyage]

@@WellGroundedGardens Sounds like you're introducing an issue. Light water-soluble fertilizer gives safe growth. Seedlings are delicate. They can get the castings and other amendments at transplant when they're strong. That's my experience anyway especially with growing microgreens for years. Micros are tons of baby plants.

[@WellGroundedGardens]

@C3Voyage for me it’s not the fertilizer (I cover that with fish emulsion and such) but rather the literal germination. For whatever reason I get MUCH faster and more consistent germination when I include worm castings in the mix. It’s less critical later as I’m potting up (and I switch over to regular potting mix, then) but I like to have it when I first start them in the cells. Plus, the gnats tend to show up either way-I’d rather get the benefit of the castings and slightly higher gnat pressure…

[@C3Voyage]

@@WellGroundedGardens When I run into situations like this, I trial it for direct comparisons. It's how I make improvements. Take your mix, and sterilize some of it. Do everything the same afterwards. What you can determine is if heating the mix to the "kill stage" by microwaving or baking hinders growth. If it does, don't do it. If it doesn't, then you can then test to see if your gnats disappear or diminish post sterilization. Test it.

[@WellGroundedGardens]

@C3Voyage my trial was years ago and sort of the reverse; I used to use a mix with no castings in it-I did have some gnats at the time, and I definitely had worse germination. I think your advice is VERY sound and people should sterilize their mixes, in general. I’ve personally chosen to take this particular trade off because for me the better germination is worth it. For others it may not be.

[@rickeversole776]

Spot on- timing wise. Michigan grower, here, woke up to white/ gray mold and watched a fungus gnat land on my flat. I will use the hydrogen peroxide treatment today after allowing today' s above average temp/ sun partially air out the overly moist condition. Once my flat is over 50 per cent germinated I will start using the fan. Thank you! I subscribed a month or so ago and enjoy learning from you!

[@WellGroundedGardens]

YES! I'm so glad that this got to you in time. I cannot tell you how much it sucked last year to watch dozens of transplants dying after I'd nurtured them for 4+ weeks.

[@Blackhuf]

Thank you! I will take care not to overwater

[@WellGroundedGardens]

Glad it was helpful!

[@ForsakenCrimmy]

This kinda missed the mark. The mold is not the problem. The over watering, poor draining, low quality(lacking organic material sufficient to prevent this) soil are the issues. Add perlite for drainage and increase soil aeration. And try a dry amendment called neem seed meal. It has completely solved my gnat problems and I do vermicomposting. I use worms to compost my food scraps indoors. It should be a gnat breeding ground right?

[@WellGroundedGardens]

The gnats are the problem; the mold is their food source. Not getting things too wet in the first place is I think the key-I do already add vermiculite for drainage. Neem seed meal will kill the gnats (I’ve also used neem oil in the past to treat affected plants in bad cases) and is another good preventative…I’d rather just avoid attracting them so that I don’t have to buy something, though I’m happy to try it if this approach doesn’t do the trick. So far, it’s working and I’ve barely had any gnats…WAY better than last year.

[@Inpreesme]

I Add hydrogen peroxide to any water that I use to irrigate my seedlings ( until they are moved outside ) whether watering from the bottom or the top along with several yellow sticky strips it’s taken a while but now I only occasionally see one or two gnats flying around per week. I think the only way you can actually stop all of them is to never bring potting soil into your house/greenhouse

[@WellGroundedGardens]

I’ve come to the same conclusion…I’ll never be completely rid of them unless I live in a hermetically sealed bubble, but like you I’ve found that I can stay on top of them 👍. Last year I underestimated the potential damage, thought of them as an annoyance, and left it unaddressed for too long.

[@anneg8319]

Ahhh. Just found fungus Kats in my sweet potato slips growing container! They haven't migrated to my tomato starts yet but would you suggest 1:4 hydrogen peroxide drench for those just in case? Tomato seedlings are doing great but I did lose them last year!

[@WellGroundedGardens]

Yup! I would do the soil drench and then let them drain well. I’m away on a trip right now and had to water my transplants more than usual before I left; I’m going to do a preventive drench as soon as I get home, as insurance.

[@anneg8319]

Thank you for your time. I moved the sweet potatoes to the garage with a heating mat, away from the other starts. I'm in South Carolina so it's cold out there but not freezing. Enjoy your trip.

[@anneg8319]

and drenched

[@bagsmode]

thoughts of using a sprayer with the hydrogen peroxide to get the full surface of the soil?

[@WellGroundedGardens]

From what I've read, it's more important to do a deep, thorough soaking of the full depth of the soil. When I pour it into the 72-cell trays they literally fizz upwards like when you're cleaning a wound; I don't know if a spray would do that as thoroughly. FWIW, I know that this soaking kills the "good" bacteria, too, but the plants get that back as soon as I transplant them.