Ask a Question forum: hydrogen peroxide to kill gnat larvae?

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Connecticut (Zone 6b)
Jul 1, 2017 6:18 AM CST
So my indoor seedlings have these itty bitty flies buzzing about them.

I read that using 1 part hydrogen peroxide and 3 parts water applied to the dried top soil will kill the larvae.

Will this also kill the flies too? Will it be safe for the roots? My seedlings are hibiscus and I only have two so it's not like there are a ton of flies but as summer comes into full swing I don't want this little problem turning into a bigger one.

Thumb of 2017-07-01/Zella/540c90

Name: Will Creed
Professional indoor plant consultan
Jul 1, 2017 6:48 AM CST
Gnat larvae feed on decaying organic matter and that includes most potting mixes that are kept damp or wet. The only effective method for treating the larvae (short of using toxic pesticides) is to allow the top layer of potting mix to dry out and to deprive the larvae of essential moisture. Applying any liquid, including hydrogen peroxide, to the soil works at cross purposes to this.

Seedlings are a problem because their roots are shallow and require constant dampness. You might consider very gently removing them and putting them into very small pots filled with a mix of plain peat moss and perlite, which should be sterile and larvae-free.

The adult winged gnats will die of "old age" in about a week. The key is to eradicate the larvae.
Will Creed
Horticultural Help, NYC
Contact me directly at [email protected]
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Connecticut (Zone 6b)
Jul 1, 2017 7:24 AM CST
Thank you!
Name: Daisy I
Reno, Nv (Zone 6b)
Not all who wander are lost
Garden Sages Plant Identifier
Jul 1, 2017 9:15 AM CST
I agree

And will add that sometimes watering from the bottom also helps get rid of gnats as you can stop short of soaking the surface soil.

Hydrogen peroxide has a chemical formula of H2O2. Hydrogen peroxide is unstable, so when added to water, breaks apart, forming water and releasing one oxygen molecule. It is great to use (a drop or two) if you are attempting to root plants in water. Or as a fungacide, also in water solution.
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and proclaiming...."WOW What a Ride!!" -Mark Frost

Name: Baja
Baja California (Zone 11b)
Cactus and Succulents Seed Starter Foliage Fan Xeriscape Container Gardener Hummingbirder
Native Plants and Wildflowers Garden Photography Region: Mexico Plant Identifier Forum moderator Plant Database Moderator
Jul 1, 2017 10:15 AM CST
Try sticky flypaper near the surface of the soil to trap the adults and lessen the larval load in the soil. There is a point of diminishing returns with very young seedlings where watering them less can work against the growth of the plants. At some point you are going to have gnats regardless, especially if they are being reintroduced through new soil or decaying banana peels nearby, but the sticky paper will reduce the scale of the problem several fold. It doesn't reduce the larval population directly but it can be quite useful for keeping the situation from getting out of hand.
[Last edited by Baja_Costero - Jul 1, 2017 10:15 AM (+)]
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Name: Philip Becker
Fresno California (Zone 8a)
Jul 1, 2017 6:59 PM CST
Your 1 peroxide to 3 water. Is good for plants, and helps kill, some fungus gnat larva you have.

I learned !!! By !
School of hard knocks !
this year !
My first year with lites !

Butt !!!
DE. Dusted on soil, a few times, will, kill, gnats, and larva. Thumbs up

Anything i say, could be misrepresented, or wrong.

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