https://www.gardenmyths.com/hydrogen-peroxide-garden/

Whenever I come across claims on the internet I do a google search of the terms and append something like "myth" at the end. You will get much better result. In this case it may work but there not much evidence for it. In fact, I never bought the whole idea of adding peroxide to soil for any reason. To me if it can kill larvae then it would also kill the fine root hairs which are as delicate and have no protection.

Do you have access to mosquito control dunks? They contain a bacteria that kill various kinds of larvae and are safe to use.

Best is to control cultural conditions: use well draining soil, do not add compost or organic fertilizer, let the soil dry out well before watering, good air circulation. Basically remove the conditions that fungus gnats love.

"Has this been proven anywhere?"

Only by the thousands of indoor gardeners that have tried this method and found it to be effective.

@tropicofcancer im actually using a slightly modified version of 5-1-1 not exactly the same because i havent got peat and the exact size pine bark.
It dries fine but then the flies will pass from the other ones which were recently watered.
In addition the basil plant is wilting so i need to water it but then its highly infested. Its like a catch 22.

Any suggestions other than BT ? Thats why i was hoping the hydrogen peroxide would work but many others have said it doesnt work. 🤷‍♀️

@gardengal48 have you personally found this effective?
I ask because so many others said its a myth plus there is so much fake stuff online i dont know if people are just repeating what others are saying without actually trying it. I did buy hydrogen peroxide and really hope it will do the trick as its starting to affect my plants

Ahhh, but it DOES work!! Very effectively and with NO harm to the plants. BTi or mosquito bits also work but the hy-per is often something one has around the house, so no need to purchase another form of control.

But you will need to trap and kill adult gnats as well (btw, you will need to do so with the BTi as well). They have a very short life span and their sole aim is to mate and lay eggs. Often the lack of apparent control the hy-per may be showing is because adults are still present and still laying eggs. The peroxide has no residual effect so this is not a once and done thing - trap for adults AND treat for larvae simultaneously.

The best control is prevention :-) Use a potting mix with minimal amounts of decomposable OM, do not over water and top the pot with a layer of coarse sand or grit to discourage egg laying.

ETA: I have only had an issue with fungus gnats once. And that was many years ago before I got wise to better potting media than what was available prebagged in stores :-) tbh, there are too many gardening websites and organic gardening pest control sources that recommend the peroxide - and too many other gardeners who have tried it - for the it to be a myth. And if you already have the peroxide on hand, give it a go and judge the results for yourself!! Just remember that you also need to address removal/eradication of the adults as well.

Are you sure the one 511 is infested with fungus gnats or just adult flying over around it? Or is it that basil is also in 511? Basil does need a bit more watering.

You can try peroxide and see if it helps.

The problem I have with h2o2 is lack of any reputable source recommending it. All the other sources are garden blogs and you tubes that seem to be passing along the same info.

A random search on university extensions produced this: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7448.html

Note how h2o2 is not mentioned at all for fungus gnat mgmt. I am asking myself why if it such a common chemical. Not even the industrial versions of peroxides. I looked at several others - nada - no mention of peroxide. Then I found one and I was excited: https://uaf.edu/fs/files/PlantCarePests.pdf But something was fishy and looked up the references it cited. One of them is univ of missouri ext: https://uaf.edu/fs/files/PlantCarePests.pdf and there is no mention of peroxide in that cited article. The other three citations are: nymag, gardenbetty and jasons-blog. So you can see where they got their info from :(((

While I was writing above another related post appeared: https://www.gardenweb.com/discussions/6081949/sick-indoor-coleus-plant-dry-leaves-discoloured-new-growth#n=0

She implies peroxide worked for her. To me it is unclear since she did replace the soil AND used peroxide.

A document I wrote/saved re H2O2:

Hydrogen Peroxide

H2O2 has an extra O atom (compared to H2O) in an unstable arrangement. It's the extra atom that makes it useful in horticultural applications. Generally, we're not concerned with aerobic forms of bacteria normally occurring in container media or on roots. Since H2O2 is an unstable molecule, it breaks down easily. When it does, a single O- atom and a molecule of water is released. This O- atom is extremely reactive and will quickly attach itself to either another O- atom forming stable O2, or attack the nearest organic molecule.

Reduced O levels and high temperatures encourage both anaerobic bacteria and fungi. Many disease causing organisms and spores are killed by O, and the free O- H2O2 releases is very effective at this. Additionally, when plants growing in water-retentive media are treated with H2O2 it will break down and release O into the area around the roots. This helps stop the O from being depleted in the water filled air soil air spaces until air can get back into them. High O levels at the roots will encourage rapid healthy root growth and discourage unwanted bacteria/fungi.

I know H2O2 comes in several different strengths, the most common of which are 3% and 35% solutions. Least expensive is the 35% product (what I use) which you dilute (to an approximate 3% solution) by mixing 1:11 with water. I have used the 3% solution at 1.5 to 2 tbsp per gallon as a cutting dip/soak, and have mixed it into irrigation water for plants in extremely water retentive soils at up to 3 tbsp per gallon, both with good results and nothing adverse apparent.

H2O2 in high concentration is a powerful oxidant and quickly oxidizes almost anything it contacts, so be careful with it if you use it. A solution that is too strong can destroy any organic molecule it contacts.

I have seen this chart posted several times as suggested strength solutions for use in watering plants. You may wish to start at a lower concentration , such as I have used, and experiment.

TO THIS AMOUNT OF WATER ADD THIS AMOUNT OF 3% HYDROGEN PEROXIDE --OR-- ADD THIS AMOUNT OF 35% HYDROGEN PEROXIDE

1 cup, add 1-1/2 teaspoons @ 35% - 7 to 10 drops
1 quart, add 2 tablespoons @ 35% - 1/2 teaspoon
1 gallon, add 1/2 cup @ 35% - 2 teaspoons
5 gallons, add 2-1/2 cups @ 35% - 3 tablespoons plus 1 teaspoon
10 gallons, add 5 cups @ 35% - 6 tablespoons plus 2 teaspoons
20 gallons, add 10 cups @ 35% - 3/4 cup plus 1 tablespoon plus 1 teaspoon

Al

Thank you all!

ToC thank you especially for the deep delve into hydrogen peroxide. And trying to find a legitimate source! It got me thinking so I decided to do an experiment myself.

I diluted 35% H2O2 with water to reach 3% and then further diluted to 1 part 3% and 4 parts water as described by sites as the correct percentage necessary.

I then found a fungus gnat larvea (wasn't hard as I have many unfortunately) using a microscope attachment for my phone.

I dropped it directly into the solution for about 30 min - 1 hour. I thought it worked but after taking it out it looked as fine as before, wriggling about contently.

Another myth propagated by the internet and touted as fact unfortunately. It would have been so convenient if it worked as its very simple and cheap.

Conclusion: HYDROGEN PEROXIDE (1 : 4parts water) DOES NOT KILL FUNGUS GNATS.

Wow Stephanie: You are the first one I know to do an experiment. The kill should have been near instantaneous if at all. Thanks for sharing your results. If you get a chance, can you do the same experiment using distilled water instead of tap water? Wondering if dissolved solids in tap water somehow negated the effect of peroxide.

I am guessing you from somewhere in Europe. Bti is available on UK amazon but very expensive. In US it is quite cheap. I use it in my small outdoor pond and can vouch for its effectiveness. By extension I can believe it will work against fungus gnat larvae too. When I buy or receive plants in Summer I sprinkle a tiny bit over the soil as a first aid till I get to repotting them.

Testing on one single larvae in a manner other than the typical usage (soil drench) is hardy the road to a scientific conclusion :-)

It may not have worked successfully for the OP - and that could well be a factor of methodology - but a peroxide solution has worked for many, many others.

@stephaniedan

Sorry it turned out to be long response.

Your approach to the problem is what will be considered scientifically correct first step. And that is to eliminate any possible factors that may interfere with the outcome and directly expose the larvae to peroxide at the recommended dilution. This is the worst case scenario for the larvae being confronted with its nemesis directly. Your results were surprising to me a bit too. Science depends on skepticism. I was thinking it should have some effect given that it is a direct exposure. So what can change the outcome? One thing is the water. That is why I suggested distilled water. Next could be the concentration. May be you can double the concentration and check again. Third, it could be the h2o2 you have lost its efficacy and is not 35% anymore.

To be fully scientific the experimented should be repeated with more larvae in the same solution, etc. You may find out half of them die but the other half lives. You may have just landed on a particularly tough candidate by accident. It is a low probability but finite considering how it is touted. I guess you get the point. In science, it is called design of experiments (DOE) and is quite involved.

Let us say after your experiments you find out it is 50% effective at 0.6% concentration. The next thing is field trial. In field condition any pesticide would be far less effective than laboratory conditions since there are many factors that will come into play that interfere with its actions. In this case the presence of bark/peat will interfere since both are organic in nature. Peroxide will disintegrate in contact with organic matter.

Now imagine what fraction of your soil organic matter is composed of fungus gnat larvae in the first one or two inches of soil where FG larvae may be present. Far less than even 1% may be close to 0.01%. Means whatever peroxide you add to the soil will first has to deal with bark and peat and if anything survives that will reach our said larvae. Peroxide, unlike any other pesticide, is extremely short lived unstable chemical. It will disintegrate extremely fast in contact with an organic molecule. It is so fast it really does not even matter if you add another round of it right after the first one. So you see I cannot get my mind around to imagine that peroxide will have any effect or if it does it will be very marginal.

On the other hand look at other contact insecticdes: alcohol, soap, oil. They are not destroyed on contact but linger for several minutes till they dry out. Peroxide reacts and is deactivated in milli-seconds.

Now let us look again at the vast number of times it is touted on the internet as being a perfect solution. I have already presented my case that not a single reputable university source ever mentions it. So what is going on. It could be one of those myths that is virally transmitted without proof. Or it could be it works but not a single evidence you can find that proves it works. Everyone of those claims goes like this:

a. I have been battling with FG and i have used peroxide, neem, soap, etc etc for the last one month and now I think I have it under control.

b. best way to deal with it is to let the top 2 inches dry out completely and then drench with peroxide. I think I got rid of them for good.

c. I have replaced the top 2 inches of soil or completely and treated with peroxide, etc and no more fungus gnats.

I have seen these over and over again. And every one of them involved manipulating more than one factor at the same time. So no way to tell what actually worked. I have also seen posts where people complain it did not work. These are all anecdotal evidence. Anecdotes are useful but far from being considered evidence let alone scientific evidence.

Curiously enough, in most such posts they will also tell you that subsequently they are battling with leaf drop, mealy bugs, and all kinds of plant maladies. Coincidence? I do not think so. As I said before if peroxide can kill FG larvae then it stands to reason that it will also destroy the fine root hairs. Root hairs are very delicate and almost single cell structures. I suspect in many cases more root hairs are destroyed than any fungus gnats.

Finally look at Al's recommendation of dilution rates he posted. They are in the range of 0.01%. At that concentration it will be very safe for the plant, probably destroy some/most single cell organisms and introduce some oxygen into the soil. Certainly it will improve upon existing anaerobic conditions.

"I have already presented my case that not a single reputable university source ever mentions it. So what is going on."

University websites - those of the extension services that provide gardening and agricultural advice - are restricted from suggesting any chemical pest control products that are not pesticides registered to control the pest in question. Hydrogen peroxide is not a registered pesticide. You will also not find recommendations for vinegar (as a herbicide), baking soda solutions (fungicide), ammonia sprays to control slugs, essential plant oils and various other "home remedies" on these websites. It doesn't mean they don't work........only that they are not an approved, registered pesticide.

This just appeared on my list for some reason and I'm sorry I missed it until now. What fun!

It may depend on the instar of the larva as well. My hypothesis? Younger, just-hatched larvae are certainly going to be more susceptible than older ones, so you may have simply gotten a third-instar larva which will naturally have a smaller chance of dying.

In practical terms, the first app does tend to reduce the numbers when I use it, but a few may reappear, consistent with an incomplete kill. The second app seems to take things out if done a bit later. I wouldn't call it perfect, but I'd call it pretty good.

It does nothing to kill adults, which can lay more eggs, so these need to be trapped separately. Fly paper or a small dish of apple cider vinegar usually takes care of that.

H2O2 will also kill cyanobacteria and moss at the surface of the soil if you want to get rid of it (I tend to ignore it as something that will go away when I stop needing to water that much), and can help battle root rot on plants (as much as anything will, root rot is a tough one). It's all-around handy stuff.

@tropicofcancer im so sorry it took me a while to answer. Unfortunately I did something rather stupid and killed the plant i was using to test the fungus gnats so I dont have a way to test them anymore! 🤦‍♀️

@morpheuspa thats interesting! Im not sure how old the larvae was and i hope that you are correct and is it a cheap and easy solution!

Stephanie: Over the last few days I did a bit of further digging. Found two experiments on youtube. In one the person used up to 3% solution on a larvae and found nothing happened. But did not wait to see if longer term exposure changed the result. In another experiment a person experimented with mosquito larva instead. Found that only at 3% strength did have an effect but not immediately. It took about 24 hrs in the solution to kill all the larvae. Just two more data points.

A side effect of my research was I found out why they are called fungus gnats. Apparently an important part of their diet is soil fungus and other soil microbes. So it is possible that the diluted peroxide is actually aimed at killing those and thus depriving them of their food source. Sounds good in theory. Not yet convinced how that works in practice.

@tropicofcancer thank you for sharing ! Thats definitely food for thought! So perhaps there is hope yet! So a possible combination of exposure time plus less food matter contributing to positive results.

If i get another infestation im going to try to test these further at different concentrations - particularly relaying back to what you said earlier in the thread about damage to roots with higher concentration levels which id like to avoid.

Hopefully it wont be an issue though as switching to 5-1-1 in as many plants as possible.

Guessing that 1:4 dilution or at 0.6% peroxide will probably do no harm do those delicate root hairs. So give it a try. But then let it dry out too. Hope it does something in one application. Otherwise seems such a contradiction. One adds peroxide to kill but makes everything wet, that we want to avoid. And then let it dry out only to apply again within probably say 3-4 days to turn it prematurely wet again.

With 511 the problem will be less. You still have to let it dry out. FG may arrive with peat depending on the source you use. Look for dry pure peat to keep the probability low.

In my case, a few of my pots may show some signs of FG mainly because of they are damp to long. Means the soil has decomposed too much and needs a full repot soon. I ignore these small isolated incidences.

"Hopefully it wont be an issue though as switching to 5-1-1 in as many plants as possible."

As noted, FGs do feed on organic matter, and kept damp or during organic decay of the 5 and 1 parts, that's...going to be able to sustain FGs happily and the root hairs, too.

It was another reason I'm testing a Perlite component of soils instead of bark (same size, more durable, but isn't organic). We'll see about the long-term effects but so far, so good. Root hairs still grow, exist, and are perfectly happy, there's still 16% peat in the mix, and I started with some FGs.

Those are now reduced but not gone on isolated plants. So "good" but not "perfect," and I'm about to abort that experiment and H2O2 the thing and return it to the main mass of plants.

I'd be curious to see what the results of the switch are without other attempts to rid yourself of FGs.

I actually found using hydrogen peroxide is super effective. I mixed 3% hydrogen peroxide 1:4 with water and the fungus knats just disappeared after a couple of days! My problem with them wasn’t too severe though. For reference I have a medium sized devils ivy.

As have I. FGs live well mostly in pots with lots of OM, the reason I'm trying to cut that back. The hypothesis there being that they won't do as well without it. It seems to work, but not perfectly.

Severity of problem does matter. Mine's varied from "nothing much" to "awful" at times. Right now, it's pretty much gone and unlikely to return, so I'd call it defeated.

Soil dwelling organisms like larva and worms feed on freshly decomposing organic matter. In fact, they are part of the chain of decomposition. They feed and poop and then lower organisms process it further. FG's primary food is fungus and freshly decomposing organic matter. Of course, bark and peat are organic matter but nowhere near the same class as leaves, seeds, fruits from the plants. Plus bark and peat both resist decomposition - bark more than peat. But that is what makes it so attractive for soil mixes. Otherwise we could have added any wood chips or other organic matter.

In case of 511, not only it is not a suitable base for FG, the top inch or two will dry out quickly making it even harder for FGs to thrive. Of course, all these can be negated by someone who habitually over-waters and does not keep the medium clean of organic debris.

All my plants are in 511, gritty or similar structured mixes. And I have not had FGs at least for a decade now. In isolated cases I do get some and always it boils down to a pot that is basically spent - a combination of other accumulated organic matter and severely decomposed bark.

People do have reported problems sometimes with FG in 511 mix. Almost all of them used processed potting mix for the peat fraction. And they are notorious sources of fungus gnats.

ETA: Made some minor corrections.

I see, but... I... I...

I give up.