If there’s one thing a hydroponic grower should come into contact with at some stage, it’s H2O2, better known as hydrogen peroxide. Normally sold in Vol. 10, 20, 30 or 40 at the pharmacy, it has quite a few uses like sterilization and whitening. It’s pretty potent stuff as the Vol 40 you’ll buy from the pharmacy is only a 12% strength solution but causes burning on contact with the skin.

If the chemical name H2O2 looks familiar, it’s because it’s very close to water. But, H2O2 is a very unstable molecule far more dangerous than water being made up of 2 hydrogen and 2 oxygen atoms. One of the oxygen atoms is loosely bonded to the molecule and breaks off quite easily, it is this that gives H2O2 it’s oxidising ability. Essentially, hydrogen peroxide breaks down into water with extra oxygen molecules in it(the extra oxygen is absorbable by the roots of the plant). The store bought 3 – 12% solutions are weak and needs to be kept from turning into water with the use of stabilizers. It’s these stabilizers are not safe for human consumption and shouldn’t be used in your grow or on plants that you consume. Best is to get hold of some food grade, 35% strength or higher which is strong enough not to need the stabilizers. But be warned, hydrogen peroxide is really potent stuff.

The 35% or 50% hydrogen peroxide is too strong for some of the things you may need it for. You’ll often see advice talking about a 3% solution but you may have a 50% solution and need to dilute it.

There is a simple equation that can be used to accurately achieve your 3% solution or any other strength solution you might need. Lets say you have a 40L reservoir that needs maintenance with H2O2. For every liter in your reservoir you’ll need 1-2ml of 3% H2O2 solution (Please note that H2O2 should be introduced to your plants in small doses and gradually increased, plants can be affected if you add too much). So for 40 liters of nutrient solution, you’ll need 40-80ml of 3% H2O2 solution. Lets say we need 80ml of 3% for this exercise. Take the % you need (3), multiplied by the volume you need in ml (80) divided by the strength of the solution being mixed from (50). 

The equation then looks like this using those variables:

  • (3 x 80) / 50 = 4.8ml 50% H2O2
  • Minus that from the original 80ml to get the amount of water to add.
  • 80ml – 4.8ml = 75.2ml water



The most popular use is sterilizing hydroponic reservoirs. 2ml of 3% solution per liter of nutrient solution, or 3-5ml of 50% H2O2 per 10L of water in your reservoir will do the trick and keep it sterile for around 4 days. The big plus for using H2O2 is also the extra oxygen molecule that gets released into your water. Expect white healthy roots when adding it to your reservoir on a regular basis. Should you be fighting root rot, then H2O2 is the answer you’ve been looking for. Be warned though that if you are planning to use it in your living soil, beneficial microbes, bacterias and fungi will also succumb to it’s effects, not just the bad guys.


The same 3% solution is needed which we then dilute with four parts of water to spray on the leaves of the affected areas. Spray in the cooler parts of the day as the sun is going down for example. An easy way of mixing 1 liter of solution for mildew, is simply adding 14ml of 50% hydrogen peroxide in 1 liter of water. Please also note that PM is never really gotten rid of, once you have it, it needs constant treating and management until harvest. At harvest time, you can give affected buds the wash.


This is a technique whereby harvested buds are rinsed in a solution of H2O2 in order to kill any invisible fungi, moulds and the like to mitigate the risk of loosing your harvest in the drying and curing phase. Once again, the 3% solution is needed, cannabis flowers are submerged in the solution and gently moved back and fourth for 30 seconds. They are then removed from the solution and are rinsed straightaway in clean water for around 30 seconds. Place your washed flowers on paper towels  soaking uo as much of the excess water as possible. Place them in a ever so slightly warmer environment for a short period of time if need be, then the normal drying process of hanging upside down should be followed.


The last use for H2O2 we’ll talk about is taking care of pests. You can read of how soft bodied insects will succumb to a solution on contact. The usual 3% solution is still advised and like with the mildew, diluted down with 4 parts of water. Repeated use is also recommended and should be sprayed at the coolest part of the day. This can also be used for the “Bud Wash” technique at harvest time but flowers must be rinsed with clean water afterwards and hung to dry immediately. For fungus gnats, spray your solution on the surface of your growing medium every couple of days.


H2O2 is a great aid in hydroponics, mainly helping your budget by being able to do more than one job. It’s very important to be cautious when working with it as it will cause severe burns. Always make use of gloves and protective eyewear and flush with plenty of fresh water if skin or eye contact is made.

Keep in mind that H2O2 should never be stored in an air tight container. The loose oxygen atom that breaks off eventually increases the pressure within the bottle. It’s at this point that opening the bottle is very dangerous as it will spray on you. When storing the bottle after use the lid should never be turned tight. Rather store it with the cap only just loose to allow any pressure build up to escape safely.

Happy Harvest stocks 50% Hydrogen Peroxide for R150. Due to the hazardous nature of H2O2 though, it cannot be delivered by courier and is for collection only. Alternatively, you can find Happy Harvest Hydrogen Peroxide in your favourite grow shop.


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Phillip · June 22, 2022 at 2:40 am

I see how everybody’s talking about how great food grade hydrogen peroxide is for sterilizing your marijuana plant soil and surrounding areas plus adding option to the soil is a fantastic idea actually a must! But what I don’t see is, safety precautions for humans is it safe put it’s on your plants that you’re going to smoke? I didn’t know about food grade hydrogen peroxide and I have killed my lady plants going on three times , once when I had powdery mildew I read about hydrogen peroxide butt of course the wife said no and so I used something called serenade and that was terrible even though it said organic it blistered my wife’s lips and mouth just buy her smelling the fumes through the door my back was she would have smoked anyway needless to say I had to kill those black as well second slaughtering of my young ladies was because of hemp russet mites the relentless and almost a third time because russet mites came back but I was able to destroy them with guy can’t think of the name right now but it was a pure soap 00 Casteel pure soap something like that ! Anyway yeah it was hell I had to drink and drink and drink some but it did show him the end result was my wife cannot smoke the pot do do a peppermint smell they put into it ! So now I must ask what happens when you smoke the bud besides peroxide transfer into your lungs

    admin · July 7, 2022 at 7:12 am

    Hi! Sorry to hear about your troubles in the grow room. The reason we use high grade peroxide is that it leaves no residue unlike other substances, so it’s just about as safe as can be. When hydrogen peroxide breaks down or evaporates, it only leaves water behind so no transfer of anything into your lungs. Generic store bought peroxide has stabilizers in it, so best to get food grade or even better, technical grade 50%. Hope this helps!

Rick Moeckel · July 26, 2023 at 9:41 pm

Can I use hydrogen peroxide mixed with water to spray on my marijuana plants. Leaves and everything?

    admin · July 27, 2023 at 5:30 am

    Hi Rick, most certainly, as long as it’s food grade or better. The dosage per litre water will depend on the grade you have, but for 50% I would start with 1ml per litre water. Peroxide does have the ability to brown the pistils on your flowering cannabis, so be mindful of that.

How To Mix A Hydroponic Nutrient Solution - Happy Harvest · April 13, 2020 at 9:00 pm

[…] Adding food grade Hydrogen Peroxide (35% or higher) to your solution will increase the amount of dissolved oxygen in the water and around the roots, a preventative measure against root rot. H2O2 will also keep your hydro setup sterile. Add it to the solution after nutrients and before pH adjustment. More on H2O2 here […]

How to Germinate Cannabis Seeds - Happy Harvest · July 15, 2021 at 10:48 am

[…] Hydrogen peroxide is used to sterilize the outer shell of the seed and increase oxygen levels around it. Oxygen is important for gemination and is not uncommon for cultivators to rinse seeds in a weak solution for up to 30 minutes. Add 3ml of 50% hydrogen peroxide to 250ml of water for this method of soaking. Be sure to rinse the seeds well with clean water after soaking. Microbial life of all kinds won’t survive this method, whether it be unwanted fungi spores and bacteria or beneficial microbes. […]

Powdery Mildew Prevention - A Foliar Guide - Happy Harvest · January 23, 2023 at 7:41 am

[…] A clean, versatile compound that is a strong oxidizing agent and disinfectant. Peroxide for short, is basically water with an extra oxygen molecule. Peroxide solutions can be effective in killing PM colonies and spores. Mix a 3% solution with 4 parts water and spray on affected plants in the coolest part of the day. Hydrogen peroxide has the potential to damage plants, burn pistils and oxidize trichomes with foliar applications so beware. Handle it with extreme care and read more on hydrogen peroxide here. […]

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