If you spend any time on social media or reading popular gardening blogs you already know that hydrogen peroxide does all kinds of useful things in the garden. You will see blog headings such as, “11 Mega Reasons why Hydrogen Peroxide for Plants is a Must” and “10 Amazing Uses of Hydrogen peroxide for Plants in the Garden.”
This stuff must be fantastic! Or not.
Not every claim is a complete myth, but many of these claims are just wishful thinking. Time to look at some science and get down to the reality of using hydrogen peroxide in the garden.
Hydrogen peroxide, or peroxide for short, is a simple chemical with the formula H2O2. It is water with an extra oxygen atom attached.
Peroxide is sold in most pharmacies as a disinfectant in either a 3% or 6% solution, but is also available at higher concentrations. When applied to bacteria or fungi, it will kill them. You might remember using it to sterilize a cut, although this is no longer recommended since it also damages tissue in the cut, making it more difficult to heal.
It is very reactive and easily loses the extra oxygen when it comes into contact with all kinds of other chemicals. Spraying it into the air, on soil or even adding it to water will cause it to degrade rapidly producing oxygen and water. Light will also degrade it, explaining why it is kept in brown plastic containers. Any mixtures for plant use need to be used right away.
When peroxide is mixed with soil it will react with living microbes as well as dead organic matter. In fact, an older method for measuring the amount of organic matter in soil used peroxide as the main reagent. During this reaction oxygen is produced and can be observed as bubbles.
“It makes plants think that plain water is actually rainwater and you know how good that is for plants!”. Rainwater does contain very low levels of peroxide. Other claims include the following.
Plants don’t grow well in compacted soil and it is claimed that pouring peroxide onto it will reduce compaction. The peroxide releases oxygen and somehow this opens up the pores in soil.
This is nonsense. Peroxide will release oxygen, but it will not build up enough pressure to open up new pores in soil. The peroxide is degraded quickly as it reacts with organic matter and in a few minutes you are left with soil that is just as compacted as before.
Hydrogen peroxide is a disinfectant, so this will work, however commercial greenhouses don’t use it. They prefer to use products that combine hydrogen peroxide and peroxyacetic acid because they work better.
Comments like, “If you find a plant disease in your backyard, use a hydrogen peroxide solution to disinfect everything that might have come into contact with your troubled plants” are misleading. Firstly, most diseases in the garden don’t pose a problem and don’t need to be treated. Secondly, it is next to impossible to disinfect the soil in a home garden.
Soil and plant material will deactivate the peroxide, so material being disinfected should be washed first.
The claim is that root rot is caused by waterlogged soil due to overwatering. The water fills the air spaces, resulting in low oxygen levels which makes it easier for root rot fungus to take hold. Adding peroxide to the soil adds oxygen thereby improving the waterlogged soil.
This problem is real and it can be caused by overwatering. “Most plants can be affected by some form of root rot, usually caused by species of water molds: Phytophthora and Pythium, or by species of fungi: Rhizoctonia, Fusarium, and Thielaviopsis. These opportunistic, soil-borne plant pathogens infect plant root systems, where they thrive under low oxygen.”
“For plants with root rot or fungal infections, use 1 tablespoon hydrogen peroxide per cup of water”. Using the 3% standard solution this results in 0.2% solution which is now so dilute it won’t add much oxygen to the soil. The other thing to consider is that this is 99.8% water. The problem being solved is, too much water in the soil. Does adding more water really help?
Trying to solve root rot on plants in the ground is difficult. The best approach is to let the soil dry out to give plants a chance to fight the infection. Then solve the real soil problems, such as compaction, high water table, poor drainage etc. Follow that with proper watering.
“You can use hydrogen peroxide to combat every kind of fungal infection on your plants.” One site recommends using a 0.75% solution for spraying plants, which is a 200 mM solution.
I am always suspicious when a product cures all kinds of fungi infections. Even commercial fungicides are only effective on some fungi.
A study looked at preventing powdery mildew from infecting greenhouse-grown cucumbers by using a hydrogen peroxide spray. They found that peroxide solutions of 15 and 20 mM concentration reduced PM from 90% to 12%. However:
Powdery mildew seems to be a difficult fungus to cure once plants have it. Most treatments need to be started early to prevent an initial infection. It is also important to recognize that the suggested concentration of 200 mM would harm the plants. Every plant has a different sensitivity to chemicals but what you find on gardening blogs is a standard concentration for “all plants” and “all diseases”.
Peroxide is a fungicide and will kill fungal organisms, so there is no doubt it does work in some cases. The problem for the gardener is to know which cases work, when to spray and what concentration to use. This kind of information is almost nonexistent.
Soak your media in peroxide and it will kill some of the microbes – but is this a good thing?
While some people are talking about disinfecting media, others are adding microbes to make plants grow better. A lot of the commercial potting soil I see has mycorrhizal fungi added. Peroxide will kill these organisms.
Even if you disinfect the soil completely, as soon as you add plants and set it in a window or on a patio outside, microbes from the air or plant will contaminate it. Trying to sterilize soil and keep it sterile is impossible without special lab equipment. Gardeners should not even bother to try.
“Let the seeds soak in 3% hydrogen peroxide for five minutes. Then thoroughly wash off the chemical by running water over the seeds for a minute.”
This recommendation has the same problem as some of the other claimed benefits, namely a one-size-fits-all approach. This advice assumes no seed is affected adversely by the soak and it assumes all seed will be totally sanitized – that is not likely to be the case. “A seed treatment method that works for one type of seed may not be as effective for another, because surface structure varies for different seed types.” The treatment can also adversely affect the germination rate.
I have grown several thousand different species of plants from seed and have never sterilized them. I do use the baggy method for germination (I have a video showing you how) which lends itself to fungal infection and yet I have had few issues. Sanitizing seeds might have helped in some cases, but in general, seeds do not need to be sanitized.
One concern with seedlings is damping off disease. The best way to prevent this is by keeping soil drier and running a fan 24/7. In wet conditions even sterilized seed will get damping off.
There is one case were seed sanitation may be a good idea and that is for growing sprouts (i.e. microgreens). “Raw or lightly cooked sprouts are a common source of foodborne illness.” Unlike the above recommendation, UC Davis suggests to, “treat the seed by heating on the stovetop for five minutes in a solution of 3% hydrogen peroxide preheated to 140°F (60°C).”
The claim goes something like this. High levels of oxygen are needed by sprouting seeds and peroxide provides a simple way to provide the extra oxygen. Or, some claim that the peroxide breaks down the seed coat and makes it easier for the seed to germinate. It is suggested to use a 3% soak for 30 minutes.
A peroxide soak has been used to speed up the germination of some seeds.
A four hour soak in 3% peroxide increased the germination rate and increased the number of seeds that germinated for Ribes cereum. However, an 8 hour soak had the opposite effect.
A soak in hydrogen peroxide did not improve the germination of Kentucky bluegrass.
A hydrogen peroxide concentration of over 1% reduced the number of lettuce seed that germinated, but even at 0.1%, the length of the radical (root) was reduced in size.
Treatment with hydrogen peroxide nearly doubled seed germination of the Atlantic white cedar (Chamaecyparis thyoides).
Peroxide plays a critical role in seed germination. As soon as water is absorbed, natural peroxide levels influence several of the key processes that need to happen before seeds germinate and higher levels can speed up the process. It is also important to note that excess peroxide results in seed deterioration and a loss of seed vigor. The seed produces this needed peroxide on its own and does not need gardeners to supply it. Adding it in the right concentration and using the right soak duration will likely speed up germination, but doing it wrong, can kill the seed.
Peroxide soaks can help with germination but the actual mechanism is complex and still being elicited by scientists. It is not as simple as providing oxygen or softening the seed coat. It does not work for all seeds and too much can harm them. The blanket statements on the internet about speeding up all seed germination is false since most types of seed have not even been tested. If you are working with seeds that germinate slowly, you will most likely need to run some tests to see what works.
Some say that peroxide makes a good fertilizer. Hydrogen peroxide is hydrogen and oxygen – how can anyone suggest this as a good fertilizer?
It is claimed that watering plants with hydrogen peroxide solution will add more oxygen into your soil which increases nutrient uptake by roots, thereby increasing growth. Use about two teaspoons of 35% hydrogen peroxide to around one gallon of water and then use it on your garden every other time you go out to water your plants.
It is unlikely that the excess oxygen increases root growth, except in cases where the plant is stressed by conditions like compaction. Peroxide in plant cells do play a role is things like potassium absorption by roots. It is possible that extra peroxide in soil has some impact on nutrient absorption by roots, but I can’t find scientific support for pouring peroxide onto the soil to enhance root growth. Remember that the oxygen provided by a peroxide soak will only last a few minutes.
Peroxide applied to roots has been found to inhibit root growth of rice seedlings.
Current science does not support this practice.
“A 1% hydrogen peroxide solution is safe to use and will keep away insects and kill any eggs. Aphids will be deterred from sitting on the leaves of your plants with just a spritz of this solution.”
Do aphids sit on leaves? I see them mostly on the stems and buds!
If you spray an insect or eggs it is quite likely it will be harmed and might even be killed.
Some claim peroxide kills larvae and eggs of fungus gnats, but I could not find a reliable source that confirms this. There is also no evidence it kills aphids.
The claim says that spraying plants with peroxide repels insects. How can this be? When peroxide is sprayed on plants it will degrade quickly as it reacts with the microbes that cover the leaves and sunlight hits it. After that, it’s just water. Last time I checked water is not very effective at repelling insects.
The claim is that a 10% spray will kill off unwanted plants. Given the above mentioned phytotoxicity levels, this is quite possible but it would also kill non-weed plants.
If this works, it is surprising that it is never mentioned in government weed control procedures. Also note this requires a higher concentration of peroxide than the normal consumer product and that inhaling this higher concentration is harmful to lungs.
“Mix some peroxide into your watering can before you take it out into the garden. As hydrogen peroxide has strong oxidation, it will remove any harmful chemicals or pesticides found in ordinary tap water. This includes getting rid of chlorine which is added to water at treatment plants.”
Hydrogen peroxide is a strong oxidizer and it does react with some chemicals. It certainly does not “remove all harmful chemicals”. It is also possible that the reaction produces a chemical that is even more harmful. Each chemical of concern should be researched and evaluated on its own merits.
Peroxide will react with chlorine in drinking water, provided the pH is above 7. The reaction converts chlorine into hydrochloric acid – is that better for plants?
The truth is that the levels of chlorine in drinking water are not harmful to plants, so there is no problem to solve.
Hydrogen peroxide is made by plants and used to control a number of internal hormonal systems. In low amounts it can trigger a plant to initiate the production of natural pesticides which in turn protect the plant from insects and diseases. For example, in pepper plants a peroxide spray can induce a plant to protect itself from a virus attack.
In high amounts peroxide is very toxic to plants.
Much of this research is still in early stages and we are just starting to understand the full extent of what peroxide does in both plants and animals.
When microgreen and lettuce seedlings were treated, some damage to leaves was seen at 0.0025%. This study recommends an upper limit of 0.01%, but this does vary by species. Mature plants are probably less sensitive since their leaves are tougher, and coated with microbes. The peroxide will react with the microbes and decompose before entering the leaf.
One popular site suggests using a spray of 0.75% to control diseases which is almost 100 times above the safe limit for seedlings.
Most of the claims made on gardening blogs and social media are not supported by science. Some are true in select cases, but the typical claim is usually stated in an all encompassing way – kills all insects – making the claim untrue.
Popular suggestions also ignore the potential harm these treatments can do. Peroxide can be toxic to plants at some of the suggested doses.
Peroxide is certainly a good way to sanitize solid surfaces. It will kill microbes, but it affects both the good and bad guys.
I would not use it except for special cases that really need a solution and where there is some scientific evidence that it works. If you have a problem it might be worth your while to do some experimentation by applying different concentrations and seeing if there is an improvement in plants.
Most people use seed that germinates fairly quickly so there is no problem to solve. I know that 10 days for tomato seed seems like a long time, but it isn’t compared to slow germinating seeds, such as clematis. Experimentation with seeds would be a good place for some citizen science work. The key here is to use controls. Some of the seed needs to be treated in a normal way to compare it to seed treated with peroxide. You should also count germinated seeds – don’t just use a gut feeling that tells you it worked. If you do such work, post it on our Facebook Group.
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