The Right Way to Use Hydrogen Peroxide in the Garden

When it comes to saving time and money in the garden, who isn’t interested in discovering new tips and tricks? Some of this advice can be a game changer, but a lot of it turns out to be ineffective or wasteful.

A lot of online advice these days points to the use of hydrogen peroxide when it comes to things like sterilizing and killing fungus. As a household product, a lot of people already have some lying around, so why not use it?

We did a deep dive on a lot of these methods and found that this common chemical compound isn’t always the answer. Usually, there’s a better alternative that works better and actually costs even less. Before you put a handful of bottles of H₂O₂ in your shopping kit, check out our advice on when to use it and when to skip it.

When Not to Use It

Nothing works as a panacea solution to plant woes. It’s important to know when hydrogen peroxide is useful in the garden, and what situations call for a different solution.

Don’t Use to Sterilize or Aerate Soil

When we think of hydrogen peroxide, we usually think of its use as a disinfectant. It’s often used when cleaning the home and for medical use, killing bacteria, fungi, and viruses. The logic naturally follows that it could sterilize your soil as well. 

Sometimes, sterilizing potting media is a good idea, usually when we’re reusing old soil in a nursery setting. New soil can be expensive, after all, so why not reuse the old stuff?

Older, previously-used soil can be full of weed seeds, bacterial and fungal pathogens, and more harmful organisms. If you’re reusing soil that you suspect may have plant pathogens from previous use, it makes sense that you’d want to sterilize it.

However, there’s a faster, easier way to sterilize than applying hydrogen peroxide, and it’s practically free. Cook it.

A Better Solution

To sterilize old soil, you can put it in the microwave or in the oven in batches, fully sterilizing the entire batch. You’d need a lot of hydrogen peroxide to sterilize a lot of soil. Will it work? Yes. Is it efficient? No.

Regarding new soil, unless you have reason to suspect suspicious with unused potting mix, there’s absolutely no need to sterilize it. Your seedlings and plants aren’t at risk if you don’t sterilize unused soil.

Once you get down to gardening, it’s just a fact of nature that your soil is going to slowly fill up with fungi and other microorganisms anyway. This is not only inevitable but also beneficial. There’s no reason to “clean” your new soil, and your efforts would just be a waste of time anyway.

What about aeration? While it’s technically true that pouring hydrogen peroxide into the soil will create pockets of oxygen, slightly aerating the soil, it’s another option that’s wildly inefficient.

If you’re dealing with compacted soil around your home that needs aeration, there are multiple ways to solve the problem, including tilling and adding compost.

The idea that you could purchase an immense amount of the chemical compound and pour it all over the soil would not only be extremely costly but also, it would damage or kill all the beneficial microorganisms in your soil.

Perhaps you are just thinking about adding a little bit of hydrogen peroxide to your seed starting mix to aerate it. Most of these mixes are already nice and airy, so this really isn’t necessary. A brand new bag of airy seed starting mix costs about $7, so it’s not going to break the bank.

Don’t Waste It On Tool Sanitation

Well surely H₂O₂ has a place in my garden shed when it comes to sanitizing tools, right? Not really!

Sanitizing tools is crucial, especially when you’re trying to prevent the spread of disease. Fungal diseases and other pathogen-spread disease can run rampant in gardens, especially in humid areas. Gardeners often have to take many steps to minimize disease.

The last thing you want to do is manually, willingly spread it. If you’re pruning diseased leaves off the bottom of a tomato plant, you wouldn’t want to use the same tool to prune a healthy tomato plant without cleaning it first.

Similarly, my zinnias are struggling with powdery mildew this year. Whenever I deadhead and prune them, I clean my pruners before harvesting my squash, a group of plants also known to struggle with downy mildew.

A Better Solution

So, what’s the best way to thoroughly and quickly sanitize tools? The most effective option is actually bleach, and you only need a tiny bit, making it super cheap. Prepare a mixture of one part bleach to nine parts water, and you’ve got the perfect tool-sanitizing liquid.

You can put this in a bowl or bucket and dunk your tools in it, or you could put it in a spray bottle and spray your tools with it. Either way, at a ratio of one part bleach to nine parts water, this method costs less than pouring hydrogen peroxide on your tools. Rinse your tools with water after, as bleach is corrosive.

In a pinch, rubbing alcohol also does a better job than hydrogen peroxide. If you’re trying to prevent the spread of disease and only have hydrogen peroxide around, by all means, use it to sanitize your tools. It works; it’s just not the best option. Next time you’re at the store, consider grabbing a bottle of bleach.

It Doesn’t Prevent Root Rot

Root rot in the garden is usually caused by a handful of different water mold or fungi species: Phytophthora, Pythium, Rhizoctonia, Fusarium, and Thielaviopsis. These pathogens all grow and spread in low-oxygen conditions, which result from overwatering. 

When you overwater your plants or when they are planted in soil that retains too much water, the roots suffer from lack of oxygen and then water molds and fungal pathogens take over. This leads to root rot, killing your plants.

Since hydrogen peroxide kills fungi, it’s understandable that some may hypothesize that it’s an effective weapon in the war against root rot. With houseplants, where you’re able to remove the plant fully from its pot, douse it, and repot it in fresh soil, this might be the case.

However, when it comes to landscape plants, veggies, and other plants that you are growing directly in the ground, this chemical compound doesn’t provide much help. You can pour some on the ground around a rotting plant to kill some of the pathogens, but it doesn’t solve the problem.

A Better Solution

The root of the problem is the over watering, compacted soil, or combination of both leading to lack of oxygen. H₂O₂ may briefly kill pathogens and briefly provide some oxygen, but if the excessive watering continues, the problem will return.

You can avoid root rot by planting in well-draining soil and monitoring your watering. If you’re not sure if soil is wet or not, try using a moisture meter. If it’s already moist, you don’t need to water it again. This is what will seriously help prevent root rot.

Use Something Else for Fungus Gnats

Fungus gnats can be a real pain when cultivating houseplants or starting seeds indoors or in greenhouses. They lay their eggs in the soil and prefer moist conditions. Since some houseplants (and germinating seeds) prefer consistent moisture, it’s only logical that fungus gnats will appear in this environment.

Some growers advocate for the use of hydrogen peroxide to kill these gnats. While it’s slightly effective, it doesn’t really get the job done. Similar to the root rot issue, killing the gnats that flock to the moist potting mix is more of a temporary solution.

The truth is, as long as your potting mix or seed starting mix stays moist, you’re probably going to be in an ongoing battle with fungus gnats. If you’re growing a houseplant that thrives in moist soil but doesn’t like to dry out, it’s only a matter of time before fungus gnats return, even if you killed some of them.

A Better Solution

Fungus gnats are sometimes part of growing water-loving houseplants. However, there is another solution for controlling that’s much more effective, and it’s called Bacillus thuringiensis israelensis, or Bt.

Bt is actually a type of bacterium that kills mosquito and fungus gnat larvae. You can purchase a product called “mosquito bits,” which includes this bacterium. You add the product to water and then use that mixture to water your plants, killing larvae without damaging the plants.

If you’ve got your heart set on indoor seed starting or moisture-loving houseplants, don’t waste your time on hydrogen peroxide. Switch to a watering routine that includes the occasional application of mosquito bits instead.

Don’t Soak Seeds Peroxide

Somehow, rumors spread that soaking seeds in hydrogen peroxide can cause them to germinate faster. Soaking seeds in water can increase their germination rate, and hydrogen peroxide can kill bacteria. Perhaps this is what people had in mind?

The truth is that there’s just not a lot of significant research to back it up. In fact, when gardeners compare seeds soaked in water to seeds soaked in H₂O₂, they usually discover no benefits. While some light studies have found it effective, others have not. It seems like the jury is still out on this one.

Soaking seeds in hydrogen peroxide can actually be detrimental depending on the window of time, and different seeds seem to react differently. There’s no established rule that shows soaking seeds in this manner is beneficial.

A Better Solution

Depending on what seeds you are trying to germinate, there are different methods to increase your success. Many germinate better after soaking in water for 24 hours. Some require cold stratification or scarification, and lots of seeds just germinate faster with a heat mat.

You pay good money for your seeds; dunking them in hydrogen peroxide isn’t worth risking their vitality.

To Kill Weeds, Use Vinegar Instead

Lastly, while hydrogen peroxide does kill weeds, it’s yet again not the most effective method.

This chemical compound will kill weeds when applied at strengths of 10 percent or more, which is not always easily found at the grocery store. Plus, think of how much you would need to clear out a serious patch of weeds.

If you have weeds throughout your lawn, in sidewalk cracks, and in other locations, you’re going to need a lot of DIY weed killer. Buying that much hydrogen peroxide isn’t going to be particularly cheap. So while it works, it’s just not that efficient.

A Better Solution

On the other hand, vinegar works extremely well. You can purchase cheap, 30-percent vinegar in bulk that will kill weeds. You probably already have some at home.

When it comes to weeding, don’t forget the old-fashioned way, either. You could always weed by hand, which is the cheapest method of all. You can hand weed on windy days, without PPE like you would need with vinegar, too.

How to Use Hydrogen Peroxide in the Garden

Here are some examples of proper ways to use this household solution for common garden issues.

Treat Powdery Mildew

Scientists conducted a study in which they discovered that spraying a solution of hydrogen peroxide and water on cucumber plants increased their resistance to Podosphaera fusca (a fungus that causes powdery mildew in cucurbits) and decreased the severity of powder mildew from 90.4 percent to 12 percent.

Best of all, the solution they used was prepared at a ratio of one part hydrogen peroxide and 60 parts water, so you only need a miniscule amount of the chemical compound to try this at home. 

If you’d like to loosely replicate this study, fill a spray bottle with water and a splash of hydrogen peroxide and keep it in your garden shed.

Powdery mildew can affect many types of plants, including not only vegetables and annual flowers but also trees, shrubs, and grasses. If you’ve been gardening for a while, you already know which plants you like to grow that are most susceptible.

If you have plants that have already succumbed to powdery mildew, try spritzing them lightly with this mixture once a week, and see if it works for you. Or if it’s still early in the season, but you just know from prior experience that your zucchini plants always gets this fungus, try misting them preemptively.

Such a diluted mixture is unlikely to do any harm and is extremely affordable, so why not give it a shot?