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Does Cinnamon Keep Fungus Gnats Away From Your Plants?

Everyone who grows plants indoors eventually runs into a kerfuffle with the dreaded fungus gnat. Fortunately for you, there’s an easy, all-natural solution to your fungus gnat problem: cinnamon.

Cinnamon is an all-natural fungicide and insect repellent. It both repels adult fungus gnats and eliminates the fungi food source that gnat larvae need to develop. Just a few teaspoons of cinnamon is enough to eliminate fungus gnats within 2-3 weeks.

But before you add cinnamon to your grocery list, let me walk you through why cinnamon works, what type of cinnamon you should get, and how to use it on your plants.

Why Cinnamon Is A Powerful Fungus Gnat Repellant

Cinnamon is a great fungus gnat remedy because: 

  1. It prevents fungus gnats from ever establishing an ongoing life cycle in your plant’s soil to begin with by repelling adult fungus gnats
  2. It disrupts any existing life cycles of fungus gnats currently existing in your soil by eliminating their larval food source

It’s a highly effective fungicide and insect repellent and it’s supported by a large body of scientific research. 

The key ingredient in cinnamon that gives it its power is a chemical called cinnamaldehyde (also called cinnamic aldehyde, pictured above). It makes up about 60-90% of cinnamon oil and is precisely what gives cinnamon its familiar smell. 

Cinnamaldehyde does a few things:

  1. It inhibits mycelial growth by inducing oxidative stress in fungal cells and interfering with the build of the cell wall (i.e. chitin content, permeability, genes involved in cell wall integrity).
  2. At high enough doses, it significantly alters metabolic gene expression in insects, especially genes related to antioxidant defense, nutrient metabolism, and regulation of cellular events
  3. At lower doses, it functions as a strong insect repellant 
  4. It does all of the above with absolutely no mammalian toxicity

Long story short, cinnamon works.

As a fungicide, cinnamon oil has been proven to be one of the most effective essential oils at inhibiting growth of yeasts, fungi, and bacteria. 

When cinnamon kills off any fungi growing in your plant soil, it eliminates a key food source for fungus gnat larvae.

In one study, researchers tested 11 different essential oils and found that cinnamon oil from cinnamomum verum (also known as true cinnamon tree) was the most effective at controlling the growth of aspergillus flavus, a type of fungi.

Cinnamon also has a strong aroma that repels insects, who often rely on their sense of smell to find food sources.

If you want to see it in action, here’s a video by AntsCanada who ran an experiment showing that cinnamon was one of the only effective ant repellants out of 9 other home remedies (cinnamon at the 5 min mark):

At even higher concentrations, it can even be used as an insecticide. You might even see cinnamon oil listed as an ingredient in commercial pesticides. (Not that you would want to make one yourself).

Now that you know why cinnamon works, let’s get practical and talk about how to actually use it on your plants.

How To Use Cinnamon To Eliminate Fungus Gnats

Step 1: Quarantine Your Plants

Before you do anything else, the very first thing you’ll want to do is bring down the COVID lockdown hammer on your plants. You should take this step as soon as you start seeing fungus gnats around your plants.

It’s probably safe to assume that if you see some fungus gnats walking around in the soil, that plant is probably infested.

Quarantining your plants will minimize the spread of fungus gnats to your other plants and give you a smaller surface area that will need to be treated.

A closed off room like a bathroom or your garage are great choices to keep your plants during the lockdown. Ideally, it should be a place you don’t visit too often so that you won’t have to physically be in the same room as the fungus gnats too often.

You will probably be keeping them there for about 2-3 weeks. So the room should have some kind of light source (like a window) to keep your plants happy in the meantime.

Step 2: Get Cinnamon

The first thing you’ll need to know is what cinnamon you should get. There are actually several forms of cinnamon you can choose from:

  1. Cinnamon powder
  2. Cinnamon oil
  3. Cinnamon sticks

My recommendation would be to get some cinnamon powder. It’s the most common form of cinnamon that you’ll see and it won’t be too hard to find in your local grocery store. It’s also really simple to apply it thoroughly and evenly on your plant soil.

If you do choose to go with cinnamon powder, you should look to get “pure” cinnamon powder to maximize its potency.

Cinnamon oil and cinnamon sticks I would only use if you happen to already have some at home and want to give them a try first. Otherwise, I would say to go with the cinnamon powder.

Step 3: Cover Your Soil In Cinnamon

Take your cinnamon and pour it over the soil of your plants. It’s probably easiest to do this with a spoon. Pouring directly is fine, but might make a little mess.

If you happen to be using cinnamon oil or cinnamon sticks, the concept is still the same. You will just have to do your best to spread the oil around the soil or break up your cinnamon sticks into smaller chunks.

Since fungus gnats primarily reproduce in the soil, covering the soil in cinnamon will ensure that their only means of reproducing is sealed off.

There’s no need to be conservative with how much cinnamon to apply. You will want to get at least an eighth of an inch/quarter cm layer of cinnamon on your soil. The exact thickness doesn’t matter, you just don’t want to sprinkle a little cinnamon on top and call it good. You want to create a thick barrier preventing fungus gnats from reaching the soil.

Try to block off any openings for them to wiggle their way through as a secondary measure, in case they somehow manage to resist the repelling effects of the cinnamon odor.

As you water your plants, some of the cinnamon will wash away. So you will need to reapply the cinnamon after every one or two waterings.

You should see your fungus gnats start to diminish within a couple of days and they should be completely eliminated after 2-3 weeks.

Step 4: Let Your Soil Dry Out Between Waterings

After your fungus gnats have been eliminated, you’re going to need to cut back on your watering schedule to avoid running into fungus gnats again. It can be hard to resist the urge to keep your plants well-hydrated, but try to allow the top 1-2 inches of soil to dry out before watering your plants. 

Fungus gnats are attracted to moist soil. Keeping the soil constantly moist with frequent waterings is an invitation to get a fungus gnat infestation.

You can stick your finger into the soil up to your first knuckle to check. If you still get some moist soil sticking to your finger, you can probably afford to wait a little bit before watering again. Plants generally tolerate under-watering better than overwatering. 

Alternatively, you can also try bottom-watering your plants to keep the top layer dry, where fungus gnats tend to lay their eggs.

How Long Does It Take For Cinnamon to Kill Gnats?

Taking fungus gnat lifespans into consideration, it’s going to take at least 2-3 days until you will start seeing a noticeable drop in fungus gnat numbers and up to 2-3 weeks until you’ve completely eliminated all the fungus gnats.

Cinnamon is more of a deterrent than an insecticide, so it isn’t necessarily going to instantly kill fungus gnats on contact. It’s going to take some time until you’ll see its effects on your fungus gnat problem.

Adult fungus gnats live about 8 days. So although you’ve hampered the growth of new fungus gnat larvae from growing into adults with cinnamon, the existing adults will still take some time to die off from old age.

To add to that time, larvae take about 10 days until they become pupae, then 4 more days to become adult, egg-laying fungus gnats. So on top of the 8 days of adulthood, there are also the young pupae that will hatch in a few days before starting their 8 day countdown.

So starting the timer from when you first apply cinnamon to your soil, it will be at most 22 days for the just-laid eggs to live out their entire lifespan assuming they somehow survive the cinnamon dark ages (which is not likely).

The good news is that cinnamon has a strong aroma that repels insects. So any existing adult fungus gnats will likely fly elsewhere to avoid the smell, thereby dropping reproduction rates and the number of gnats you’ll see around your plants.

If it’s been longer than 3 weeks and you’re still seeing fungus gnats, it’s possible that there wasn’t enough cinnamon coverage on your infested plants or you missed an infested plant somewhere.

There’s also no guarantee cinnamon will be effective on all 2,200 species of fungus gnats. Maybe your particular fungus gnat species just happens to be immune to the effects of cinnamon.

If you do think your fungus gnats aren’t responding to the cinnamon, it’s probably time to try another measure like hydrogen peroxide.

If you want to see how to use hydrogen peroxide to get rid of fungus gnats, check out this article here.

Why Else Put Cinnamon On Plants?

Cinnamon has a few other benefits other than getting rid of fungus gnats which I’ll list out here in case you’re interested:

1. It Prevents Mold Growth

If there’s enough moisture, mold can start to grow on the surface of your soil. There are mold spores everywhere! It’s both not a good look and it might not be the best for your plant if the growth gets out of control.

As a fungicide, cinnamon is a great way to discourage mold from establishing a foothold in your plant soil.

2. It Repels Other Unwanted Guests

Cinnamon doesn’t just work on fungus gnats. There are also a few other critters that are turned off by the smell of cinnamon including:

  1. Ants
  2. Squirrels
  3. Mice
  4. Spiders
  5. Cats (But it can be harmful to their health)

So in case you run into other pest problems in the future, cinnamon can be a versatile home remedy for that.

3. It Can Protect Cuttings or Open Plant Wounds

If you’ve been pruning your plants, applying some cinnamon on the open cut can help protect them by preventing fungal infections. 

If you’ve heard about the importance of sterilizing your pruning tools, adding some cinnamon to the cut is doing essentially the same thing. You’re essentially just sterilizing the cut with some anti-fungal powder.

This is why cinnamon is often cited as a rooting agent. Its natural fungicidal properties help prevent fragile cutting roots from rotting.

This is especially useful for new plant cuttings, which tend to be extra sensitive to bacteria and fungal growth.

Can Cinnamon Damage Plants?

Even when applying cinnamon powder directly to plants, it is completely safe and non-toxic both for plants and people.

Cinnamon oil, however, is a little bit of a different story. It’s a more concentrated form of cinnamon that potentially has phytotoxic properties when applied to plants. There are even some commercial herbicides that use cinnamon oil as one of the ingredients.

But it’s important to note that cinnamon oil is only dangerous at higher concentrations. Even then, it’s usually combined with other ingredients when the intended effect is to make a herbicide.

Most recipes for using cinnamon oil on plants recommend you dilute it with water, for example 20 drops of cinnamon oil to 2 cups of water.

If cinnamon is applied in moderation (as most household uses are) it’s perfectly safe for plants. I don’t think it’s really possible to reach dangerous concentration levels without getting some kind of special lab-grade cinnamon extract or directly pouring a bottle of pure cinnamon oil onto your plant.

I hope you’ll give cinnamon a try on your fungus gnat problem!