I Started Sprinkling Cinnamon on My Houseplants and I Haven’t Seen a Fungus Gnat Since
Adding a regular dusting of cinnamon to my houseplant care routine has helped me win the battle against those pesky little flies.
It’s that time of year when those pesky little houseplant flies appear and a certain online retailer sees a huge spike in sales of hideously bright yellow sticky traps. Fungus gnats are a common indoor plant pest and once they’ve moved into one pot, they quickly spread to others, making them tricky to eradicate. But a couple of years ago, I started dusting the soil surface of my houseplants with cinnamon powder, and I haven’t seen a fungus gnat since.
Does using cinnamon on your houseplants get rid of flies? I’m no scientist, and the cinnamon powder certainly wasn’t a miracle cure-all but, used alongside other measures, I’m pretty sure it helped get rid of those annoying little critters, and is helping to keep them away for good. It’s well known that cinnamon has anti-fungal properties so it makes sense that, as fungus gnat larvae primarily feed on fungi as well as algae and decaying organic matter, this spice might be a useful aid. Cinnamon has been used for millennia: evidence exists of it being traded 3,000 years ago, ancient Egyptians used it for embalming mummies, and it was once more valuable than gold. So it has to do something, right? So that’s why, at my wits end trying to get rid of the flies on my houseplants, I gave it a go.
Through trial and error, I learned a few vital details that seemed to be the difference between cinnamon powder simply adding a pleasant scent to my houseplants and being an effective part of my fungus-gnat-free program. Here’s what I discovered.
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1. Fungus Gnat Larvae Live in Houseplant Soil
Fungus gnats are sciarid flies, and females lay eggs just under the surface of damp compost, which hatch into larvae within a few days. These maggots eat dead plant matter and fungus for a couple of weeks, then pupate for 3–7 days before hatching into adult flies. This short lifecycle is why a few fungus gnats on one plant can quickly turn into a full-on houseplant infestation. It also means that for every critter you see flying around your plant, there are probably hundreds more living in the soil.
So my first step was to refresh the potting soil in all my houseplants. I scraped away as much of the compost as possible, washed and dried the pots, then repotted the plants using a purpose-made potting mix for indoor plants, like Back to the Roots Organic Indoor Potting Mix, available from Amazon.
And yes, I did wear exam gloves like these from Amazon because I was scared of the maggots.
2. Cassia Cinnamon is More Effective Than Ceylon Cinnamon
Turns out, not all cinnamon is created equal. Cinnamon is made from the bark of various species of Cinnamomum trees. Cinnamon produced from Cinnamomum aromaticum trees is known as cassia, while premium Ceylon cinnamon comes from Cinnamomum verum trees. While Ceylon is considered far superior to cassia for culinary use, it contains a much lower concentration of cinnamaldehyde, which is the anti-fungal compound in cinnamon.
It’s important to realize that cinnamaldehyde is slightly phytotoxic – harmful to plants – in very high doses. But that’s why it’s effective as an anti-fungal agent. This does mean it’s vital to use it very sparingly on your houseplants, though.
3. Organic Cinnamon is Best
Regular cinnamon production can involve synthetic pesticides, chemical fertilizers and fumigation. I like to know what I’m giving to my precious houseplants, so I use organic cinnamon. You can buy cost-effective 1¼lb resealable bags of powdered organic cassia on Amazon, or it’s even cheaper to buy organic cassia sticks such as these on Amazon and blitz them into a fine powder yourself.
4. A Light Cinnamon Dusting is All You Need
You’re not adding cinnamon to the soil to kill the fungus gnat larvae, but simply using its anti-fungal properties to create an environment where fungus is less likely to grow, so removing the maggots’ food source. So all you need is the lightest dusting of cinnamon on the surface of the potting mix. I dusted with cinnamon every couple of weeks during the infestation, then once a month or so afterwards. To get a super-fine dusting, I used a small matcha sifter like this from Amazon.
It’s important not to use too much cinnamon because of its phytotoxicity but also to allow the potting mix to breathe. If you cover the surface, that can suffocate the soil and increase mold, so making the problem worse rather than better.
5. Use Sticky Traps to Break the Flies’ Lifecycle
During an infestation, you need to use sticky traps as well as cinnamon to break the lifecycle. Do put up with the garish bright yellow ones, as that bright color apparently triggers a landing response in insects. I prefer to use several smaller sticky traps like these from Amazon that you can push into the soil all around the houseplant, rather than one larger trap.
Leave the traps in place for at least a month after you stop seeing the fungus gnats, so they catch any new flies that emerge from the soil, and the lifecycle is broken.
6. Quarantine Recovering Plants
I hate to tell you this but one fungus gnat can lay around 300 eggs in its 7–10 days of adult flying life. That means plants can easily be reinfected. If you’re serious about getting rid of sciarid flies for good, it’s best to have three stages of quarantine for your infected houseplants. Your intensive care wing is for plants with visible flies. Your recovery wing is for plants with no visible flies but still have sticky traps to catch hatching adults. Your rehabilitation wing is for plants you’re confident are fully fungus-gnat-free but are keeping away from the rest of your houseplants for an additional month to make doubly sure.
Sciarid flies are weak flyers, which is why you’ll often see them scuttling around pot rims and soil surfaces, so having your three quarantine wings in different rooms is fine, though obviously the further away the infested intensive care unit is from your healthy houseplants, the better.
7. Use Preventative Measures in Addition to Cinnamon
As well as giving the surface of potting soil an occasional light dusting of cinnamon, the best preventive measure against fungus gnats is not overwatering your plants. Compost that’s continually moist creates perfect conditions for eggs to hatch and plant material to decay and provide food for larvae.
Simple moisture meters such as these from Amazon will tell you whether you need to water or not, so there’s no guesswork involved, or trying to remember when you last gave a houseplant a drink.
These moisture sensors are left in the soil for continuous monitoring and the core turns from blue to white when it's time to water. Each stick lasts for 6–9 months and replacement cores are availalable.
This set of six 6-inch-long ceramic worms provides a playful way to monitor moisture levels in houseplant soil. Because the worms are made of a porous material, they are a darker color when the soil is full of water.
Stick this ladybug's 9" probe into potting mix to find out whether or not it's time to water. You'll need to wipe the probe dry after each use to prevent rusting, but reviews indicate its accurate as well as adorable.
8. Keep Houseplants Healthy
Because your houseplants aren’t sitting in soggy soil any longer, they’ll suffer less root rot so there’s less decaying plant matter for fungus gnat larvae to feed on. Keeping your plants in tip-top condition will also help keep roots strong, and a few simple measures can make all the difference to their wellbeing.
Check what light conditions each plant needs and move it to an optimum spot. Regularly dusting leaves will allow the plant to make full use of that sunlight with efficient photosynthesis, and a pair of microfiber dusting gloves such as these from Amazon makes it a fun task. Feed with a soluble fertilizer, too. If you mostly have foliage houseplants then a feed that’s high in nitrogen such as Smart Grower Genius+ House Plant Food, available from Amazon, will bring you big, green leaves. If you have a mix of houseplants, look for a more evenly balanced all-purpose indoor plant food such as Purived Houseplant Food, also available from Amazon.
There you have it, houseplants that are free from those darn fungus gnats! As you can see, cinnamon is just one part of my plan to keep my plants fly-free. On its own, cinnamon won’t get rid of a fungus gnat infestation. Nor will cinnamon prevent sciarid flies moving into an unhealthy houseplant that’s providing a perfect family home to raise a few thousand babies. But I do think cinnamon’s anti-fungal properties are helping to create an environment that’s not conducive to fungus gnat infestation, which is a very valuable element in my overall houseplant care plan.
