Getting rid of Fungus Gnats with Drying Out Soil, Cinnamon, Sticky Traps, and Nematodes

Rainman was right. Kmart sucks. At least the Kmart in New York City at 34th Avenue sucks.

For those of you who are used to beautiful, bright Kmarts, this one happens to be in the lower LIRR levels of Penn Station in Manhattan, whose three levels are sort of like Dante's first three levels of hell. Products strewn all over the place, customers with sour looks on their faces and employees who clearly would rather be doing anything else but helping you (the nice young gentleman in the camera department being the exception).

As you know, I've been buying a lot of plants at that Kmart, mostly because they keep giving me free points and I figure, instead of using them to buy junk food I'll use them to rescue the poor, dying houseplants they have in 4" pots and give them a little love, Charlie Brown-style.

A few months ago I bought a big bag of potting soil from them and put it into a corner without thinking. Within a few days, I started noticing tiny flies all around the house. I Googled it, and realized they're fungus gnats. I looked all over for where they might be breeding, and finally stumbled upon that bag of soil in the corner (which I hadn't even opened). But out of tiny holes in the bag little fungus gnats were flying in and out, like some kind of high-rise gnat apartment.

The funny thing is, I searched on Google on how to get rid of fungus gnats, and stumbled upon this post I wrote in 2010 that talked about how to get rid of them.

Now just to recap, fungus gnats aren't tiny flies, they're not mosquitoes (even though they kind of look like miniature versions of them), and they're not fruit flies. They don't bite, they just get really, really annoying. They have a tendency to fly into your face and up your nose because they're attracted to the carbon dioxide you like to breathe out.

The fungus gnats' name sounds a little nastier than it really is. They don't spread fungus, they just like to eat it. They look for it in decaying plant matter and potting soil, including the aforementioned bag of soil from Kmart. And months after getting rid of the bag, I still see them, as they managed to fly their way into a couple of my plants, including a Corn Plant that I'll be writing about shortly.

As for my office, as you know I bought a whole bunch of plants from this same Kmart. Both the Golden Pothos and Warneckii you've already seen came from there, as did a few other plants I bought and haven't posted about yet. Nearly all of these plants, I found, came infested with fungus gnats. Seems that if you're a fungus gnat, 250 West 34th is like Studio 54 for gnats--they're snorting fungi, they're fooling around and having babies. They say a gnat can lay 200-300 eggs at a time, which is why getting rid of them can be a major pain. The only saving grace of gnats is that they don't have a very long lifespan. As larvae (when they actually can do the most damage to your plant roots) they develop for 2-3 weeks on or near the surface of your plants, and they live for 7-10 days as adults. The problem comes when you have hundreds of them flying from plant to plant raising multiple overlapping generations at a time.

So following my own advice from 2010, I did the following

1) I stopped watering all my plants, at home and in the office, until at least the top layer of soil was completely dry to the touch. Now most of these plants are pretty hardy and are known to withstand large periods of drought in the wild, so most of them are okay. But sadly, one of the plants I bought was a small 4 inch Boston fern, and ferns love moisture. I'd hoped to nurse it into a giant plant, but it looks like the gnats may be winning this one.

The browning of the leaves indicates two things--that larvae have infested the root system and are eating away at them until they rot, and that my holding off on water isn't helping the plant any. The problem is, if the eggs and larvae are in the roots, watering the plant won't do anything to kill them, but if I let the roots dry out, the plant will not survive. So I'm playing a bit of a balancing act to try to find the right timing where I can kill the gnat larvae and not be too late to water and mist the plant again. We'll see if the little fern pulls through...hang on little buddy.

2) I placed yellow sticky-fly traps all over the place both at home and in the office. I knew which plants at the office here infested, but at home I placed traps in multiple plants to see where they were breeding the most. I read a great piece of advice on the Web that said that you should place trap vertically to catch flying ones, but also horizontally over the soil that catch youngsters as they hatch and fly into the world for the first time (seems kind of cruel, but hey, their mama shouldn't have picked indoor houseplants as her maternity ward).

The advice turned out to be great as far as determining the source of the infestation--one morning I woke up and while the vertical fly trap had one or two gnats, the horizontal one over the corn plant at home had about half a dozen. It's not surprising to me because I tend to overwater that plant and the root system doesn't seem to suck in water as much as it does with my other plants, so the soil stays damp for a long time. At work, the culprit was the Dracaena Warneckii, so I segregated that plant from the rest of them.

Then one morning on both plants, those same fly traps had DOZENs. Warning--the next few pictures are not for the faint of heart.

It was sort of a fungus gnat Vesuvius, where a bunch of gnats looked like they were just in suspended animation. Interestingly, I find that when they first get stuck they stand upright, because as long as they're still alive they try to fly off. But once they die, the stick flat to the yellow trap. Again, sounds cruel but remember their brains aren't very big and they only live for about 10 days anyway.

A few days later, here's what it looked like.

I told you these pictures weren't for the faint of heart. The yellow traps were working to kill the adults as they hatched--and putting the yellow cards horizontally did a great job of catching dozens of them. But it seems that there were so many of them that at least some were getting through and continuing to lay eggs--and all it takes is one to lay 200 eggs.

All in all, I think Yellow Sticky traps are more useful for supporting other efforts to eradicate gnats, but they won't get rid of them all on their own (although for every one you trap, that's upwards of 200-300 eggs that won't be making it into your plants). For example, you can use them to monitor how well your other methods are working, and of course the more gnats on the yellow traps the less will be laying eggs. I brought myself a new batch of 25 Yellow Sticky Traps from Amazon (these are the same ones they sell at Gardener's Supply).

3) I did the old trick of dusting a layer of cinnamon on the top of the soil, something I had forgotten all about from my post in 2010. I took my big McCormick tub of cinnamon and just spread it to cover the soil on all the plants I knew to be infested, and separated the other plants from them. I don't know if it's the fine powder or the smell or both, but cinnamon is known to repel female gnats (rather than lay eggs on the cinnamon they'll fly around to try to find somewhere else). From what I could tell it's working in my case. And because cinnamon is a natural substance (it's just grated from bark of the Cinnamomum verum or a similar tree), you don't need to worry about chemicals harming you. Plus the lovely scent puts me in the mood for coffee cake every time I walk by the plant. A co-worker of mine dubbed my office "The Cinnamon Lair".

After a bit of research, it seems another advantage of cinnamon is that it kills the surface layer fungi (again, which grows on dead organic material). Now some say that having some fungi is actually a good thing for plants and you don't want to kill it all, and I've also read conflicting reports of how some people say the oils in cinnamon bark aren't great for plants if in excess, so it's best to just give it a dusting. But in my desperation I just poured a good 1/8th inch layer of cinnamon on the soil (it helps that years ago I didn't end up baking all the apple brown betties and apple pies and coffee cakes I thought I would when I bought that McCormick cinnamon years ago).

Sadly, while the cinnamon did seem to lessen the amount of gnats, it didn't wipe them away either. There were some larvae that were just deep enough in the roots that the kept coming.

4) Something else that helps at home is having a vacuum cleaner handy. As I said in 2010 I like to use my Dyson Hand-held because it's got powerful suck and it has a little clear plastic bin where I can see if I got the little guys. At work I don't have that luxury so I have to wait for them to fly by my face and then I do my Mr. Miagi on them (sans tiny chopsticks).

5) Sadly, after trying all of the above, I still had gnats flying around in both the office and at home. I'd think they were all gone and then one day one would fly up my nose. Exasperated, one day I scraped off the top inch or two from the corn plant soil and put that soil in the oven.

In desperation, I took a hair dryer in an attempt to dry out the top level of soil, used Saran Wrap to cover the top of the pot, and used duct tape to block the holes on the bottom.

As you can imagine, these didn't work either, again because the larvae were too deep in the root system to be killed by just wiping out the top, contrary to what a lot of blogs say.

6) Exasperated, I went to Amazon and searched for "nematodes". For those who don't remember from my earlier post what nematodes are, they're microscopic little parasitic worm-like animals that attack larvae of bugs like fungus gnats, but are completely safe for humans, animals, and even beneficial insects.

It's really fascinating to hear how they work--when you apply them to the soil (thousands of them at a time), they crawl through your soil looking for larvae, and once they find it they burrow into the larvae through its body openings and release a toxic bacteria into them. The larvae are killed in a day or two from blood poisoning, and then the nematodes reproduce inside the larvae. Once they reproduce, the exit the dead body and seek out new larvae to infect.

Okay, I know what you're thinking...words like "parasite" and "bacteria" and "toxic" and "infection" sound a bit scary. And it's true, there are some species that are harmful to animals and plants. But again, the ones you buy called "beneficial nematodes" (Steinernema feltiae) are perfectly safe for you, your pets, and your plants. So much so that the EPA doesn't even regulate them. In fact, they occur naturally in the soil but usually not at enough volume to kill larvae--so you're basically helping boost the number of them in your houseplant soil.

I ordered the one from "Dr. Pye". I thought of ordering the one I got last time from Gardener's Supply Company, but that mixed only one gallon and I needed a lot more.

I got the package yesterday. It came with a cold pack which was completely melted at at room temperature by the time I got it. Supposedly, though, nematodes have a shelf life of several weeks if refrigerated properly; mine had an expiration date stamped on it of September.

Here's the Scanmask product I got in the mail. It's basically a small zip-lock bag of what looks like fuzzy off-white powder. 

This company packages most of their nematodes for spraying outside, but I got the smallest package (1 million nematodes) for use indoors. They said to combine the contents of the bag with 1-5 gallons of water. I decided to cut the bag in half, and use half for my houseplants at home and half for the office. 

Once you fill your watering can with water, you dump the nematode "dust" in and wait 15 minutes. Then you agitate the can. This will make sure that the hundreds of thousands of little worms are spread evenly through the water.

You can't really see them with your naked eye, but if you pour them into clear glass dish or container and hold them to the light, you'll see the specks floating in the water. If you look real closely (it helps if you have a microscope or a magnifying glass), you'll see these specks wriggling. That's when you know your nematodes are alive. They may look creepy, and surely enough their cousins are known to be pretty nasty to plants, animals, and even humans, but these are the good guys. Unfortunately with this batch I couldn't really tell--it when I looked closely it looked like a few of mine were moving but most of them seemed awful still.

You do need to pre-water your plants to make sure the soil is moist, and then after that you water the plants with the nematode-filled water. Some people like to apply the nematodes directly to the soil, some like to dig a little hole in the soil to give them a head-start. Finally, you top it off with another watering to make sure they're in there good.

Ironically, at this point the important thing is to keep your soil moist the make sure the nematodes don't dry out but have a good environment to breed themselves, which sort of defeats #1 above.

After weeks of being tormented by gnats, I liked the idea of them munching away at the larvae. Unfortunately, after a while it was clear that these particular nematodes didn't work, probably because most of them had died during shipping. The corn plant (at home) and the fern and Dracena Warnekii (at work) were still so infested that the yellow sticky traps were still full of dead gnats each day, just a few at first but a few days later back to their usual amount. I ordered a new batch of nematodes from Gardener's Supply to see if that would help.

7) Finally, it was time to call out the big guns. I've read on various blogs how you can use hydrogen peroxide to kill gnats. You start out by getting a 3% solution (the kind you can find in the drugstore), and then diluting it with 4 parts water.

Then, you water your plants with the solution, covering the whole top layer of soil and letting the water run out of the bottom of the pot. The soil will start bubbling (sort of like it fizzes when you put it on an open wound)--what's happening is that because H2O2 is an unstable molecule, when it touches the soil it'll break down into an oxygen molecule (O) and a water molecule (H2O). Fungus gnat larvae will purportedly die on contact with the hydrogen peroxide, and as a bonus, the oxygen that's released will help kill fungus and mold that attracts it as well as prevent root rot by providing oxygen to the soil.

When I did this, I saw a flurry of gnats fly out. I was hoping that they didn't come back and their larvae have gone to that giant Golden Pothos in the sky.

Unfortunately, the next day I came to the office to find--you guessed it--a number of fungus gnats still buzzing around. I looked at the soil and saw plenty of creeping things in there. I've learned in my research that soil mites and springtails (both of which I saw in the soil) aren't bad things and one needn't obsess with getting rid of them. But I thought for sure the hydrogen peroxide bath would have at least cleared away some of them.

And so, I'm still fighting to get rid of these things. I'll try another batch of nematodes when I get them, but other than that I think just continuing to let the soil dry out is going to be the best approach.

The most important lesson I learned--DO NOT buy potting soil from the Kmart in Manhattan. Sadly, they are not doing a thing about cleaning up their store--look at what I saw just the other day in a section of Miracle Gro bags of potting soil (again, not for the faint of heart).

I feel sorry for anyone who buys soil from them because they're almost guaranteed to be bringing hundreds if not thousands of gnats and gnat larvae with them.

In fact, if you buy plants from them, keep them far, far away from all your other plants until their soil has completely dried out and you're sure there are no gnats nor gnat eggs.

And if you do buy potting soil anywhere, make sure the bag is sealed. If it has aeration holes, you might want to open the bag outside and let it dry out if you have any suspicion it might have gnat larvae (some have reported for brands like Miracle Gro that their sealed bags actually come with gnats in them).

I think over the years I've dealt with all the common houseplant pests--aphids, whiteflies, fungus gnats. The one common thing is to catch infestations before they start, by identifying where bugs are coming from (it's usually a plant) and cutting off the source, and then being persistent in fighting them before they take over your house or office, which can completely defeat the purpose of raising plants to help you have a clean, peaceful environment.