Garden, House

Get rid of fungus gnats (those annoying little black flies on your house plants)

This is going to be a bit of an awkward post to write because I know you guys really won’t want to see lots of pictures of beasties. But it’s also a big topic and something I’ve been asked about a lot, so I think it’s important we cover it here.

Basically, if you have ever kept house plants or a greenhouse, you will at some point have experienced those tiny little black flies. You know the ones, they hang about near the plants and occasionally stray further and generally annoy you? Yup, as you guys know, I have a tendency to overwater my plants, meaning sometimes the soil is perfect for these little flies to come and lay their eggs and before you know it, a few annoying flies is a deluge. It’s infuriating.

Well over the past few months I have been experimenting with the best ways to deal with them, without giving in and throwing your plants in the bin. So here we go, the video below or text version for non-video folks 🙂

Fungus Gnats, what are they?

Well as I mentioned, they are tiny little flies that you’ll see flying around near your plants, crawling on the leaves or on the soil. The flies themselves don’t actually do any harm, they are just annoying. However, they lay their eggs in damp soil (one fly can lay 300 eggs) and their larva hatch and then go to work eating any decaying organic matter in the soil. Mostly they don’t harm your plants, but occasionally they can damage plants roots or baby plants so you don’t really want to leave things be if you have them.

Now like I say, I’ve been on a bit of a journey of learning with these little guys, finding out what works, what doesn’t and worst of all, how easily they spread from one plant you bought at tesco to the whole house! Not a happy bunny. So I’m am going to share my 2 prong approach that absolutely works and I’ll tell you about some of the other things I tried that didn’t work so well.

As usual, when I have a problem like this, the first thing I do is start to research. I read other bloggers posts, I look up gardening magazines, I hit youtube. Firstly, phew!! What a relief to see that I wasn’t the only one being driven nuts by these little things.

I watched a few videos online and something I was seeing mentioned a lot was vinegar traps. Basically, this involves leaving a small dish of cider vinegar or wine vinegar beside or on the soil if you can. The idea being it attracts the flies and kills them. I’ve got to be honest, although I sometimes found dead flies in the dish, they pretty much didn’t do anything. Apart from making the room smell. So that only lasted a few weeks. I also tried soapy water, which did kill the flies on contact but didn’t seem to have as much effect as I’d hoped as it didn’t affect the larva (I kept getting more and more flies). I found a neem oil drench did, but again the smell is awful and I didn’t want it in the house if I could help it. So back to the internet and the next thing that was recommended… toppings on top of the soil, which is meant to deter the flies from laying eggs. Again, I didn’t notice any real difference in numbers by doing this and after these failed experiments, my fly problem was becoming a real pain.

So what did work? Well… this is something I saw on a youtube video from someone in the states. It’s a product called Mosquito bits. The ingredient in these is BTI which is a bacteria that targets the larva. Completely organic (bacteria not chemical) and safe for humans, pets, plants and aquatic life. But I did have to find my way of using them though…

The recommended way to use the bits is to sprinkle them on the soil of your plant and water over them. This releases the bacteria into the soil and so kills the larva in 24 hours. However… I found the bits themselves (which are corn) got mouldy. I don’t want mouldy stuff on my plants!!! So I found the best thing for me was to soak the bits overnight in the water I was going to use to water the plants. Bingo!!! This lets the active ingredient get into the water so when I watered my plants it went right down into the soil. No yuck!!

I found this worked brilliantly, overnight, but remember that this only kills the larva, all it takes is one wee fly and you are back to the start, so this is where my two-pronged approach comes in. I also use yellow sticky traps on all the plants. They are not attractive, I admit, but this way you get the adult flies too and honestly, it really, really works. No more flies! I even use these in the greenhouse when I got fungus gnats there in the water bath which then spread to my tomatoes.

So the Eli method of dealing with the little sods?

  • Step 1. Soak (drench) the soil using water that you have soaked mosquito bits in.
  • Step2. Leave yellow sticky traps around all your plants, as close to the soil as you can (but watch out, they are VERY sticky – long hair is not good)

Now, if you have a really severe infestation, this will work but may take a bit of time. So in extreme cases and I did do this with one plant, I binned all the soil and washed the plant in a bucket of water to get rid of any larva and eggs there could be in the soil or roots of the plants. I then potted it up in fresh soil, drenched with the mosquito bit water AND gave it a sticky trap. It was quite a bad one though so this was an extreme measure. I don’t recommend something this extreme every time. But it works because the flies lay their eggs in the top layer of soil and the larva then burrow deeper, so getting rid of the soil, gets rid of both eggs and larva.

But I am happy to say that we seem to have got the problem under control, now it’s just about being vigilant and not letting even one wee fly take hold. I am a woman possessed!

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  1. THANK YOU!!!! This information has been incredibly useful and I have now ordered the required equipment to slay the hordes of fungus gnats who have taken up residency in my house plants. Their days of distracting me whilst I try to work are numbered, down to 2 days to be precise because that’s when the items will arrive in the post! Brilliant advice thanks again.

  2. Oh I feel for you… we had an absolute nightmare with them.

    I’ve since also discovered that hydrogen peroxide is fantastic. A 4% solution diluted down to 1 part in 4 parts water works very fast. It’s a bit weird though as it fizzes in the soil. Also… an unexpected benefit, it aerates the soil (the fizz) nd my plants really perked up afterwards.

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