Houseplant help!

[Amethyst]

I think I have fungus gnats near my house (and office) plants! The problem seemed to start when I repotted several of my houseplants. The problem has gotten so bad that I had to bring all my office plants home because it was too dreadful having people waving gnats away when they come to my office. But now I have lots of houseplants at home which are infested and I have an urgency to get rid of them. 
 

I used potting soil that was an opened bag (closed up, but not brand new) that had been sitting in my garage over the winter. A co-worker suggested that could have been infected with the fungus. 
 

I have used sticky traps but all that does is catch them. I want to prevent them. Kill the larvae, I guess. 
 

I read that hydrogen peroxide mixed with water will kill the larvae, so I’ve tried a few treatments of that, but I don’t know how often before I’ll see a difference. And if I use it too often, I’ll be overwatering my plants and/or creating a moist environment for the fungus. Ugh. 
 

Is the only solution repotting ALL my plants? And will that really solve the issue if there are still gnats hanging around? Will they just lay their eggs in new soil? 
 

Any help/experience/insight/suggestions appreciated!

[BeachGal]

A lot of potting soil mixes are contaminated with fungus gnats because they are loaded with organic matter that they eat and it's moist.

One way to begin killing the larvae that are in the soil is to allow the soil to dry out completely for as long as possible without killing the plant or stressing it too much. The gnats crawling and flying above the soil are no longer eating, just mating.

• Keep using the yellow stickies during this time to catch the adults. One adult will lay about 300 eggs in the week or so that they are alive and above ground. If you can catch even one, you're preventing lots of future gnats.
• Add about an inch of sand on the top layer of soil in the pot at the same time because flying gnats want to lay their eggs in the moist soil, not dry sand. Small jagged rocks would work too. You can buy both at places like Home Depot. They're usually indoors around the fertilizer and seed starting section.
• Water from the bottom if you can so that the top layer of soil dries out more quickly and discard any water in saucers after watering.

This process takes awhile to work, at least a month or longer, of cycles of drying out and watering but I have found it to work fairly well.

https://www.homedepot.com/p/Mosser-Lee-5-lbs-Desert-Sand-Soil-Cover-1110/204370350

Another option is to use beneficial nematodes. You can buy these from a company called Arbico. They don't harm humans or pets. I have not used them but I think you'd want to find out if the soil needs to remain moist for the nematodes. Again, you'd have to give it some time as they kill the larvae in the soil.

https://www.arbico-organics.com/category/pest-solver-guide-fungus-gnats

You can also buy small plants that eat adult fungus gnats. I wouldn't use these exclusively if you have a severe infestation but you could grow them to help control future problems.

https://www.apartmenttherapy.com/plants-kill-fungus-gnat-problem-36976874

I have not been able to get the mosquito bits or hydrogen peroxide soaks to work at all.

 

After you stop seeing the gnats, you might want to water your plants using a rooting hormone in case the gnats were eating the roots. Some of my plants had severe damage that took a few years to correct because so many roots had been eaten.

Edited May 7, 2023 by BeachGal

[Amethyst]

Thank you for this very thorough reply BeachGal! I only water my houseplants every three weeks, so I don’t think the problem is that I’m overwatering. 
 

I do notice that whenever I buy potting soil, it is moist. I wonder why, especially if it’s a good place for fungus. 

Edited May 7, 2023 by Amethyst

[Amethyst]

2 hours ago, BeachGal said:

Add about an inch of sand on the top layer of soil in the pot at the same time because flying gnats want to lay their eggs in the moist soil, not dry sand. Small jagged rocks would work too. You can buy both at places like Home Depot. They're usually indoors around the fertilizer and seed starting section.

This is a great idea. I’ll try this! (Dh is telling me to throw them all out and buy new plants! Waah. I really hope I can control these soon!)

[eagleynne]

You can use mosquito bits to kill the larva. You soak the bits in your watering can and then water as usual. The bacteria in the bits kill the gnat larva. It's completely safe for people and pets. Amazon sells it here or buy it from most lawn and garden centers.

[kbutton]

I haven't completely conquered the gnats, but I think they are knocked back to a survivable level.

I now cook all my soil in the oven before using it. I moisten it and then cook it until it's steamy all the way through. I think you can do it in the microwave too. 

3 hours ago, BeachGal said:

After you stop seeing the gnats, you might want to water your plants using a rooting hormone in case the gnats were eating the roots. Some of my plants had severe damage that took a few years to correct because so many roots had been eaten.

Great idea! Thanks!

[Amethyst]

1 hour ago, kbutton said:

I now cook all my soil in the oven before using it. I moisten it and then cook it until it's steamy all the way through. I think you can do it in the microwave too. 

I have never heard of this suggestion! Is this something that people do because it is well know n that pitting soil harbors larvae? I have so much to learn!

[Amethyst]

1 hour ago, eagleynne said:

You can use mosquito bits to kill the larva. You soak the bits in your watering can and then water as usual. The bacteria in the bits kill the gnat larva. It's completely safe for people and pets. Amazon sells it here or buy it from most lawn and garden centers.

I have never heard of mosquito bits before until BeachGal mentioned them. I think I’m going to try a combination of things. I’ll try anything! I’d really like to avoid throwing out plants - some of them are real beauties! 

[kbutton]

4 hours ago, Amethyst said:

I have never heard of this suggestion! Is this something that people do because it is well know n that pitting soil harbors larvae? I have so much to learn!

Yes, I can't remember if I heard about it here or on some kind of plant youtube.

[BeachGal]

Heating soil for a period of time would probably work, too. I was going to do this but then I read a thread on a horticulture forum that was confusing and made me change my mind. Apparently, if the heat is too high, that can cause problems? Maybe read about it for temp and time instructions. @kbutton has done this successfully so there you go!

I am curious why potting soil is not sold dry, too. Maybe there’s a benefit to the moisture? Anyone know more about this? I’m thinking of opening my big bags of potting soil in our basement, spreading it out on trays and just letting it dry out during winter — hoping that would kill the fungus gnats.

If you decide to repot, carefully rinse the roots well. Eggs and larvae will likely be there.

Good luck!

 

[kbutton]

I don’t think all potting soil arrives moist.

I cook mine at a low temp and stir it often. I can’t say that I’m positive it works, but it makes me feel better! 

[BeachGal]

21 minutes ago, kbutton said:

I don’t think all potting soil arrives moist.

I cook mine at a low temp and stir it often. I can’t say that I’m positive it works, but it makes me feel better! 

I am going to try the cooking method. Anything to get rid of them or at least reduce their numbers!

[MercyA]

My last three bags of Miracle-Gro soil (two bags for succulents, one bag for African violets) were all infested with fungal gnats. I'm avoiding the brand from now on!

[popmom]

It's hard because--just like in every microbiome--there are good microbes, too. So if you cook your soil, you kill the good stuff, too. Some microbes are important for the plants to take up nutrients from the soil. 

The mosquito dunks are good. I use that ingredient in the rainwater I collect. This is what I have. It's easy to mix into a watering jug.

For those who need to cook or sterilize their soil--because sometimes you just have to-- here is a product to consider adding back in afterward. You can probably find it cheaper somewhere else.

I had a terrible time this winter with my seedlings damping off. So frustrating. That made me want to cook my soil mix!

 

Edited May 10, 2023 by popmom