Dealing with Fungus gnats in Potting Mixes

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I picked up my bags of potting mix soil early this year. It felt even a bit crazy, to be driving to go get them, when there were very cold temps outdoors that day, yet, it was a very sunny winter day and I was enjoying that part of it. But I did pause to think, “Am I getting these too early?”

Last year, I used more bags of potting mix than I expected, and I am constantly potting up plants, moving succulent babies into small pots, repotting larger plants, then comes seed starting soon where both my potting mix and seedling mix is used in trays. I also use some in my container installs later in the season, and it is kind of endless. I even repot houseplants for people from time to time. So, I need potting mix, and lots of it.

One thing I have noticed a lot lately, on various Facebook group pages related to gardening, plants, and interior houseplants, is the constant frustrations by people, some are newbies, some are not, on how they get fungus gnats in their houseplant pots inside their home. Everyone suggests a remedy they have tried from putting cinnamon on the soil to using hydrogen peroxide, misquote bits, and other home remedies. Whatever their remedy is, there is a growing (no pun intended) frustration with issues such as fungus gnats in the pots of plants and it has to be difficult for people to accept, especially when it gets out of control. They want to succeed and keep their plants healthy, but they encounter something like this and it drives them nuts.

Here’s a fact sheet about fungus gnats from Cornell, if you wish to read about them in formal, technical terms: http://www.greenhouse.cornell.edu/pests/pdfs/insects/FG.pdf

No growing mixes are immune to fungus gnat infestation, but fungus gnat numbers can vary among growing mixes. Adults are strongly attracted to microbial activity in soil/media. For fungus gnat management, avoid immature composts (<1yr old), including composted pine bark mix. Mixes with any compost are usually more attractive to fungus gnats than pure sphagnum peat. Good sanitation is vital.

Fungus Gnat Biology and Control
J.P. Sanderson

I thought this statement above from the Cornell fact sheet by J.P. Sanderson was key, especially when he wrote, “no growing mixes are immune to fungus gnat infestation” and also the part about mixes with compost are usually more attractive to fungus gnats. In other words, to me anyhow, the more sterile and clean the mix, the less likely you will get the dreaded FGs (fungus gnats) in the potting mix when the conditions are right for them to start up in the pot (such as moisture and temperature).

For monitoring adults, yellow sticky traps placed horizontally at soil level are most effective. However, fungus gnats can be monitored along with most other flying greenhouse pests by positioning yellow sticky traps vertically just above the crop canopy.

Fungus Gnat Biology and Control
J.P. Sanderson

Many times, on these gardening Facebook pages, people will say, “I use those yellow sticky tapes” to get rid of them, but in my opinion, those do not get rid of them – they MONITOR them. As noted above, these are useful to keep track of the amount of a FG infestation you may have and they also should be placed relatively close to the plant’s surface area. If you have a yellow sticky cards filled with them and you see FGs flying all around your kitchen, you definitely have a major problem. Usually the best place to start is thinking about the potting soil mix you used in the first place, and also your habits on how often and much you water your plants in pots inside the home.

“However, the primary reason why fungus
gnats are abundant in homes is related to
changes in moisture levels associated with the growing media of houseplants. Fungus
gnat adults are highly attracted to moist growing media. Furthermore, as the growing medium ages or degrades it tends to retain more moisture, which will also attract fungus gnat adults. In addition, decreased day length and cooler temperatures slow plant growth and water usage. If watering practices are not altered….

— by W.S. Cranshaw and R.A. Cloyd*

The link for the quote above and the full article is from Colorado State University Extension.

But one reason I pulled this quote from the article above (in the black box) is because I feel I see more of the FG problem in the winter as well. As they are pointing out, three things occur: the FGs like moisture and they note growing medium (potting mixes) sometimes hold onto too much moisture (meaning if in a pot for years and it doesn’t drain well for whatever reason), and lastly, plants tend to slow down growth in the winter months inside the home, so less moisture is being used up by the plants now in the winter months (so it stays too damp and maybe even too cold).

Cathy T (me) in my greenhouse

Another article noted that people notice FGs more so in the winter because they are inside with their plants, versus the plants perhaps being located outside during the summer, or you are outside more in the summers than winter. Houseplants in particular grow slowly and roots may not take up as much moisture such as a big tropical plant would in the summer outdoors. You get the idea. But they are pointing out, you may just notice them more in the winter months than summer.

Whatever the reason, these FGs are a real PIA to people. And they ain’t pretty either when you get into sowing seeds in a few months for your warm season vegetables, such as tomatoes, in your seedling trays. So, it is critical you pick the correct seedling mix in the first place. And have the right temperature, environment, clean pots, and good potting or seedling mix, in this case when you start your seeds. Perhaps correct is not the right wording, you want quality seedling mix.

Usually my number one rule, when you go to buy potting mix or seedling mix, is to be sure you are purchasing fresh mix that is in good standing condition in the bag. If the bag is a mess, at whatever location you are getting them from, I’d stay away. I’ve talked about this in the following post:

https://containercrazyct.com/2019/04/15/cathy-ts-5-must-dos-for-successful-container-gardening-and-patio-pots/

When I see people upset that they can’t get rid of the FGs in their planted house plants in pots (from these posts I’ve been reading), and they feel they’ve done everything right, such as let the soil dry out on the top (since the FGs prefer moisture), they bought reputable potting mix from a reputable place, they keep their pots and areas clean, their pots have drain holes, they been careful with watering, and they are truly trying to be the best plant parent they can be, I feel their pain. Why, oh why, can’t we count on potting mixes in the retail market place to be problem-free?!

I remember once telling a guy at a garden show years ago that when I get my greenhouse I’m going to keep it super clean, and he laughed and said, bio-diversity is just as important. True I’ve learned, and no matter how clean, you just can’t sometimes prevent nature from stepping in. That is true.

And I agree with the above article too, if the soil is too damp, AND cold, and in winter, I think there is more of an issue with FGs (fungus gnats) appearing. The best thing you can do is ACT on it immediately when you see the little buggers flying around your potted plant.

However, I do everything I can to start with good, quality potting mixes. And I see that question asked often on Facebook gardening like group pages, well, what is the BEST mix?

Oh boy, that one gets all kinds of responses. I know which I like and which I don’t. I don’t get any paid advertising on my (this) blog at all, so if I listed them, it would not be for the purposes of gaining that but I also sometimes don’t want to list them because they can seem like the best, and whack, you get FGs anyhow!

One year, I offered a Seed Starting Workshop and we decided to test various seedling mixes. Each attendee had 32 cell trays and we split up the rows by using a different seedling mix per row. We used some of my professional potting mix and other potting mixes on the market, and two different types of “seedling mixes”. One type of seedling mix, for some reason, completely failed. The seedlings in those rows were so tiny and did not grow much. All the other rows were just fine. All was sown at the same time and in the same growing conditions, and I was watering the seedlings after the class for a few weeks for the attendees. I was so excited that I even thought of testing out the various potting and seedling mixes in this experiment without knowing the results would show us a really good example of how you might think a mix is great (and I did think the brand and place I got from was reputable) but it failed. Why? Hmmm. One thing was that mix did have more heavy components (bark) even though it was listed as “seedling mix.” As per the note in the first listed article above where they said mixes with more compost are more attractive to FGs, so the mix was heavier and probably a bit more compost like. Where all the other mixes were primarily sphagnum peat moss and perlite.

Seedlings in Cathy T’s Greenhouse

I never went back to the retail store to say, hey, we used this mix – and it really didn’t work well compared to the others. But what would the store owner say? He’s probably think I was a nut job. Not knowing, I’m Cathy T. LOL.

For years, I’ve been saying, I’m going to write a full blog post about potting mixes and which are the best (in my opinion based on use). And make it very technical, but I still really haven’t done that. I always just repeat what I’ve always said, use a fresh bag of quality potting mix (or seedling mix if sowing seeds) and follow these watering instructions (provided with my kits or in my workshops, etc.) and all should be fine.

I think probably the most important thing is the potting or seedling mix needs to be fresh, sterile, and well-draining. Mixes with additional components are sometimes too heavy for container gardening, seed starting trays, and patio pots. It also greatly depends on what you are growing (seeds versus a perennial plant). Heavier potting mixes are needed for certain types of plants. But I’m generally referring to seedlings right now and smaller plants, or those in smaller pots, such as houseplants being a good example.

As you can see, it is a hard topic to narrow down.

Because I am vigilant at cleaning my greenhouse and keeping watch of my plants, and my pots, and picking up debris, using good potting or seedling mix to start with, I rarely have a potting mix issues or major fungus gnats. However, recently I had one big succulent plant in a single 6-7″ pot, and I saw them, saw those FGs flying around that particular plant, and I immediately put a yellow sticky pest trap card near the surface of the succulent plant on a little stake inserted into the pot, and yup, I knew right away that plant had an issue when I checked that card one or two days later. There were quite a few FG attached to it in places. I tried treating it, the soil, with various methods. Then I checked again, the problem was not going away. What did I do? I repotted the whole plant into new fresh potting mix and even a new fresh pot, and I’ll keep watching. I did it because I love that big succulent, and it was worth it to redo the potting of it, at least it was worth it to me. Plus I didn’t want it to spread to other plants.

Because this topic is big and is difficult to write about, I’m going to stop writing right here and continue more on this later. It is time for my second cup of coffee!

BTW, my seedling kits come with my professional well-draining fresh potting mix (the one I trust) and planting instructions, growing charts, when to sow timing, seedling trays and seeds! If you are local to my areas, check out my offerings on www.WORKSHOPSCT.com. I am offering free delivery to areas near me!

Thank you,

Cathy Testa
860-977-9473
containercathy@gmail.com

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