My Miracle Gro Had Fungus Gnats

When I began building my terrarium, I went shopping for dirt. Seems funny that we actually have to buy dirt but I figured if I purchased a fairly well-sealed bag that it would at least be free of pests and dry.

The hardest part was finding something sealed that didn’t contain water retention pellets or large amounts of fertilizer. (Everything I read about terrariums suggested that you didn’t want to over-fertilize; so I figured dirt with fertilizer already in it would be a bad idea. And, I figured water retention pellets would throw off the ecosystem I was trying to create.) The only thing I could find without a bunch of junk in it was a large bag of Miracle Gro™ dirt.

Too bad I didn’t read the reviews on Amazon about that Miracle Gro™ dirt – before I purchased it and put it in my terrarium.

The Hatching

About a week after I planted my terrarium, I noticed it had tiny residents of the flying kind.

I did a bunch of research and found that Miracle Gro™ had a history of selling fungus gnats along with their dirt. Fungus gnats themselves don’t do that much harm. But, fungus gnat larvae like a damp environment and enjoy dining on young plant roots. I had created their preferred habitat in creating a slightly over-moist terrarium. (If you’re not familiar with millimeters, 3 mm is roughly the same as the height of 3 dimes stacked on top of each other.) [photograph courtesy of Peter Rühr licensed under CC BY 3.0]

The worst part is that people have complained for years on Amazon and other sites about fungus gnats. Yet, the company apparently has done nothing about it.

Getting Rid of Fungus Gnats

Suggestions varied on how to treat them. One of the well-known home and garden website’s experts said they would die off on their own – a comment I have come to suspect had more to do with keeping an advertiser happy than offering good advice. Another site mentioned setting a clove of garlic in the soil. Another site said the garlic was someone’s idea of a bad joke.

I thought if I killed the adults as they hatched that I could stop the cycle. After all, I never saw more than 2-3 at a time. I think I did slow them down a bit but, problem is that the darn little buggers lay their eggs almost as soon as they hatch themselves. By the time I saw them, they had more than likely already created the next generation of gnats.

Cooking the Dirt

A number of sites recommended baking the dirt in the oven to kill the gnat larvae. My understanding is that it does work. But, all that fertilizer, water retention and other junk they put in the soil is particularly stinky when it is cooked. Several folks reported that their house smelled for days.

Cooking the dirt will kill the beneficial stuff too. But, one terrarium expert suggested that once the terrarium got going and fallen leaves degraded into the soil, it would be fine. A few crushed leaves on the surface could jump start things; particularly if your terrarium plants don’t seem to be dropping leaves.

Mosquito Dunks

I should have tried Mosquito Dunks. They had worked with stuff like this in the past for me. I think they even worked on white flies and those darn teeny shiny bugs that live in dirt. But, I didn’t want to add more moisture to my terrarium. If I had been smart about it, I would have watered the plants with ‘Dunk-treated water and left the lid off for a few days to let the excess moisture evaporate out of the terrarium. I had nothing to lose at that point, unfortunately, I just didn’t realize it.

I think a better choice might be Mosquito Bits – little granules made of the same stuff as the Mosquito Dunks. They can be sprinkled on the soil and watered-in. You can do that with the Dunks too but you have to break them apart yourself and I was always afraid the cats might find a stray bit of the stuff that got away. I’m thinking that the Bits might be the best choice – several of the reviews on Amazon mentioned they worked great for fungus gnats and fruit flies. It also makes it easier to use if any of our other house plants get unwelcome visitors.

Might be a good idea to sprinkle a small bit of granules in all of the houseplants once a year or so to prevent future outbreaks. I’ll certainly be giving any new houseplants we purchase at least one treatment before placing them on windowsills or shelves where we already have plants.