Avoiding fungus gnats in houseplants
August 29, 2017 12:46 PM   Subscribe

I just had to throw out all my houseplants (8 of them) because of fungus gnats. This all started with one plant that transmitted them to the others. I've read up on how to make one's existing plants less hospitable to fungus gnats, but how do I identify plants that may be harboring them in the first place and avoid bringing them home?
posted by ocherdraco to Home & Garden (7 answers total) 5 users marked this as a favorite
I can't answer your question, because in my case fungus gnats came in packaged potting mix. But I can say that I had great success using Mosquito Bits to kill off the larva of fungus gnats. I'd soak the bits in water and then use that to water plants. Takes a week or two but it does work.
posted by Neekee at 12:58 PM on August 29, 2017 [3 favorites]

Any plant with low water usage will be much less hospitable to gnats. Succulents, cactus, euphorbia, but also thinks like Sansiviera or Ficus can go bone dry plenty long enough to kill the gnats off.

If they spread from 1 to 8 pots, you may have been overwatering.

A few good spiders will go a long way toward gnat control, as will the mosquiticide treatment above.

In my experience, fungus gnats are a minor and treatable annoyance, not something that requires careful screening, quarantine and culling.
posted by SaltySalticid at 1:03 PM on August 29, 2017 [1 favorite]

Beneficial nematodes will do the job!
posted by xo at 2:16 PM on August 29, 2017 [1 favorite]

This soil drench works like a charm.
posted by Lexica at 2:38 PM on August 29, 2017

If your infestation was not a "minor and treatable annoyance," it might be the fungus gnat's even more evil doppelganger, the root aphid. Root aphids also have a winged form, and are even harder to get rid of - they are the worst!

To try to get them out of my houseplants this summer, I used essential oils and safers soap, peroxide, nasty pesticide containing pyrethrins, and a hot water soak of the naked, soil-less roots (120 degrees, I think, using an immersion circulator meant for cooking sous vide). I lost a lot of plants, but some are coming back nicely.

When I replanted them (in sterile pots and new soil) I top dressed them with fine gravel (traction sand, washed clean, is a nice midpoint between sand and gravel) and used beneficial nematodes, and left sticky straps in the pots to catch any sign of re-infestation.

Horrors--a couple of days ago I found a couple of winged aphids on the sticky trap! I threw that plant out, and I'm going to re-up on the nematodes for the rest of the plants. Cross your fingers for me.

If I wasn't so attached to my plants I think it would have been better to throw them all out, clean the house top-to-bottom, and wait a good while before getting new ones.

To get to the point of your question: it's hard to quarantine a plant, since the insects and eggs can be hard to see and even dormant. When you get a new plant, keep it well away from the others, and examine it closely, regularly. Consider putting some sticky traps horizontally over its soil, as a warning system. Most importantly, I think, is paying attention to the source: plants from my nearby boutique garden centre are more expensive, but that place is staffed by experts, and they absolutely need to maintain a good reputation. Plants from Home Depot or Ikea or the grocery store or whatever? Never again.
posted by Edna Million at 4:55 PM on August 29, 2017

What SaltySalticid said, plus:

You can't really identify plants that are likely to have them exactly, because there aren't exactly plants that are more likely to harbor them than other plants. It's more that some soil mixes are more likely to have fungus gnats than others. Peat moss in particular tends to hold moisture long enough for fungus gnats to stay comfortable, and it's a cheap, moisture-retentive way to bulk up a soil mix so it's widely used in wholesale production of tropical plants, which are grown [mostly] outdoors in warm, tropical climates where they dry out quickly, so being able to hold water longer saves on the number of waterings they need. The cheaper the plant at retail, the more likely the soil mix is mostly peat moss; the more peat moss, the more likely there are fungus gnats.

So you're actually fairly safe to buy whatever tropical plants you want; you just need to change the soil after getting them home. Often "cactus and succulent" mixes are good for regular tropical plants, and it's helpful to add some coarse sand or gravel to a cactus and succulent mix, if you plan to grow actual cacti or succulents in it. Even a good soil mix can still sustain a fungus gnat population if you keep it too wet.

Certain bagged soils are both mostly peat moss and always come with free fungus gnat eggs, every single time, to the point that (last time I looked) the bags had a printed disclaimer explicitly not guaranteeing that the soil inside would be free of insects. (Brand rhymes with "spherical crow.") There's a specific bagged soil I like personally, but I think it's only distributed in the midwest and south so it probably isn't useful to you as a recommendation; MeMail if you want to know anyway.
posted by Spathe Cadet at 5:20 PM on August 29, 2017 [2 favorites]

Nthing any product that contains Bti (Bacillus thuringiensis israelensis) - a bacteria that harms gnat larvae but is otherwise completely harmless. Cleared up our houseplant gnat problem in 2 weeks. No need to pitch the plants in the future, sorry that you already said goodbye to them :-(
posted by rouftop at 7:56 PM on August 30, 2017

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