Gnats in the soil...?

SixSilver(Sahelian)July 14, 2014

Having just moved into an apartment all on my own, I decided to get plants and we always had some on our balcony when I lived with my Mom. My apartment is actually on the ground floor so I have no balcony, but I figured I could have plants indoors. As naive as it may sound, bug infestation never even crossed my mind; now, in barely two months, I've had to treat my plants for gnats, spider mites and mealy flies.
The flies and the spider mites, I'm convinced came with the plumerias I've bought, mainly because they never came close to my other plants (a bunch of succulents, haworthias, mainly, and a couple of adeniums).

I was able to get rid of the mealy flies fairly fast with a homemade treatment and daily observations of my plants; the spider mites, on the other hand, weren't much impressed by anything, so I sacrificed the plumeria they had infested (and that was dying anyway --- from the mites or my treatments or both...).

The gnats, I thought I got rid of when I sacrificed the flowers I was trying (and failing) to grow. They had infested only those and the haworthia I had repotted in purchased soil. As I learned a little too late that cacti and succulents need a special type of soil, I unpotted it, switched to semi hydroponics, and moved it from the living room to my bedroom: I place all the healthy, non infested plants in my bedroom in hope to stop the pest from spreading too all my plants...

The thing is, I'm finding gnats again in the living room... and I have no idea where they come from: I haven't purchased any other plants since I realized I had these pests infestations. They certainly didn't come from the clay pellets and I never, ever see them around the plants that I haven't repotted: I think the soil is too compact. Too dry too. So I doubt they came with the plants...

I do wonder about the window, though... but I have mosquito nets on all of them. Except they're not the finest nets either. And the living room's windows over look a small garden. I haven't really checked for gnats infestation but I don't think there are any, especially as the garden isn't watered much. Not every day, in any case... the soil does go dry between waterings.

The only other thing I see is the soil I've purchased --- the one that's not for succulents and cacti (I'm still a beginning, making stupid beginner mistakes): I had some left in a small container that had a few succulents leaves, experimenting with propagation. I did water it regularly but I think I got the soil a little more moist the last time I did; and shortly afterwards, gnats were in that container and in the adenium's semi hydroponics pot (that one is still the living room).

Now, I'd like to know just how crazy my hypothesis is: that the gnats are actually in the soil I've purchased; not the adult forms but eggs, perhaps. Or do I have to blame the mosquito net on my windows for not keeping the gnats out? I want to make sure that once the gnats are gone, they are really gone (perhaps 'forever' is not very realistic, but I can't have them coming back every month); and I can't do that without knowing where they come from (although if it is from the window, that might be a problem)...

Would anyone care to share thoughts on that...?

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ked1985(7a NoVA)

I have no idea what the exact solution to this is, or 100% where they might be coming from, but have you tried watering from the bottom of your potted plants instead of the top? I had gnat issues at work and this is what got rid of them. Actually dealing with gnats in a plant recently bought as we speak... I let everything dry out (to the point the plant is wilting or until I see a decrease in buggies) and then water from dishes.

Is it possible the gnats could be coming from somewhere else, like the kitchen or cat/dog food or something?

    Bookmark   July 14, 2014 at 5:03PM
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No, not coming for the dog food... but I'll be extra careful anyway. I'll check the kitchen too; but the only place I see them are around the plants in the living room... and, again, it just seems to strange to me as I never leave any doors open and all the windows have mosquito nets; none of the plants I've bought and haven't repotted are infested; just the ones I've repotted with the purchased soil...

I'll switching to semi-hydroponics with a water reservoir at the bottom... it should be the same as watering from the dish... hopefully, the gnats will not be able to reproduce.

And I'm going to stop using that soil... just in case.

    Bookmark   July 14, 2014 at 7:14PM
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ken_adrian Adrian MI cold Z5

i think you would benefit greatly from two other forums.. houseplants... and container gardening [its hard to learn how to water containers] ...

there is so much you need to learn about potting media that i dont know where to begin ...

except.. in my learning curve.. i learned to sterilize my potting media... regardless of what the bag claims... such as at the link .. gallon baggie and the microwave.. moisten well ... then heat until it steams a bit .. use the next day ... its just so much easier.. to avoid a problem.. than solve it.. for 60 seconds of nuking ... [steam forms at 212 degrees... soil is sterile at 160 .. so close enough as soon as you see water vapor forming]

and second... let you pots NEAR dry .. in between watering ... if the plant wilts.. you are one day late .... if it never wilts.. you are watering to much ... and that is why you have fungus gnats ... and watering varies thru the year... e.g if the furnace is running in winter.. or a/c in summer ...

you should plan on a day .. and get all your plants outdoors.. and repot them all .. starting with fresh new sterile media ... and insuring that there are no bugs in any of your soil... repotting one plant ... really doesnt mean they wont be back in the new soil from other plants.. within hours ...

you can come back to the clinic as often as you like.. but again.. learning how to avoid the problems.. will be found in other forums ... as to the gnats .... the cause is water management.. the effect is the resultant bugs ...

and make sure.. you never bring home a plant with bugs.. take the time at the store.. to really look it over.. and perhaps.. isolate it from your collection for a week in a different room.. while you observe ...

good luck ....


Here is a link that might be useful: link

    Bookmark   July 15, 2014 at 6:58PM
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Thank you, Ken, I will check those two forums and definitely sterilize the soil if/when I need to use it again --- baking the soil in the oven had crossed my mind but I didn't know if it were a good idea and/or whether it would break down all the nutrients... I figured I'd google it when the time came, so it's good to know it is possible. I find also very interesting that soil becomes sterile at 160°...!

I already had read about letting the potting media dry between watering; I'm just so worried about the plants surviving on too scarce water than thriving, that I thought I'd be safe with semi hydroponics... guess not. I'll do more reading and figure it out --- getting it right is a learning process...

Last but not least, I definitely agree with you: it is better to avoid the problem than solving it and GardenWebs is a great place to understand these things, thanks to people like you; I really enjoy reading threads and I am amazed at all the things I learn... just need to make everything work, now. Hoping there won't be too many trials and errors along the way!

    Bookmark   July 16, 2014 at 5:07AM
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