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I planted some sweet potato tubers that have been growing vines for a few months now. The plant is potted in the same cactus soil I use for my cactus plant-- very high drainage-- in a tin pot with a dozen drainage holes on the bottom. I water it with about half a cup of water + plant food per week. I feel that this is not too much, but please let me know if it is.

I recently got an infestation of fungus gnats. I didn't realise what it was at first, so unfortunately I didn't start treating until there were about 50-60 (based on the number of adults I've killed with sticky tape/ a makeshift sticky swatter). In summary, I'm catching the adults, I've mixed up the top inch of soil with about a quarter cup of cinnamon, and daily I spray the top of the soil with 3% hydrogen peroxide solution.

Are there any other preventatives measures I can take? I had fungus in the pot about a month ago, which I'm sure lead to this situation, but I water so infrequently I'm not sure what to do about that. Should I start watering it with a <3% hydrogen peroxide solution as opposed to regular filtered water? Not sure if long term that would harm the plant. Also please note that my cactus doesn't have gnats, so I don't think it's a soil quality issue.

  • Fun observation-- I've also sprayed the 3% hydrogen peroxide solution on my cactus as a preventative measure. It doesn't foam on the cactus soil, but only when I sprayed the sweet potato soil the first couple of times, which I think is pretty indicative of the problem. – Ryan Oct 26 at 11:40
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This site has some good information on organic control; personally, I prefer the vinegar & sticky traps that they mention. I would use at least one of the methods documented in this site (which you already have - hydrogen peroxide). For clarity for future readers (in case that site goes down), these methods are:

  • Reducing the frequency of watering so that the soil dries thoroughly between waterings
  • Putting sand on top of the soil (controls larvae)
  • Vinegar traps (controls adults)
  • Potato traps (controls larvae)
  • Homemade or purchased sticky traps (controls adults)
  • Diatomaceous earth (controls larvae)
  • Watering with a hydrogen peroxide solution (controls larvae)
  • Sprinkling cinnamon onto the soil (controls larvae)
  • Bt (Bacillus thuringiensis v. israelensis) - (controls larvae)
  • Beneficial nematodes

So, you have a ton of options, many of which are free or low-cost and some of which you presumably already have in your house.

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  • I did not know H2O2 could be used here to water.. what about watering with vinegar? – ina Nov 22 at 2:32
  • I wouldn't try watering with vinegar because that will lower the pH in a rather shocking (to the plant) manner. You could use vinegar traps, which won't affect the soil's chemistry. – Jurp Nov 22 at 14:16
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I have these fungus gnats also often, and especially in indoor plant containers they are a nuisance. Very annoying. When I am fed up with them (i.e. become a pest) I always use a biological control method to get rid of them. It is a treatment with nematodes (Steinernema feltiae), which parasite on the larvae of these fungus gnats. You can buy them online, I recently bought them and then the next day they are in the mailbox. You'll have to dissolve them in water and then water your plants with it. Keep the soil a bit moist for a week or two. Coincidentally, I just treated al my plants last week, and now a week later I see only still a few of them flying around. I am sure they'll disappear within another week from now, I have used it before.

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