I have been at war with them for 6 months, and have run out of ideas.
I have 4 plants in my office: a Peace Lily, a Croton, an Ivy, and a Spider Plant. They are pretty good size.
Here's what I've tried...
1) I cut water down to a minimum, not allowing water to stand in the pan at the bottom. Watering about once a week.
2) I put sand in the top inch or so of soil, and covered that with cedar mulch.
3) I soaked mosquito dunks in the water. Then I started crumbling them into the water. I did this for a few months.
4) I put yellow sticky traps out, lying them flat on top of the mulch. They catch them in bunches.
I did all of the above for 3-4 months, and it did not help.
5) I quit using the mosquito dunks and instead started watering with a solution of 1 part 3% Hydrogen Peroxide to 4 parts water. I did this for a month or so.
6) Then I decided to try watering from the bottom up (with the Hydrogen Peroxide solution) for the last 2 months, thinking if I kept the top of the soil dry it would stop them. But they are somehow still multiplying.
The gnats are just as bad now as they were 6 months ago. I replace the sticky traps every month or so, and within week or two they are covered with dozens and dozens of them.
I'm out of ideas. Any other suggestions?
My last resort is to take them all home and wash all the soil off the roots and repot completely with new soil. But I've been putting off doing it because they are in heavy pots and it would be a significant undertaking. I work on the 11th floor of a highrise building, and would have to carry them a long way. Plus, I don't want them going into shock and losing all their leaves from stripping off all the soil.
They otherwise appear healthy.
Thanks for any suggestions.