Eight plants, all in the same room. I have been bottom watering only for three weeks, yellow sticky cards above every pot, and I let the top of the soil dry out properly between waterings. I catch maybe 20 gnats a week on the cards and there are always more flying up when I move a pot. Adults do not bother me but I assume the larvae are eating roots. What breaks the cycle properly?
Check for a reservoir you have forgotten: a cover pot with standing water, a propagation jar, a saucer under the radiator. Eight plants in one room with a persistent population usually means one wet thing is breeding most of them.
Also worth knowing that gnat larvae mostly eat decaying organic matter and only chew roots when the population is huge. If your plants look healthy, this is an annoyance rather than an emergency, which takes the panic out of it.
The other half of the fix is physical: a 2cm layer of coarse sand or fine grit on the surface. Adults will not lay through it and emerging adults struggle to get out. Combined with a larvicide it is close to a permanent solution rather than a management routine.
Second this, but use actual horticultural grit rather than decorative pebbles. Big pebbles just create dark humid gaps, which is a gnat hotel.
@invoice_ivy · 2w ago · 3 replies
Sticky traps catch adults, which is the least important part of the population, and bottom watering keeps the top dry but leaves the middle of the pot permanently damp, which is where larvae live happily. What actually works is treating the larvae: a soil drench with a BTI product, the mosquito bits sort, steeped in water and used as your regular watering for three consecutive cycles. It targets larvae specifically and breaks the generation chain. Three weeks of trapping adults will never win because each female lays 100 or more eggs.
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@braise_or_bust · 2w ago
Do I need to keep bottom watering while doing the drench, or is that counterproductive?
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@invoice_ivy · 2w ago
Top water for the drench cycles so the treatment reaches the top few centimetres where the eggs are, then go back to whatever suits your plants. Three cycles about five days apart covers the life cycle.
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