This is for AutoPot growers with small black flies drifting up off the pots. Bottom feeding keeps the surface drier than hand watering does, which works against gnats, but it won't clear a population that's already breeding, and a few points in every run keep the top wet anyway. Close those gaps, trap the adults, and release soil predators at the stage when the media can hold them.
The short answer
- Once the system is on, nothing gets poured over the top of the pot.
- Cover the media with 1 inch of coarse sand or fine gravel.
- Hang yellow sticky cards for adults and use a raw potato slice to check for larvae.
- Put soil predators in at potting up, while the media is still damp from the first soak.
- Keep live biologicals and larvicides out of the reservoir.
Why a dry top helps, and where it falls short
Fungus gnat larvae need continuously moist media and feed mainly in the top 1 to 2 inches, so extension services treat a drying surface as the primary control. An AutoPot tray fills about an inch deep at the base of the pot and the plant drinks it down before the next fill, so water moves up from below and the surface is the driest layer in the pot.
That helps, but bottom watering alone doesn't cure an infestation that's already established. Purdue Extension also notes larvae can turn up at the bottom and along the edges of pots, not only near the surface.
| Stage | How long | Where |
|---|---|---|
| Egg | About 3 days at 75°F | Damp media, at or near the surface |
| Larva | About 10 to 14 days | Mostly the top 1 to 2 inches; also pot bottoms and edges |
| Pupa | About 4 to 6 days | In the media |
| Adult | Lives about 7 to 10 days | Media surface and lower leaves |
UC IPM puts one generation at about 17 days at 75°F, faster when warmer. NC State notes indoor generations overlap, so every stage is present at once, and anything that hits only one stage has to keep working through several generations.
Where an AutoPot's top stays wet
The establishment phaseGnats show up in the first weeks after potting up.
- Cause. Before the system goes on, AutoPot has you water each pot from the top, outside the tray, until it drains. AutoPot USA's rule is to keep top feeding until the leaves reach past the pot rim, so this is the one stretch of the run where the surface is wet on purpose.
- Fix. Water again only when the pot feels light, since a heavy pot still holds moisture. Model-by-model timing is in How to Set Up AutoPot for Your First Run. This is also the window for releasing soil predators, covered below.
A missing or thin drainage layerMedia stays dark and damp after the system is on.
- Cause. AutoPot calls the drainage layer crucial: it lifts the substrate above the tray's fill line so the pot doesn't become oversaturated. Without one, the bottom of the mix sits in every fill.
- Fix. At the next potting up, add an AirBase or up to 1 inch of pH-stable clay pebbles, pea gravel, Growstones, or Perlite No. 3. The 9 and 13 gal XXL fabric pots are the only AutoPot pots meant to run without one.
A peat-heavy or compacted mixThe mix is mostly peat, or straight coco or soil with little perlite.
- Cause. AutoPot warns against 100% coco or soil and peat-heavy mixes because they compress. Purdue Extension adds that female fungus gnats are highly attracted to media containing peat moss and pine bark.
- Fix. At the next run, blend about 50% perlite or clay pebbles into the soil or coco.
Organic top dressing or living soilCompost, castings, or dry amendments on the surface.
- Cause. UConn Extension notes that no potting mix is immune, and that adults are drawn to microbially active media such as compost and vermicompost.
- Fix. Start sticky cards the day the dressing goes on. Liquid supplements for living soil go in by tray feeding, not over the top; AutoPot hasn't yet published its guidance on dry amendments.
Don't drench an established plant from the top. Salt builds up in the top inch of the substrate, and AutoPot warns that water poured through it can damage roots. That applies to gnat treatments too; the reasoning is in finishing a run without top watering.
Physical controls that suit a dry top
A 1-inch layer of coarse sand or fine gravel over the media discourages egg laying. Spread it with the pot out of the tray and wipe the pot sides and base afterward, the same way AutoPot has you clean pots before they go in, so no grit reaches the valve.
Yellow sticky cards catch adults and show whether numbers are falling. Clemson Extension suggests hanging them upright just above the canopy to monitor, and laying them flat over the media to catch adults as they emerge from pupae. For larvae, press a slice of raw potato into the media, leave it one to two days, then lift it and look underneath.
Traps only thin the adults. UC IPM points out that gnats spend most of their lives as larvae and pupae in the media, so the controls that end an infestation target those stages.
Soil predators, and when an AutoPot can support them
Happy Hydro's What's eating your plants? page has a fungus gnat recommendation and the full list of soil options. The AutoPot-specific catch is moisture: every one of them works in the media and asks for damp conditions at release.
The Stratiolaelaps mite page says to keep the top layer lightly moist and not let it dry out completely for the first two weeks. The rove beetle instructions start with watering the media about 10 minutes before release, and the Steinernema feltiae nematode drench calls for moist media before and after.
In an AutoPot, that points to potting up. The establishment soak and top feeding supply the moisture those products ask for, at the one stage AutoPot already waters from the top. Sprinkle predatory mites on after the first soak, or use the nematode drench, mixed per its label, as that watering.
On an established plant, the mite and beetle go on the surface dry, with nothing to mix. For a drench-type product, tray feeding is the only route AutoPot supports for liquids once the system is on; the steps are in feeding an AutoPot. Neither AutoPot nor the product page says how far up the pot a tray-fed drench carries, so count on it reaching the lower root zone, not the surface layer where most larvae feed.
Nothing live goes in the reservoir. The nematode page says not to run the mix through fine filters, and every AutoPot reservoir feeds through an inline filter.
Parts that fix this
-
Stratiolaelaps scimitus
Predatory mites that settle into the top of the media; release at potting up.
-
Dalotia coriaria rove beetles
For heavy pressure; winged adults spread themselves from pot to pot.
-
Steinernema feltiae nematodes
A drench that fits the establishment soak, before the system goes on.
Questions
Oklahoma State Extension allows 3 to 4 weeks of drier surface conditions plus a sand or gravel layer to get gnats in check. Adults already flying live out their week or so; the drop you're watching for is fewer new ones on the cards each week.
Fungus gnats look like tiny mosquitoes, with long legs and clear wings. Shore flies are stockier, like a small housefly, with smoky wings marked by five white spots, and their larvae feed on algae. UConn Extension manages shore flies mainly by preventing algae growth.
Not in the reservoir: we found no AutoPot or manufacturer guidance for BTI in AQUAvalve lines. In the tray, AutoPot's test for any product is whether it dissolves fully and stays dispersed. Mosquito bits are granules on a corn-cob carrier, which doesn't dissolve, so anything steeped from them needs straining before it goes near the valve.
Possibly. The Stratiolaelaps page says very dry rooms may need a second release, and an AutoPot surface runs drier than a hand-watered pot. University of Arizona Extension gives 2 to 4 weeks as the interval to reapply these mites if pressure continues.