Pests

Fungus Gnats in Greenhouse Seed Trays: How to Break the Cycle

Fungus gnats are the small, dark, slow-flying flies that lift off your seed trays every time you water. The adults are mostly a nuisance. The larvae, living in the top layer of wet mix, are the real problem: they feed on organic matter and fungi and can damage the roots and stems of young seedlings and cuttings. A greenhouse full of warm, moist propagation trays is their ideal home. The fix is less about spraying and more about changing the conditions they need.

First, make sure they are fungus gnats

Several small flies turn up in greenhouses. Getting the ID right matters, because the controls differ.

Shore flies feed on algae, so their control is mostly about removing the algae (drier benches, cleaner floors). Fungus gnats are about the mix itself. If you are not sure, a hand lens and a sticky card will help you compare.

A simple larvae check: push slices of raw potato into the surface of a few trays. After a few days, lift them and look for tiny translucent larvae with shiny black heads on the underside.

Why seed trays are a perfect nursery

• Constant moisture: propagation trays are often kept wet at the surface, which is exactly where gnats lay eggs.
• Warmth: heat mats and warm benches speed up their life cycle.
• Organic mixes: peat, coir, compost and bark all provide food for larvae.
• Domes: humidity domes keep the surface wet longer.
• Contaminated mix: bags left open or stored damp can arrive already infested.

Step one: change the watering

For most growers this does more than anything you can buy.

1. Water less often, more thoroughly. Let the top layer of mix dry out a little between waterings while the root zone stays moist. Lift trays to judge weight instead of watering on a schedule.
2. Bottom water. Set trays in a solid tray of water for a short time, then drain off the excess. The surface stays drier than with overhead watering.
3. Take domes off sooner. Once most seeds have germinated, vent or remove domes so the surface can dry.
4. Improve airflow. A small circulation fan over the bench dries the surface and strengthens seedlings.
5. Drain saucers and trays. Standing water underneath keeps the bottom of the mix soggy.

Step two: trap the adults and monitor

Yellow sticky cards catch adult gnats and give you a running count. Place them just above the mix surface, angled or laid flat near trays, not high up in the roof. Replace them when they fill up. If the count drops steadily over a few weeks, your other measures are working.

Traps alone will not end an infestation, because larvae keep developing in the mix. Think of them as a gauge, plus a modest reduction in egg-laying adults.

Step three: go after the larvae

Bti drenches

Bti is a bacterium that targets the larvae of flies such as mosquitoes and fungus gnats. It is sold in products for mosquito control and in products labeled for fungus gnats in potting mix. Growers commonly use it as a soil drench. Use only a product whose label covers this use, and follow its directions for rate and frequency. It works on larvae, not adults, so repeat applications are usually needed to catch new hatches.

Beneficial nematodes

Steinernema feltiae nematodes are widely sold for fungus gnat larvae. They are applied as a drench into moist mix. They are living organisms, so storage, water temperature and how soon you apply them after delivery all matter. Follow the supplier's handling instructions closely.

Predatory soil mites

Predatory mites that live in the top layer of the mix (often sold under names like Stratiolaelaps or Hypoaspis) feed on gnat larvae and other small soil pests. They suit growers who want ongoing biological control in a propagation area. See our biological pest control guide for how to introduce beneficials without wasting them.

Step four: barriers and better mix

• Top dressing: a thin layer of coarse sand, fine grit or vermiculite on the surface dries quickly and makes egg-laying less attractive. Keep it thin so you do not bury small seeds.
• Fresh mix, stored closed: keep bags sealed and off the greenhouse floor. Many growers store opened mix in lidded bins.
• Do not reuse infested mix for new seedlings without treating or composting it properly first.
• Clean as you go: remove dead leaves, fallen seedlings and algae from benches and floors.

A note on cuttings and plug trays

Cuttings rooting in a warm, humid propagator are often more vulnerable than seedlings, because the base of the stem is soft and the mix is kept constantly moist while roots form. Use a free-draining rooting mix with plenty of perlite, avoid letting cuttings sit in water, and vent the propagator a little each day once roots begin to form. Plug trays with small cells dry out faster, which helps, but they also hold less buffer if you let them go too dry, so check them more often rather than watering heavier.

What to avoid

Also avoid broad-spectrum insecticides as a first move if you use or plan to use beneficial insects elsewhere in the greenhouse. They can wipe out your helpers along with the gnats.

A simple four-week plan

Prevention checklist for next season

• Store seed-starting mix in closed bins, off the floor.
• Clean and sanitize trays and inserts before reuse.
• Start with a free-draining mix suited to seeds.
• Plan to bottom water from day one.
• Put a couple of yellow cards near the propagation bench from the start.
• Run a small fan over the bench once seedlings emerge.

The bottom line

Fungus gnats are a moisture problem first and a pest problem second. Dry the surface, water from below, open the domes and add airflow, then use sticky cards to track progress and a labeled larval control to clean up what remains. Do that consistently for a few weeks and the clouds over your seed trays fade out.

If seedlings are still struggling once the gnats are gone, check our guide to common seedling problems.