Plant care guide
Boston Fern Fungus Gnats: Control Without Drying It Out
Updated
Why does my Boston fern have fungus gnats?
Fungus gnats breed in moist potting mix rich in organic matter, which is exactly what a Boston fern grows in. The adults are a nuisance. The larvae in the mix feed mostly on fungi and decaying matter but can damage roots when numbers are high.
What to do first
Confirm the flies are coming from the fern with yellow sticky traps or potato slices. Then keep the mix evenly moist but never soggy, let only the surface dry, empty saucers, and drench the mix with a Bti product to kill the larvae.
Fungus gnats are small, dark flies that hover around houseplants and run across the soil. University of Maryland Extension’s fungus gnat guide calls them annoying but harmless to most indoor plants. Their larvae breed in moist soil with lots of organic matter.
That makes a Boston fern (Nephrolepis exaltata) a likely host. NC State Extension says the fern needs moist soil that should never be allowed to dry out. Clemson Cooperative Extension says ferns grow best in organic potting mix. The usual advice to “let the soil dry out” needs adjusting for this plant.
Confirm it’s fungus gnats, and that they come from the fern
Adults: Clemson’s houseplant pests factsheet describes delicate flies about 3 mm (1/8 in) long. They run across or fly near the soil surface. They are weak flyers and are drawn to light, so you may see them at windows.
Larvae: whitish with shiny black heads, up to about 6 mm (1/4 in) long, Clemson says. UC IPM’s fungus gnat pest note adds that larvae may leave slime trails on very moist mix.
Find the source:
- Yellow sticky traps in or near the pot catch adults. UMD and UC IPM both recommend them for monitoring.
- Raw potato slices, cut side down on the mix, attract larvae. Check under them after a few days. UC IPM describes this test.
- Cover the pot with fine screen or cheesecloth for a few days. UMD suggests this to see whether flies emerge from it. Gnats can also breed in drains with moist debris.
Are the gnats hurting your fern?
Usually not much. UC IPM says adults don’t damage plants or bite people. Larvae mainly eat fungi and organic matter, though they also chew roots. In large numbers they can stunt growth, especially in seedlings and young plants. Clemson notes that a heavily infested plant may lose leaves from root feeding.
UC IPM also warns that a wilting plant is far more often suffering from too much or too little water, or root rot, than from gnats. Check the roots if your fern is declining.
Get rid of them without drying out the fern
- Water less often, not less thoroughly. Clemson’s indoor ferns factsheet says ferns should never dry out completely. UC IPM recommends letting the surface of container soil dry between waterings. For a Boston fern, let just the top of the mix dry before watering again. Wisconsin Extension advises keeping the mix evenly moist but not saturated.
- Remove standing water. Empty the saucer or cachepot after watering. Clemson says not to let water stand in fern pots, as it damages roots. Standing water also feeds gnats.
- Drench the mix with Bti. Bacillus thuringiensis subspecies israelensis (Bti) kills fly larvae in the mix. UMD and Clemson both recommend soil drench products labeled for indoor plants. UC IPM notes that Bti does not persist indoors, so repeat about every five days. Bt sold for caterpillars does not work on gnat larvae.
- Trap the adults. Sticky traps and potato slices reduce numbers while the drench works on the larvae. Replace the potato slices every few days.
- Keep going for a few weeks. UC IPM says a generation can develop in about 17 days at 24 °C (75 °F). Larvae already in the mix will keep emerging as adults for a while.
About sprays: Clemson warns that pesticide sprays injure many ferns. UC IPM says insecticides are rarely warranted for fungus gnats in homes. If you use any product, check that the label covers houseplants and test it on a small area first.
Stop them coming back
- Water by feel. Keep the mix moist, never soggy. NC State advises reducing watering in winter, when the fern is dormant and overwatering can lead to root rot.
- Use fresh, pasteurized potting mix when you repot or divide. UC IPM warns that incompletely composted organic matter is often already infested.
- Improve drainage. UC IPM suggests adding more perlite or sand to the mix. Clemson says fern potting mix should be porous and drain quickly.
- Don’t overfeed. Wisconsin advises half-strength feeding every 4 to 6 weeks in active growth, cut back sharply in winter. UC IPM warns against heavy organic fertilizers, which attract gnats.
- Raise humidity around the fern, not in the pot. NC State suggests a tray of wet pebbles. Wisconsin suggests double potting in moist sphagnum. Neither adds more water to the root zone.
For a wider view of gnats on any houseplant, see fungus gnats in houseplants. For watering and humidity, read the Boston fern care guide.
Related guides
Other boston fern problems we cover: brown fronds, shedding leaflets, spider mites, yellow leaves, root rot.
References
- UC IPM Pest Notes: Fungus Gnats
- Clemson Cooperative Extension: Common Houseplant Insects & Related Pests
- University of Maryland Extension: Fungus Gnats on Indoor Plants
- NC State Extension Gardener Plant Toolbox: Nephrolepis exaltata
- Clemson Cooperative Extension: Indoor Ferns
- University of Wisconsin–Madison Extension: Boston fern, Nephrolepis exaltata ‘Bostoniensis’
How this guide was put together: UC IPM, Clemson Cooperative Extension, University of Maryland Extension, NC State Extension and University of Wisconsin–Madison Extension guidance.