Plant care guide
ZZ Plant Gnats: Control Them by Watering Less
Updated · First published
Why does my Zz plant have fungus gnats?
Small dark flies running over a ZZ plant's mix or hovering near it are fungus gnats. Their larvae need mix that stays damp, so gnats in a ZZ pot almost always mean it is being watered more often than a water-storing rhizome plant needs.
What to do first
Stop watering until the mix has dried out completely, empty any saucer or cachepot, and trap adults with yellow sticky cards. If larvae persist, drench the mix with a Bti product labelled for houseplants and repeat as the label directs.
Gnats around a ZZ plant are irritating, but they are also useful. They tell you the mix is staying wet, which matters more to a ZZ than the flies do. University of Maryland Extension calls fungus gnats annoying but harmless to indoor plants, and says overwatering feeds the fungi their larvae eat.
Confirm they are fungus gnats
Clemson HGIC describes adults as delicate flies about 3 mm (1/8 in) long that run across the mix or fly weakly near it (HGIC 2252). Check for:
- Larvae in the top of the mix. The UC IPM Pest Note describes a shiny black head and a whitish to clear, legless body.
- A potato test. UC IPM says raw potato chunks pushed into the mix attract larvae.
- Another source. Iowa State Extension says gnats do not always come from plants; check unused drains and condensation trays (Iowa State: Insect Pests). UMD suggests covering a pot or drain with cheesecloth for several days to see where they emerge.
Why a ZZ pot breeds them
A ZZ stores water. UF/IFAS says its thick rhizomes, fleshy stems and waxy leaflets give it extremely low water loss, and that it can survive 3 to 4 months without watering (EP252). That means a ZZ uses water slowly, and its mix stays damp for a long time after each watering.
NC State Extension advises watering only once a month in winter, and twice monthly in summer only if the mix has dried out completely. It adds that wet feet are not tolerated. Iowa State says a thorough watering every 7 to 14 days usually works, but problems develop when the mix stays constantly wet (Iowa State: Zamioculcas).
Iowa State calls fungus gnats a good indicator of overwatering. UC IPM notes that larvae also feed on decaying organic matter, so a pot with rotting roots can keep feeding them.
What to do
- Let the mix dry completely. Clemson says letting the soil dry between waterings kills the larvae. NC State adds that a ZZ can be treated much like a cactus or other succulent.
- Empty the saucer and cachepot. UF/IFAS warns against letting ZZ roots sit in water.
- Trap adults. UC IPM suggests cutting yellow sticky traps into small squares on sticks in the pot. UMD uses the same traps to monitor numbers.
- Treat larvae with Bti. UC IPM, Clemson and UMD all list Bacillus thuringiensis subsp. israelensis (Bti) for larvae in the mix. UC IPM says repeat applications are commonly needed.
- Consider nematodes. UC IPM names Steinernema feltiae for moist mix at 16–32 °C (60–90 °F). On a ZZ you will be drying the pot down, so Bti usually fits better.
- Skip sprays. UC IPM says insecticides are rarely warranted for fungus gnats in homes.
Check the rhizomes
UC IPM stresses that wilting is far more often caused by watering problems, poor drainage or root decay than by gnats. If stalks are yellowing, leaning or soft at the base, slide the ZZ out of its pot. University of Maryland Extension describes rotted roots as brown to black and soft or mushy, with browning or blackening at the crown (UMD: Root Rots of Indoor Plants). It advises clean pots, sterile potting mix, and discarding severely infected plants.
Pets and people
UF/IFAS says all parts of the ZZ plant are toxic if ingested. Keep sticky traps, Bti products and trimmings out of reach of pets and children.
Prevent a return
- Use bagged, pasteurized potting mix. UC IPM warns that poorly composted organic matter often carries gnats.
- Water by dryness, not the calendar. NC State’s rule is to water only once the mix has dried out completely.
- Keep a gnat pot apart. UC IPM advises against moving it near other plants while adults are emerging.
Our fungus gnat guide covers the life cycle and treatments in depth. For watering and light, see the ZZ plant care guide or the ZZ plant hub.
Related guides
Other zz plant problems we cover: brown spots, mealybugs.
References
- UC Statewide IPM Program: Pest Notes: Fungus Gnats (Publication 7448)
- University of Maryland Extension: Fungus Gnats on Indoor Plants
- Clemson Cooperative Extension HGIC 2252: Common Houseplant Insects & Related Pests
- NC State Extension Gardener Plant Toolbox: Zamioculcas zamiifolia
- UF/IFAS Extension EP252: Cultural Guidelines for Commercial Production of Interiorscape ZZ
- UF/IFAS Gardening Solutions: ZZ Plant
- Iowa State University Extension and Outreach: Zamioculcas
- Iowa State University Extension and Outreach: Diagnosing Houseplant Problems Caused by Insect Pests
- University of Maryland Extension: Root Rots of Indoor Plants
How this guide was put together: UC Statewide IPM Program, University of Maryland Extension, Clemson Cooperative Extension, NC State Extension, UF/IFAS Extension and Iowa State University Extension guidance.