How to get rid of fungus gnats in houseplants for good

Step by step: how to get rid of fungus gnats in houseplants for good

1. 1

   Find the infested pots

   Push a few 1/4 inch slices of raw potato into the soil surface of each plant. After 2 or 3 days, turn them over and look for tiny clear larvae with shiny black heads. Treat every pot that has them, and keep the rest away from the infested ones.

2. 2

   Let the top of the soil dry out

   Stop watering until the top 1 to 2 inches of soil feel dry to your finger, then water thoroughly and let it dry again. Dry soil kills eggs and young larvae and makes the pot unattractive to egg-laying females. Most houseplants tolerate this easily.

3. 3

   Empty saucers and cachepots

   Pour out water that collects in saucers or decorative outer pots within about 30 minutes of watering. Standing water keeps the bottom of the soil wet, which is where some larvae survive.

4. 4

   Set yellow sticky cards

   Push a yellow sticky card into each infested pot, close to the soil. Adults are drawn to the color and get stuck, so fewer females lay eggs. Replace the cards when they are covered.

5. 5

   Treat the soil with Bti

   Soak Bti granules or a crushed dunk in your watering water as the label directs, then water the plants with it. Bti kills the larvae but does not last long indoors, so repeat about every five days for a few weeks.

6. 6

   Repot plants that stay soggy

   If the mix is old, broken down or never dries, repot into fresh potting mix with extra perlite for drainage, in a pot with a drainage hole. Remove dead leaves, rotting roots and old bulbs, which feed the larvae.

7. 7

   Seal your bagged potting soil

   Close open bags of potting mix tightly or move them to a sealed bin. An open bag in a garage or basement can breed gnats and reinfest every plant you repot.

8. 8

   Keep going for a full month

   At room temperature the fungus gnat life cycle takes three to four weeks, so keep the soil dry on top, the cards out and the Bti going for at least that long. The catch on the cards should fall steadily week by week.

How do you know they are fungus gnats?

Fungus gnats are tiny black flies with long legs and long antennae that look like miniature mosquitoes and hang around plant pots and windows. They do not bite.

Colorado State University Extension describes adults as about 1/8 inch long, delicate and black, with a Y-shaped vein on the wing, and notes that they are weak fliers that stay near the infested plant. UC IPM adds that their long antennae set them apart from shore flies, which are more robust with short, bristle-like antennae.

Not sure? Our guide to getting rid of gnats in the house has a full side-by-side comparison. Note that the University of Maryland Extension says fungus gnats can also breed in drains with moist debris, so if you have no plants, test your drains.

Which fungus gnat treatment works best?

Drying the soil is the most important step; Bti and sticky cards make it faster. Use them together, because each one targets a different life stage.

UC IPM notes that Bti products such as Mosquito Bits and Gnatrol are sold in garden centers, and that because Bti does not persist indoors, infested pots may need treatment at about five-day intervals. Colorado State recommends insecticides only if gnats persist several weeks after you have changed your watering. If you use one, choose a product labeled for houseplants and follow it exactly: the EPA notes that using a pesticide in a way its label does not allow is against federal law.

Why are fungus gnats worse in fall and winter?

Because plants use less water when days are short and cool, so soil watered on a summer schedule stays wet for days, which is exactly what fungus gnats need.

Colorado State lists three reasons: plants brought in from outdoors carry gnats inside, people notice them more when they are indoors more, and, most of all, the potting mix stays moist longer. Old mix also holds more water as it breaks down. Water by touch, not by calendar, from October to March, and check plants that spent the summer outside before you bring them in.

Mistakes to avoid

• Watering on a fixed weekly schedule in winter, so the soil never dries.
• Using only sticky cards, which catch adults while larvae keep developing in the soil.
• Applying Bti once and stopping; it breaks down within days indoors.
• Leaving water standing in saucers and decorative pots.
• Repotting into the same old, waterlogged mix or into a pot with no drainage hole.
• Setting vinegar traps, which attract fruit flies far better than fungus gnats.