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Garden Q&A: Overwatering breeds fungus gnats

| Saturday, Nov. 2, 2013, 6:29 p.m.
Jessica Walliser
Potted houseplants, like this young bay laurel, are prone to developing infestations of fungus gnats if overwatered.

Question: I am having a problem with gnats in the dirt of my houseplants. My plants were fine until I brought a new one home from the nursery. They are not killing the plants, but quite the nuisance flying around my house. I've tried sprays, layering sand on top of the soil, removing an inch of the soil and replacing with diatomaceous earth, and completely replacing the soil with organic mix. My last attempt was to remove the plant from the soil and pour boiling water through the dirt hoping to kill the gnats and their eggs. I then covered the soil with plastic wrap for a week or so. I'm not sure if it has worked. Any suggestions? I enjoy having live plants in my home and would hate to go to artificial/silk.

Answer: When I was a kid, my older sister had a scrawny Norfolk pine in a mustard-colored, glazed ceramic pot sitting on a table in her bedroom. I never much cared for it. It wasn't the kitschy '70s container that made it unlikable, nor was it the plant itself. My distaste for that puny pine stemmed from the scores of little bugs plaguing it. Every time you walked past that pot, a cloud of tiny, black flies exited the soil surface. Inevitably one ended up in your nose … or eye … or hair.

I understand your frustration with this tiny insect.

Adult fungus gnats, like those in my sister's room and at your house, become problematic when their populations reach conspicuous levels, especially on houseplants. Individually, they are barely noticeable; by the hundreds, however, they are hard to miss. Their gregarious nature makes them a classic example of a nuisance pest.

Mature gnats measure a mere eighth of an inch and live for about a week. During this time, females lay eggs in soil fissures. The resulting translucent, minute larvae feed largely on the assorted fungi growing in the potting soil, though they also can feed on fine roots and plant debris. In a few weeks, they pupate into adults within the soil and the cycle continues, with several generations occurring together at any given time.

What my mother never seemed to figure out is that overwatering inevitably leads to a fungus gnat issue. Constantly damp soils promote fungal growth, which serves as an excellent food source for the larvae. Simply cutting down on watering should readily solve your problem.

Water infested houseplants deeply — but not frequently — and only when the soil is dry, even if that means only watering every two or three weeks. The life cycle of the gnats will be broken within a few months of reducing the frequency of watering. Be sure the pot itself has good drainage and the saucer underneath doesn't house standing water.

If changing your watering routine doesn't clear up the problem after a few months, re-pot all your house plants with new, sterile potting soil, gently removing as much of the old soil as possible without damaging the roots.

You also can trap adult fungus gnats on yellow or blue sticky cards placed an inch or two above the soil surface.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. She is the author of several gardening books, including “Grow Organic” and “Good Bug, Bad Bug.” Her website is www.jessicawalliser.com.

Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 503 Martindale St., 3rd Floor, D.L. Clark Building, Pittsburgh, PA 15212.

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