In the Garden with Pat Rubin

Bee garden writer Pat Rubin writes about everything that grows, from flowers and trees to vegetables and lawns. Pat volunteered for several years as a Placer County Master Gardener and has written about gardening for many national and regional publications. In addition to gardening, she spends time raising and showing miniature horses and miniature donkeys.

In the Garden will include news, events, advice and other gardening tidbits. Pat will also answer reader questions.

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April 25, 2008

Drat those gnats

Q: I have gnats in one of my houseplants. Any suggestions on how to get rid
of them?
Phyllis Percival, Sacramento

A: They’re more of a nuisance than they are destructive to the plants, but I agree, the gnats need to go. I’ve had them in house plants, too.

Called fungus gnats, they aren’t directly harmful to plants, says Seth Taylor, sales manager for Capital Nursery on Freeport Boulevard in Sacramento. “They’ll lay eggs in the soil, and the larva feed on decomposing matter in the soil and on some of the plant’s root hairs. They aren’t harmful to the plants unless you have a large infestation or a really sensitive plant.”
The gnats we see flying around live only a few weeks, but the larvae can live for two or three months in the soil.

Fungus gnats show up when we water too much. They’re attracted to the moisture, Taylor says.

“To control the gnats, let the surface of the soil dry out between waterings, and they’ll start to disappear,” Taylor says. Adding a layer of sand to the soil surface can also help, he says, since it dries out so quickly. Next, make sure the plants don’t sit in saucers of water. He suggests using sticky whitefly traps placed near the plants to attract the gnats.

Chemical controls are available. Bonide Systemic Houseplant Insect Killer, which sells for about $6, is a granule that kills fungus gnat larvae.
For an organic approach, you can use beneficial nematodes, which are microscopic organisms that can control fungus gnat larvae.

The nematodes are available in a sponge-type product that costs about $17.
“It’s a sponge impregnated with the nematodes,” Taylor says. “You put it in water and the nematodes are released into the water and then you water the plants with it. A little goes a long way.”

Bottom line, however, is to cut back on the water. Not only does overwatering attract fungus gnats, it will eventually kill the plant.

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