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Houseplants with Fungus Gnats

Gnats like moist soil

By: Sandy Feather 2006
Penn State Extension

Q. A number of my houseplants spend the summer outside on our patio. After I brought them back into the house, I noticed a lot of gnats hovering around the plants, particularly when I'm watering them. They seem to be getting worse. Can you tell me what they are and how I can get rid of them?

A. Your annoying little guests are probably fungus gnats. These common houseplant pests are primarily nuisance insects that can be seen moving across the soil surface and on the foliage of potted plants. They will fly around your house, in your face and drown themselves in whatever you are drinking. They also like to congregate on windows. Fortunately, the adults do not damage plants. Fungus gnat larvae primarily feed on decaying organic matter in the potting soil.


Other Problems

Gnats can also feed on plant roots, which may cause problems with seedlings and cuttings. Mature plants are less likely to be affected, unless the fungus gnat population is extremely high.

 


Solutions to Gnats

You can repot plants into fresh potting soil, which will get rid of existing eggs and larvae. Covering the soil surface with a layer of sand discourages egg laying by adults. Avoid overwatering, and do not allow plants to sit in saucers filled with runoff water.


Watering Techniques with Gnats

If possible, allow the soil to dry out pretty thoroughly between waterings. Never take it to the point where the plant wilts, though. Fungus gnat larvae require a moist environment and may die if you can allow the soil to dry out thoroughly.


Biological Controls like Bt

There is a Bacillus thuringiensis (Bt) formulation sold under the name Gnatrol that controls fungus gnat larvae. It is used as a soil drench. Ultra-fine horticultural oil may be sprayed on the foliage to control the adults. Bayer Advanced All-in-One Potting Soil contains the insecticide Merit (imidacloprid) that is also labeled to control fungus gnats. You can use it when you repot the plants instead of plain potting mix.


Sandy's Garden FAQ

Q. I saved seeds from some of the annuals I grew in my garden this summer, including four-o'clocks, castor beans and marigolds. What is the best way to store them?

A. Once the seeds have dried, place them in an airtight container, such as a film canister or small jar. You can add a packet of silica gel -- those packets that come in new shoes or electronic equipment. Then store the jar in your refrigerator for the winter.

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