Expert: Will Creed - 9/15/2005

Question
My house is infested with knats. How or what can I use to get rid of them without harming my plants?  Someone told me to spray the plants with a small amount of dishwashing liquid and water, but that didn't work or I did it wrong. Help!  They're eating my plants and annoying my family.  

Answer
Hi Joan,

Adult fungus gnats fly around and are an annoyance, but they are not harmful to people. Each gnat lives for about 5 days. The trick is to get rid of the next generation - the gnat larvae that live in the top layer of the soil. The larvae feed on decaying organic matter. Decaying pine bark in potting mixes and decaying plants roots feed the larvae.

Try to keep the soil as dry as possible. Remove all loose soil from the surface and put a light layer of coarse coir (coconut husk) or sand or diatomaceous earth on the soil surface. These substances have sharp edges that carve up the larvae. (Recent studies indicate that fine-textured peat moss also deters gnat larvae.)

Another safe technique is to place ½ inch slices of raw potato on the surface of the soil to attract the larvae. After a day or so, discard the slices along with the larvae inside. Repeat this until there are no more larvae in the potato.

For more serious infestations try Knock-Out Gnats to treat fungus gnats available from Gardens Alive for about $20. See

http://www.gardensalive.com/item_display.asp?ProductNumber=1962

Another bio-control method is Gnat Not, a parasite that destroys gnat larvae and other soil pests. It comes on a sponge in plastic (5 weeks shelf life) that is rinsed into water and applied to the soil. For information, go to http://www.goodbug-shop.com/gnatnot.htm

Detection trick: Add a little water to the soil and then look very closely for tiny fungus gnat larvae swimming in the water as it pools on the surface. You need good light and good eyes to see them. If you don't, then your plant is probably gnat free.

Prevention is often the best remedy. Use sterile potting mixes that are free of bark chips. The potting mix should have ample drainage material, such as perlite so that it drains well and allows the soil to dry out frequently. Fungus gnats can nearly always be traced back to overwatering and/or poor soil quality.

Please let me know if any of this is unclear or if you have any additional questions.

Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

You can E-mail me directly at: wcreed@HorticulturalHelp.com

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