Question
After reading previous questions, I discovered that I am overwatering my dracaena marginata. Now that I know better, I can stop. In the meatime, I have an associated pest problem. There are gnats around my houseplant and on the soil. How can I safely get rid of them? And are the gnats hurting my plant or just a symptom of the overwatering?

In earlier answers spraying with Lysol and watering with rubbing alcohol were recommended. I thought that watering with anything but water would kill the roots? I guess I was wrong. Please offer reassurance that these methods are a safe way to go.

Thank you for your time.

Answer
Hi Beth,

There are other "Experts" on this site that offer advice that is based on personal experience, not professional practice. I am not responsible for their advice. I would never recommend spraying with Lysol or watering with rubbing alcohol. Both of these treatments are potentially toxic to the plants and the alcohol treatment simple compounds the problem. So let's start from scratch.

Adult fungus gnats fly around and are an annoyance, but they are not harmful to people. The gnats are usually a symptom of overwatering and overwatering is definitely harmful to your plants, although the gnats themselves are usually not.

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. Therefore, 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.)

Any liquid that is added to the soil will simply exacerbate the problem of soil that is too moist.

Another safe technique is to place ?inch slices of raw potato on the surface of the soil. 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

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.

To attract and capture the adult flying gnats, pour a little red wine into a small glass (cooking wine will work, if you don't want to waste the good stuff!) and leave it out for the day. Repeat the treatment as necessary.

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

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Will Creed, Interior Landscaper
Horticultural Help, NYC

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