Question
QUESTION: I have a variety of houseplants and ever since I've gotten them, tiny gnats have been all over the house (mainly around the plants though).  How can I kill them safely?  I don't want to spend a lot of money (I would prefer a home remedy that works if there is one) and I also have kids and pets at home so I need something safe for them.  

Thanks.

ANSWER: Hi there,

When you have fungus gnats in several plants, it is  a sure indication you are not allowing your plants to dry out enough in between waterings. Fungus gnat larvae live in the top surface of the soil, develop wings and then fly out of the soil as adults. The adults live for about a week before dying. The key is the eliminate the next generation by treating the larvae in the soil.

The first thing you need to do is remove all loose soil that is not in direct contact with the roots. This will eliminate many of the gnat larvae because they mostly live in the surface soil. After removing all loose soil, you can add a thin layer of builders sand. The sharp-edged particles of sand work physically (not chemically) by carving up the tiny larvae as they move through the soil.

The most important thing to do is to allow the soil to dry as deep into the pot as possible. How deep will depend on the plant species and the size of the pot. Most folks do not allow the soil to dry deep enough into the pot and that is where they get into trouble with root rot as well as gnat larvae. Using a pot that is too large also contributes to this problem.

Gnat larvae depend on lots of moisture to survive. Deprive them of that moisture in the surface soil and they will die without any further treatment. More importantly, by allowing the soil to dry out more, you will save your plants from root rot, which is fatal.

If all this fails, and it shouldn't if done properly, then you can use a biological agent (Bt) to kill the larvae. It is found in a product called Gnatrol that is sold through Amazon.

I have written an article on treating indoor plant pests that I will email for free to you (or anyone else) who sends a request to me at [email protected].

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

If this information has been helpful, please click the Rate Volunteer bar below and enter a rating and nomination for me. I am a volunteer on this site so Ratings are the only compensation I receive for answering plant questions.

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Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

Visit my website at: A link to HorticulturalHelp.com



---------- FOLLOW-UP ----------

QUESTION: Thank you for your answer...I wonder though as some of my plants are tiny (seedlings) if it would be a bad idea to put sand on them.  (I'm constantly trying to get them to grow so I keep planting more seeds and they get transplanted a lot as they are little plants)  And I have some plants that are in rather large containers, so it would take a lot of sand to cover them.

Answer
Hi again,

I think you are relying too much on sand to solve the gnat problem. If you are growing seedlings then you are probably using a soil mix that is infested with gnat larvae. I suggest you use 3 parts of peat moss and 1 part perlite as your mix to start and grow seedlings. It is also important that you plant the seeds in small pots that dry out quickly.

If you start with clean potting soil that has no larvae, you will not have a problem and will not need sand or any other treatments. Go to the source by using a sterile potting mix.

~Will