Getting rid of flies coming from potted plants?

I have two pots of house plants that somehow a certain type of small flies are coming from the dirt in the pot. How do I get rid of these flies? I hope it does not require replacing the dirt.

 

They are annoying little gnats but not harmful. They live in the moist soil. Watering less frequently and letting the soil dry before watering is the best defense.

 

So, just curious. How do the soils have these gnats to begin with?

 

Soil gnats can be present in almost any soil, even potting mix that you buy from a garden centre. In fact, unsterilized commercial potting soil is one of the most common vectors for many plant diseases and infestations. Another way is using gnat-contaminated soil from your own garden to pot a new plant. Yet another way is to bring a gnat-infested plant into your house; the gnats come along for the ride and lay eggs in the soil of your other house plants. And finally, they can actually just fly into your house and burrow into the soil of your house plants.

Pilot Dane mentioned the most obvious way to control them: Don't overwater your plants. Gnat eggs must stay moist to hatch. Therefore, an easy way to kill them is to turn the soil with a trowel every day, exposing the top 2" to fresh air, causing the eggs and larvae to dry out. The eggs hatch on a one-week cycle, so you may have keep turning the soil for a couple of weeks to make sure that you get them all.

Once you think the gnats are dead, you should start to bottom-water your plants. Since gnats and their eggs live in the top 2" of the soil, watering from the bottom ensures the top part of the soil remains dry. So, fill your sink with enough water to reach halfway up your planter, and let it sit for at least 15 min. or until you can feel moisture at the 2" mark (by sticking your finger into the dirt 2" down and feeling for moisture). This watering method will also promote a deeper root system which will strengthen your plant.

Another way to kill them is with insecticidal soap. To make your own, mix one cup of vegetable oil with a tablespoon of dishwashing liquid. Put two teaspoons of that mixture into a cup of warm water and stir. Then, pour enough of the mixture onto the soil surface to saturate the top 2". Putting the insecticidal soap in a spray bottle and spritzing the leaves will also protect your plant against certain diseases such as leaf fungus and mold.

Of course, those methods can be tedious if you have a lot of plants. So, there's a much easier way to get rid of soil gnats. Remove the top inch of soil from the planter and replace it with an inch of sand. The gnats don't like sand because it doesn't hold moisture as well and it's harder to dig through. Any larvae that are already in the soil should either dry out or suffocate. It's not as effective as the methods that I mentioned above, but definitely requires the least effort.

Good luck and good gardening!

[Sorry about the wordiness of my reply. I've had to deal with pests in my own garden and house plants, so I know a fair bit about getting rid of them. ]

 

If they are really bad, you can probably find Gnatrol locally or on the 'Net. Works like a champ at killing any larvae and controlling them in the future.

When I was having problems after moving to a new home, I started baking all the potting soil I had before use and then kept it in sealed containers.

 

This is awesome, thank you! I will give these suggestions a try.

 

Thanks! Is this the one, Gnatrol WDG, you use?

 

Yep, but what I used was the older liquid style. This looks like a good change as it has a longer shelf life.