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Gnats in Potting Soil

How do I get rid of household gnats in potting soil?

By Tammy from VT

Recent Answers

By Dona11/20/2010

First I soak planters in mild chlorine bleach then rinse. This water is poured over the gravel walk behind house and kills the moss/mold on pavement. We have plants lining walk and this has never bothered plants.
I have problems with soil gnats every year when I bring my household plants back into house after they've lived outside, plus, I have 3 cats. Finally found a solution ~5 yrs ago and works every time.

After repotting, add a thin layer of larger particle activated charcoal (can get at pet/fish shop) just on top of the soil. I then cover this with plastic mulch - some colors like blue or yellow enhance the colors of the plant. My favorite is blue,
the green plants and/or pink blooms seem more vibrant.

The charcoal takes care of the gnats (as I've read they like moist soil) and the cats won't disturb the plastic mulch. If you have really large planters, I'd try the volcanic rocks that you might use for landscaping or grills. Had these in some really large planters with taller inside bushes. Cat could get into planter, but, jumped out quickly when she touched the rock.
Happy indoor planting !!

By Jennifer02/07/2010

I was looking for a solution to my gnat problem. I opened a brand new bag of potting soil for some seedlings, and gnats came swarming out of the bag. I talked to my Aunt who has worked at a florist for years and said they microwave their soil for a few minutes and let it cool down before planting. It kills the larvae. Be careful, microwaving for too long can cause the soil to catch fire. Also the house will get a woodsy smell to it after doing this, she suggested microwaving some lemon juice to get rid of the smell.

By Julie01/29/2010

Those are fungus gnats, and they are difficult to get rid of completely. Thankfully they seldom really harm the plants, though they are a real nuisance. In addition to the tips below, hanging a sticky fly strip near your plants will trap a lot of them. Another tip I have seen is to put raw potato cubes near your plants and change them every three days. I think they are supposed to lay their eggs there instead of in the soil. (The worst infestation I ever had was when I had a 50 lb bag of potatoes near my plant lights in the basement, and there was a bad potato near the bottom that I didn't find for some time, so I guess they really like potatoes.)

By LARRY FLUITT01/28/2010

After you repot in clean soil, you might add a thin layer of sand to help keep them from getting to the potting soil. Best to use a growing sand that will be salt free.. a fast remedy is to mix up any liquid insecticide that is safe for the plant, even diluted down a couple of times and then use this mixture to water the plant, this will kill the larvae in the wet soil. There is also a product called Gnatrol you can use. It is Bt and only effects the larval stage so is slower to gain control.

By Dorothy Myers [17]01/28/2010

I have this problem no matter how many times I've changed the soil. I have no solution as of this date.

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Archive: Gnats in Potting Soil

By Ellen Brown

Question:

Has anyone ever had problems with gnats in their potting soil? I recently opened a bag of potting soil and used it to sow some Spinach seeds in a pot indoors, and now little tiny gnats are all over the place. I tried spraying the top of the soil with insecticidal soap, but that didn't kill them. I spend a lot of time squishing them with my finger. Help.

Hardiness Zone: 7a

Kathy from Canton, NC

Answer:

Kathy,

The little gnats you're seeing are probably fungus gnats. They seem to suddenly (and mysteriously) appear out of nowhere soon after watering plants or after opening a fresh bag of potting soil. Adult fungus gnats like to lay their eggs on moist soil. Soon after they are laid, the eggs hatch into larvae, which feed on fungus growing in the damp soil. After a short period of feeding, the larvae pupate and hatch into adult fungus gnats and the cycle starts all over again. In the case of gnats showing up in potting soil, I suspect that the eggs get laid in the soil before it's packaged at the plant. The moisture, the nutrient-rich soil and a lack of air circulation combine to create the perfect conditions for growing fungus. It's either that, or we all have adult fungus gnats in our homes that fly about undetected until we open a fresh bag of potting soil. Seeing as how gnats have such a short life span in the first place, it's hard to imagine that they would wait around for us to open up a new bag of potting soil.

In any event, the good news is that without food (fungus) to feed on, the gnats cannot develop into mature adults. One way to help halt (or at least disrupt) the fungus gnat reproductive cycle is to let the soil dry out a bit between watering. It also helps to make sure the top layer of your potting soil is exposed to plenty of sunshine and fresh air.

I once had a huge crop of fungus gnats hatch inside a terrarium I planted. When you first plant a terrarium it's easy to add too much moisture. Sometimes it takes a few days to figure out the right amount of moisture needed to create a self-sustaining environment for the plants. Although I didn't see any visible signs of fungus growing on the fresh potting soil, I soon had dozens of fungus gnats flying around the inside of my terrarium and getting stuck to the water droplets running down the sides of the glass. The whole project looked terrible and I was understandably horrified and mystified as to where the gnats came from. I ended up taking the terrarium outside to let the gnats escape. For several weeks after that it seemed like they kept right on hatching anyway. I guess they ran out of fungus, because eventually their reproduction cycled stopped and they all died.

Ellen

About The Author: Ellen Brown is our Green Living and Gardening Expert. Click here to ask Ellen a question! Ellen Brown is an environmental writer and photographer and the owner of Sustainable Media, an environmental media company that specializes in helping businesses and organizations promote eco-friendly products and services. Contact her on the web at http://www.sustainable-media.com


RE: Gnats in Potting Soil

I know you hate those awful bugs. Since you have already planted Spinach there is not much you can do. If you did not use all the bag of soil put the rest of the soil in the oven for an hour at 300 degrees. That will get rid of the gnats remaining in the soil. (10/12/2006)

By Ella

RE: Gnats in Potting Soil

Try this, put a few drops of dish soap in a small dish with about a cup of water. Mix so that it suds, then pour on top of the soil. The soap kills the gnats and should not harm the plant if used lightly. (10/13/2006)

By Jess

RE: Gnats in Potting Soil

I recently learned from a Master Gardener that all you have to do is to put a thin layer of clean sand on the top of your soil so the gnats can't get to the soil easily. Water drains through the sand, but the surface will dry out, inhibiting the gnats (who like the damp surface of the soil). Given the life cycle of the gnats this makes sense. Hope this helps. (03/30/2008)

By Chris

Archive: Gnats in Potting Soil

I have at least 50 houseplants. I re-potted many over the years and have never had a problem with gnats, now I do. I try to let the soil dry out as much as I can without harming the plants. These little stinkers won't go away. How can I get rid of them? Should I poke holes in the soil for ventilation? Would it help to dry the soil out? Is there some kind of bug poison I can use? Thanks.

Hardiness Zone: 10a

By nerakk from Hesperia, CA


RE: Gnats in Potting Soil

I had a terrible problem with them tried several things, then I decided to use a flea collar in the plants, I cut it in sections and wraped it around in each plant, it worked for me. However, you need to change when the labeling indicates it is expired. This is usually 3-6 months. (07/18/2009)

By Marcia S

RE: Gnats in Potting Soil

Bacillus thuringiensis subspecies Israelensis (BTI) that kills the larvae of fungus gnats, mosquitoes, and black flies . When BTI is eaten by these larvae toxins are released into their gut which causes the larvae to stop eating. Without food the larvae die. This interrupts the life cycle of these pests and the population then dies off. (07/20/2009)

By luvnaz

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