You don't want to use garden soil in pots. It will not drain well. It will also probably have it's share of pests in it. I have heard that cooking soil will kill pests, so try if you'd like--it won't hurt. I use little sticky traps on a stick with my potted herbs to try and keep the fungus gnats manageable. Or larger ones taped to the wall. Letting the top layer of soil dry out thoroughly will help a bit.

Is your pot deep enough for the dill?

Regarding cooking the soil, do you think it's necessary to use more than the top few inches in the pot, considering that's where the bugs will be, or is it necessary to cook enough soil to fill the entire pot?

It's a completely different prospect to have to cook a 40lb bag of soil than it would be for only the amount needed for the top two inches of each pot.

The sticky traps worked for me, too, but really only to keep them at bay - they still managed to attack the plants and made my life miserable. This is in a 2x4 grow tent, so it would be difficult to tape around the wall.

I also played with letting the top layer dry out, but that became a difficult timing game, resulting in some plants suffering.

For the dill, what sized pot do you recommend? And the same for cilantro? I have a larger pot, perhaps a 12", but it takes up so much more space.

When I pulled the roots from last season's plants, they weren't more than six inches deep by maybe six inches wide. Is that not sufficient?

Thanks so much for your help. I hope others can engage in this conversation too.

Do you have a fan on your plants? If not, get one. Get a cheap oscillating fan and it should go a long way towards solving your problem.

Fungus gnats won't breed like that if there isn't something for them to feed on. I think you may be overwatering just slightly. A fan will help to reduce humidity and improved air circulation will also help cut down on the occurrence of both fungus and gnats. Fungus gnats are not strong fliers, just the fact that they are getting blown around will help cut down on their ability to breed. And, its just better for the plants.

You can treat your pots with Bt (bacillus thuringienis), or spray the top of the soil with a pyrethrin insecticide. Either course of treatment will probably have to be repeated several times. I think the interval is 5 days for Bt, I'm not sure for the pyrethrins.

Instead of trying to cook your soil in the oven - which I have done in the past and its just a lot of work, smells up the oven, and a general pain - you could use solar sterilization to sterilize your potting mix. It may be too late in the season for this year - you need warm or hot days with plenty of sun.

You can "solar sterilize" your potting mix but it takes time. Put your potting media in a large clear plastic bag, 2mil or thicker if you can find it. You don't want the potting mix to end up thicker than about 6". Make sure the potting mix is thoroughly wetted. Not standing-water wet, but thoroughly wet. Squash all the air out of the bag so the plastic is right up against the mix. Now leave it to sit in full sun all day for about a month. It should be either on a cement patio or deck, or if your sunny space is in the middle of the yard, set it up on a pallet or something so it is not in direct ground contact. The ground is a HUGE heat sink and your soil won't heat up as much or as quickly if it is in ground contact. Use more than one bag if necessary.

Sterilizing just the top 2" won't work. You need to sterilize all of it. But fungus gnats won't occur, or at least won't get out of control, where the environment has been made unfriendly to them. The adults are a nuisance but its the larvae that can do the actual damage. Just take a many-pronged attack stance -

• fans to blow them around and reduce fungal growth due to stagnant air and high humidity

• either dispose of infected plants or treat them all using either Bt or pyrethrin soil treatments

• remove all possibly infected potting materials for solar sterilization

• clean all pots, flats, and tools with a 10% bleach solution

• wipe down all flat surfaces with the same solution, including the outsides of any potted plants you've chosen to retain

• bring in fresh potting mix

• get a moisture meter to help determine when to water

I plant cilantro in a long planter about 6" to 8" in depth. I have planted basil in the same sort of container.

A 12" pot for a single cilantro plant is overkill,...

Thanks so much for spending the time to write this.

I have a grow tent with six T6 bulbs in it (30k lumens) and the dill did wonderfully - in fact, much better than it did alongside my basil outside this summer outside.

I did have a fan running, as well as a fan inside the grow tent, and it kept the inside very dry, yet the gnats continued.

I tried to keep the top 2" of soil very dry, in an effort to kill the gnats that were already there, and trying to find that balance between giving the plants the level of water they needed and being _too_ dry, was very difficult.

I had a 12" pot for cilantro, and as you mentioned, about six plants in the pot. It did really well. I would just cut them off, leaving about three inches of the bottom of the plant remaining, and it would sprout again without difficulty. It was only once I started playing with the levels of soil dryness and a concoction of hydrogen peroxide, vinegar and dish soap to kill the gnats that it really became problematic.

I have a 20lb or so bag of Miracle Gro potting soil that I plan to use for this when I start again in a few weeks. Would you recommend mixing some perlite in with that, or is it okay as-is?

Should I just expect these gnats to come back again, or was the problem I had uncommon?

It's already way too cold to try your clear plastic bag outdoor experiment, but that would have been nice.

Thanks again.

I hate to say it, but given the circumstances I would not use any potting mix that you already have. I would set that aside until you can soil-sterilize it next summer and get new. BUT

not until you clean up and sterilize. Otherwise it is likely the gnats will just come back. Was there visible damage to your plants? Are you still seeing the same level of infestation? Have you tried a pyrethrin soak of the soil?

The potting soil I have is in an unopened bag - it was the previous bag that I believe is where the fungus gnats originated. I just assumed they would also be in this new, unopened bag as well. Is that not the case generally?

It's also been a year since I've used any of the tools, pots, or other materials. I'll also likely be using different pots anyway.

Where can I find pyrethrin soak? Is there a commercial name? Can I get it on amazon?

Does everyone have this problem with fungus gnats? I'm not doing anything all that extraordinary that I would think would cause such a problem.

I had an absolute infestation. I managed to get it subdued, but it remained until I pulled all the plants at the end of the growing season. I tried everything - sprays, vinegar, hydrogen peroxide, more sprays, etc... Nothing every completely eliminated it, and it came to a point where it was consuming so much time just to continually try new things...

A few years ago I had fungas gnats in my house plants. I used very hot water on the soil and it didn't hurt the plants at all, but did get rid of the fungas gnats.

I'd try it with a couple of your plants to make sure it doesn't harm them.

I've heard that baking the soil stinks, literally. Miracle Grow is notorious for fungas gnats, I'd eat my losses and toss the unopened bag.

Fungus gnats, like aphids, have a bit of a spontaneous generation aspect to them :-) If conditions are right - they just appear. They were not necessarily infesting your potting soil, although that was likely one of the factors that contributed to the problem. MG potting soil is c**p - it is too finely textured and far too moisture retentive. And the last is the primary problem. Fungus gnats like damp or consistently wet soils and they will breed profusely if those conditions are present.

You need to back off on your watering. Allow the soil to dry between waterings. And a faster draining, coarser potting soil would not go amiss. Also, if you dress the top of your pots with coarse sand, the gnats will not lay their eggs and they will die off rapidly. They only live about a week or so. And while they are a nuisance, they really do no harm to the plants.

The gnats don't do any damage. A heavy infestation of the larvae, however, will damage plants as they are down there gnawing the roots. The problem is - and you saw that with your last go-round - no matter how many of the adults you trap or kill, they've already laid eggs and the larvae just keep coming.

I don't much care for MG or most pre-bagged potting mixes, but these days I can't seem to get decent peat moss at all. It all seems too fine textured and dusty.

You can add pumice, if you can get it, to your soil mix to improve drainage. It is water-retentive, but helps to maintain aeration as well. You can get it fairly inexpensively as Stall Dry from a feed store. It needs to be rinsed, it is crushed and very dusty. Otherwise use the coarsest perlite you can find.

But back to treating your plants, if you want to save them.

One option is Bt-israeliensis available as Gnatrol. Follow the directions. It will take about 3 weeks. It needs to be Bt-i - not the "regular" Bt, that's a different strain. Gnatrol is the right one, though there may be others. Follow the directions - I'm pretty sure you have to apply it every 5 days for at least 3 weeks.

And you still need to sterilize all your surfaces and pots and tools to be sure you're going to start out gnat-free after whatever treatment you choose is done.

There are several Bonide products that contain Imidacloprid - an insecticide that I am pretty sure is NOT organic, but it is effective. Just follow the directions on the label.

I have not been able to locate a specific pyrethrum product to kill fungus gnats, but according to the U of CO, what you want is:

The most effective treatments are those that are persistent; killing adults for up to three days. A number of pyrethroid-based insecticides, with extended persistence, are available for use on houseplants including those containing the following active ingredients: bifenthrin, cyfluthrin, permethrin, and lambdacyhalothrin.

There used to be something called Ecotrol? I think? That would have fit that bill - but I don't think it is made any more.

It probably IS in your unopened bag of potting mix at this point - those bags always have holes and tiny tears in them SOMEWHERE. So I would remove that from the area - like into a shed or the garage, on a porch, or even just set in the yard somewhere, but away from the area.

You can solar sterilize it later. You really COULD do it in your kitchen oven, but its a royal pain, and it smells to high heaven. That much potting soil is likely to stink up your kitchen for a week or more.

After you've completed whatever course of treatment you have chosen - and it must be said, complete disposal of all plants and the soil they are in ought to be considered - you still need to sterilize everything.

Clean all your pots and tools and surfaces to get rid of all loose debris and dispose of it. Then sterilize...

Thanks again for your help. I should have made it more clear that the grow tent (a 4'Lx2D'5'H reflective enclosure with a 6-bulb T5 fixture producing 35k lumens) has been idle and inactive since last May, when I moved everything outside.

There's no plant remnants or other materials that could foster the growth of gnats since then.

This means that, as of right now, I have no gnat problem and therefore don't need any gnat control products - I'm just assuming I will have them again since I don't think my set-up was anything out of the ordinary, and would like to be prepared. I also obviously want to mitigate as much as possible of these gnats coming back by planning ahead with the proper soil, grow tent setup (fans, watering, etc) as possible.

It is too late to do the solar sterilization, but I wonder if I left this 20lb bag of MG outside in the winter, for it to freeze to a solid block, would that be enough to kill any larvae, etc?

Do you have any recommendations on soil that would be suitable for this purpose?

And it would be a bad idea to use compost from my garden outside, correct?

Thanks!

Correct - don't use compost for any containers :-)) And any fast draining, good quality potting soil will work - and it's my opinion, as well as many other container gardeners, that Miracle Gro has neither of these characteristics.

You really do not need to go to any extremes to control fungus gnats. Just read up on their life cycle - lots of online info available - and do not over water, allowing potting soil in your containers to remain overly wet. It is the wetness that encourages the gnat activity. And most herbs prefer to be on the dry side anyway.

Should you discover you do develop fungus gnats in any of your containers, just repot them in clean, fresh soil. And try that topdressing with sand; diatomaceous earth works in much the same manner by preventing the adults from laying eggs and any existing larvae from exiting the soil. Also, cinnamon sprinkled on the top is a natural antifungal and destroys the larval food source so they starve.

Ah! I had seen that about cinnamon before. Knowing how it is supposed to work now I am less skeptical of it, LOL!

I tried Gnatrol with little luck, but maybe I gave up too fast. I then used a layer of Saigon cinnamon that I bought from Costco and no more gnats. I used it on a large potted houseplant and just covered the top of the soil completely, My house did smell like a Cinabon factory.