Fungus gnats - cedar mulch - soil food web

Will a top layer of aromatic cedar mulch, which repels fungus gnats but has fungicidal properties, hurt mycorrhizae or other beneficial soil organisms?

I use a fir bark mulch in the containers on my balcony in Brooklyn, NY. We've had a rainy Spring/Summer here, so the mulch is a little fungy, which I want (right?). But Brooklyn is lousy with fungus gnats from the rain. Plus, I'm brewing a fungal tea, which basically seems to act like a neon "All-You-Can-Eat" sign for the gnats.

Breaking the local breeding cycle on my balcony isn't working because there are plenty more gnats around. I've tried intensive BTi. I keep mosquito dunks (in stretched and tied nylon knee socks) in my dechlorinated watering cans. No help. Nematodes (ScanMask Steinernema feltiae) seem to work for a few days, then not.

I think I don't want a physical barrier like a layer of sand under the fir mulch because I want the mulch to communicate with the soil, right? So that leaves repellents.

I can't stand citronella. On the other hand, I love cedar. I've had some short-term success controlling the gnats by painting GoGnats (33% cedar oil, plus water and a surfactant (sodium lauryl sulfide)) onto the outside of the containers, but the cedar oil evaporates after a few days or gets rained away, and after several applications the surfactant is building up and making my balcony slippery after it rains. Also, I'm concerned about the effects if the surfactant gets into the soil, and it's labor-intensive to paint it on. Plus, I'd like to have the repellent on the entire top surface itself, instead of just around the area.

I'm thinking about using hamster chips. Thanks for any thoughts.

I used to have a problem with fungus gnats when I grew African Violets. It was the peat moss that attracted them - when I switched to coconut coir, the gnats disappeared.

Yellow sticky traps will attract the adult gnats.

Sorry, I don't know if using cedar mulch will have any adverse effect on your potting soil. I kinda doubt it.

But I gotta ask- why are you mulching your containers if you have such a gnat problem? They need a damp environment, with decomposing organic matter for reproductive success, and I would think that moisture retaining mulch would be way too condusive to that. One of your best bets (aside from controlling this bizarre constant rainy weather we've been having, LOL, which isn't helping at all either!) is to get the top 1/4 to 1/2 inch of soil as dry as you can most of the time.

I'd scrape off the mulch. Maybe even repot with fresh soil & move the pots under a shelter when it rains to control the moisture. And if the sun ever comes out, I'd gently cultivate the top layer to get some more light & air going to dry it out. Any pot small enough should be watered from the bottom.

Some people cover the surface of the soil with sand 1/4 to 1/2 inch too. With your situation, I'd try that, but I'd also mix in ground dunk bits into the sand. That might help.

As to having the mulch touch the potting soil- you're really not going to get much organic benefit from mulch breaking down in container culture. And meanwhile, it's just holding onto moisture, (and possibly adding a larvae food source) which in this case is not helping your situation any...

I've had decent fungus gnat control success watering with hydrogen peroxide that you can get at any pharmacy. Mix 4oz per gallon and use it to water all the time. But that was indoors, with a fast draining potting mix- you have a real challenge on your hands there, being outdoors & all.

Some useful info: http://www.hort.uconn.edu/IPM/greenhs/htms/fngnatser.htm

You may want to check out the Container Gardening forum here too. Lots of good info there too.

Good luck!

Thanks HoneyBee. I'll try replacing my fir with coir.

Did you use chips, chunks, fiber, granular, or a mix?

How thick a layer did you use?

Oop! I just found this-

http://www.herbcompanion.com/your-fungus-gnats-are-showing.aspx

where they do recommend sand under the cedar. Though there's a bit of a disclaimer: "most insects hate the smell"....not exactly a 100% endorsement, LOL. But hey, maybe it'll help.

Thanks for all of your good advice Ned. The mulch is definitely a larval food source as you say. I'll also try the chunks of dunks.

I mulch because I'm growing vines on on a windy, south-facing balcony, and I'm using shallow planters, so the soil dries very quickly. I want to use a woody mulch for the nutritional benefits and the esthetics (as opposed to say pebbles, which would reduce drying but look too ornamental). But I'd definitely forego the nutritional benefits to get rid of the gnats.

I can't move the planters because they're fixed to a rail and the vines are all tangled together on a hanging net (photo attached).

Thanks again!

jnrubin, I used the finer coconut coir mixed with worm castings, perlite and vermiculite.

I love your vines! What are they, the little flowers are adorable?

Thanks Honeybee! On the right, Cypress Vine, Sweet Pea, Black-eyed Susan Vine, and a few types of Morning Glory are blooming, with some purple Cobea not quite there yet. There's also a variety of Nasturtium that's supposed to climb but doesn't. On the left I have some Scarlet Runner Bean. My favorite is the Cypress Vine - bright red tiny flowers and delicate feathery leaves.

I love the space- very pretty. It must be frustrating to go out there and meet up with those gnats! Yuck.

And certainly the location/conditions have their challenges. I bet those window boxes bake!

Good luck with it~

jnrubin - another question - do they hold up well to the sun? I know Sweet Pease like cool weather, but I am not familiar with the others.

I have a covered open porch that faces due West and would like to grow a curtain of flowers like yours. I purchased netting for this some time ago. I grew gourds on it the first year and they did fine, but I would much prefer some pretty flowers.

Thanks Ned, it's a challenge fer sure!

Honeybee, the Black-eyed Susan and surprisingly the Sweet Pea hold up very well. Each flower lasts a few days. The Cypress Vine and the Morning Glory flowers start to wilt by mid-day in my location unless it's cloudy. But in in less exposed places in my neighborhood I see Morning Glories last longer (although never more than a day). The Scarlet Runner Bean flowers hold up the longest - several days.

jnrubin - think I'll try the Scarlet Runner Bean in that location. I know humming birds and bumble bees love the flowers and they are a beautiful dark red. I grew up in England, and we always grew these beans for food. The first year I had a garden here in NC, I planted Scarlet Runners and we had a nice crop in the fall - the beans don't mature during the hot weather, but the flowers do.