"Coast of Maine" soil w/fungus gnats

Discussion in 'Growing Organic Marijuana' started by vashigopal, Sep 24, 2010.

  1. I have a single week-old seedling in this Coast of Maine Bar Harbor blend soil:
    Bar Harbor Blend Premium Potting Soil

    I mixed the Maine soil with perlite, plus a small amount of "Wonder Soil" coco coir in the center, for germination. The seed germinated (woo hoo!) but after several days the plant developed fungus gnats. I know it was from the soil because I used the perlite and the coco in my previous grow and didn't have gnats. I'm not over-watering, and the humidity is low where I live (too low, actually). I've never had fungus gnats before, so I read up on them.

    Following some advice I read here on GC and also the internet, I scraped off the top layer of soil and put a thick layer of perlite on top. I killed most of the adult gnats by hand and with duct tape. That was yesterday, and so far no more gnats.

    Right now the seedling is in a 4" pot and I'll be transplanting in another week or so into a 3 gallon container. I'm wondering if I should dump the soil and buy new, or if I should get some diatomacious earth and continue using the soil.

    From what I've read, the gnat cycle goes like this: There are eggs in the soil. When the soil is watered the eggs hatch into larvae. The larvae eat organic material in the soil and morph into gnats. The gnats lay eggs in the soil and the cycle starts anew.

    I've read that the larvae can eat plant roots. If I continue to use the soil and add a layer of diatomaceous earth on top, the larvae will not be able to develop into gnats. BUT won't they still feed in the soil until they morph and die? Is my plant at risk from the larvae if I treat only the surface with diatomaceous earth? At some point all the eggs in the soil will be gone, and no new eggs will be laid since there will be no adult gnats, but will that first generation of larvae do enough damage to worry about? Should I quit worrying and just buy new soil and hope it doesn't have gnats?
  2. Seriously... Don't worry about it at all. Reuse your soil. It is better. Just do the sand trick and the fly strips. Duck tape works too. You sound like you have it under control. Get on it by week 3 of 12/12 just to prevent the sticky buds from acting like fly strips... Ewww
  3. Thanks, I poked my finger into the perlite layer to feel for moisture and a gnat flew out. The leaves are looking a little mottled but I don't know what the reason is. I'm going to start a journal tonight, so I'll have pictures there.

    I read that fungus gnats in soil are cause by compost amendments that are not mature (ie. not fully broken down yet). Here's a quote:

    "It should be noted that in 1985, Dr. Dick Lindquist of Ohio State showed that fungus gnat problems are most serious in potting mixes amended with composts lacking in maturity (not completely composted). Microbial activity is excessively high in such mixes, and fungus gnats thrive."

    Here's the link
    Floriculture: Fact Sheets: Pest Management: Fungus Gnats and Shore flies
  4. They are not a big deal, don't water so often and have a small fan blowing gently over the top of the pot, GENTLY! This will speed the drying out of the top layer of soil and they will no longer hang out. A narrow necked bottle with old wine or any viniger in the bottom will attract and kill them. Yellow stickey strips work good also.
  5. Woodsman, thanks for the tips! I do let the soil dry out between waterings and I have a fan on it. I didn't think fungus gnats were a big deal, either, until I got them! I've read that they can eat young roots, but I hope my plant is beyond the vulnerable stage. Still, there a nuisance at best. I hate them!

    I'm going to try the vinegar in a bottle thing, and get some fly paper. Is that the same thing (in a different form) as the yellow sticky traps?
  6. I have these and have been dealing with them for months. I go on a gnat hunt every morning!! They started off as a big pest problem, but once I dryed out my soil it only become 2 or 3 ill see and kill. I picked up some gnatraul, I heard it worked well and it did but it droped my ph lower then 5 and stoped almost all growth completly. Its organic, but it messed my shit up for a while still. Even after all that I still have 1 or 2 ill see and have to kill. Any thoughts on catching theses last one or two before they cause more issues???
  7. I have the same problem. I dont think they pose a huge threat to the plant, because I've had a problem with them for over a month now and its doing well.

    Ive tried scraping topsoil off and all that did was limit the number of gnats for a small amount of time. I heard sand works well, but how much should i put on top?

  8. about an inch- 1 1/2" depth on top of the of soil in each container will deprive the gnats of a home and breeding place
  9. Introduce predators.

    There are certain bugs that you can introduce to eat all your gnat, mite, and aphid problems away.

    Read up on them!
  10. My gnats disappeared after the transplant, but now, 11 days later they are back. I saw a couple yesterday, and a at least a dozen today. I killed a few, and put a fly paper-covered stick in the closet. Tomorrow I will clean the closet and add a layer of perlite, but I don't have much room on top of the soil. Someone on another forum recommends a dish soap solution:
    Fungus Gnats
  11. Only within the past few grows have we finally conquered the fungus gnat. Stuff like keeping top layer dry, covered with perlite, sand, diatomaceous earth, yellow sticky traps (i use blue too and they are about as effective as yellow), all helps, (sand and dte can inhibit o2 exchange) i used to enjoy running my hand across the top of the perlite and smashing them as they move around under and in the perlite and as they emerge. Now i enjoy going into my grow room and seeing no activity other plant growth. yes, it does affect root/plant growth and just f's up my soil. No doubt had many, maybe all infestations start with organic soil mixs, but in my local they come in anyway as adults. So, MOSQUITO DUNKS, little bit goes a long way sprinkled across the soil top, sticky traps for the adults, kill em if you see em, end of story.
    Promise, you will see the difference in the plant, flowers size and health and in particular when you see the roots after harvest.
  12. the problem i've been having lately in that the gnats are actually coming from the drainage holes in my pots (~4gal black plastic nursery pot). The drainage holes are quite large and are located on the sides of the pot, right at the bottom. i see the bastards flying in and out of there...not really sure what i can do other than dust lightly with DE in hopes of getting it in a good enough spot.

    Maybe i'll just fill my runoff discs with perlite too
  13. Be careful not to inhale any DE dust. The sharp diatom fossils are very bad for the lungs. Sounds like a good idea to take a tiny spoon and try to jam some DE in the drainage holes. Or, take a strip of cloth and wrap it tightly around the bottom, covering the holes? You'll have to change it every time you water, I would guess.

    I'm having a hard time finding soil that doesn't have pre-mixed nutes in it. I may have to order Foxfarm ocean forest online (about $22 for 12 quarts, which isn't too bad but I need it now!) My plant is in a 6 inch container and really needs to be transplanted. I'm thinking of baking the gnat-infested soil to pasteurize it, but I've heard it will stink up the place.

    When I went to one of the hardware stores that sells a lot of plant stuff, all they had was Miracle Gro, and I told the guy I didn't want that and he said, "How come no one likes miracle gro?" ha ha -- I told him why.

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