1. I've wondered about using DE and planned to if needed.
Did you put it on right after watering?
2. Was it completely absorbed after the next watering or
does it take a few waterings?
3. Did you need more than one applications?
4. Where there any negatives with it?
Thanks for the info?
When using DE as a fungus gnat "trap", it has to be applied as a dry top dressing. It has to remain dry. Between the dry DE on top of the mix, and the lack of water that will be given (intentional drought) the fungus gnat breeding cycle can be broken.
The first step is fighting fungus gnats is stop watering. Moist mix (peat based) is a great place for gnat larvae to grow up. Plenty of organic material/roots to eat. Stop watering, and when the top layer of the mix looks dry add on a layer of DE covering the entire surface of soil(less) medium.
The lack of water and the addition of DE are successful based on a couple of factors. The first is your plant's drought tolerance combined with the medium's ability to hold moisture. The better drought tolerance, the longer it can go without water. The gnats won't stick around / persist if the mix is dry. The second factor is more a constant
. The dry DE powder is thought to combat gnats in 2 ways. The first way being the DE sticks to the gnat's abdomen and other body parts. This can dry the gnat out, or render it grounded. The second way the DE fights gnats is by drying out the medium and blocking the eclosion (emergence) of the young gnats. Oh shit, and there's a third way. Gnats can't lay eggs on the DE, and if they do the eggs will dry out.
To directly answer your questions...
1. Wait for the top of the mix to look dry, then add on a dry layer of DE.
2. Although it will "Absorb" some, you won't water over it. Other than washing it away, it will lose its ability to dry out the gnats. Another idea is that dry DE is like knives to the gnats.
3. I think the last time I fought the gnats, I had to top dress with DE once then the plants went 2 weeks without a watering. I watered, added a bunch of DE on top the next day. 1 reapplication, 4 weeks, 0 gnats.
4. a.There are 2 kinds of DE. One is pool grade, and the other is a higher grade. Pool grade has way more silica in it, and less Calcium Carbonate and minerals. Either one kills fungus gnats. One is just better as a mix amendment as it has more stuff and less filler (silica).
b. I used the cheap pool grade last time because I had it laying around. The stuff flies everywhere, gets on everything. Although it was cool blowing some in the air at a flying gnat, and watching the gnat fall out of the cloud of DE. Anyway, all of this DE led the recycling bin to be dusty when fully dried out. Amendments were added to counter-act that dusty trait.
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