By the way - learn from my mistake, though few experienced gardeners would ever make this one.
Don't try to surface-sow tiny seeds on a coarse medium. I tried to sow a 128-cell plug with 13 kinds of petunias, using my own mix with fairly coarse pine bark added.
Of course the tiny seeds fell into the cracks and never saw any light. If any DID germinate, they never emerged from the surface.
If you surface-sow onto coarse medium, sprinkle a layer of finer vermiculite on TOP of that medium, to support tiny seeds!
I'm guessing that nice, big vermiculite like 1/10" and 1/8" nuggets would need a little finer vermiculite on top to support WSing small seeds or very small seeds.
On the other hand ...
http://www.seedsite.eu/article... (3/4 of the way down)
@JonnaSudenius is the expert on WS with vermiculite in sealed containers. Her website
doesn't say that you need a layer of fine vermiculite to support small seeds, though she does say:
"Gently press the layer with the bottom of another container to settle the vermiculite. "
"Use a spoon to lightly cover seeds that require it, and if necessary, gently tamp down the surface with the bottom of another container. "
Also:
"- Seeds that need light to germinate are exceptionally good candidates for sowing in vermiculite. Just scatter the seeds on the surface; the continuing expansion of the moistened vermiculite will provide good surface contact.
...
It may seem that some areas are covered deeper than necessary due to the clumping action of moist vermiculite, but don’t worry; vermiculite admits greater amounts of light than soil mixes."
Since vermiculite lets more light penetrate than bark-and-peat mixes, maybe it is less important to keep small right on the top of the surface.
P.S. to Jonna: if you would prefer that I not take direct quotes from your website, let me know and I will delete them or re-word them (paraphrase them).
