This applies both to WS and in door sowing (and, I guess, even to outdoor, direct sowing):
Don't sow tiny, "surface" seeds on a coarse seedling mix.
Esepcially if they need light to germinate.
For example, Petunias or Lobelia.
I like very fast-draining seedling mix, so I add a lot of coarse amendments, mainly pine bark. However, if the surface is too rough, tiny seeds can fall into the cracks between chunks of mix and never germinate (or never emerge).
if you like a coarse mix, spread a little fine stuff on top, and firm it before sowing, to support very tiny seeds ON the surface. I would use medium or fine vermiculite.
Coarse vermiculite might be an exception to this theory.
Maybe pine bark is the only coarse amendment that has that problem, and indeed, I only lost 1-2 trays of seeds that way, so I haven't really proved that "coarse" is the problem.
Jonna Sudenious says medium/coarse vermiculite lets enough light reach seeds so that they sprout. She starts all her seeds that way - in pure vermiculite, in sealed tubs.
http://www.seedsite.eu/zaaiins...
http://cubits.org/ellasgarden/...
