Thanks Mark, Is 'Dry-stall' significantly different from Horticultural pumice? I thought I had hit upon a great thing because it is much cheaper but seems the same as Horticultural pumice I have bought. I do top with DG (decomposed granite.) re the moss ... I do keep the seed trays in shade, covered with a humidity dome until the seeds germinate ... then remove the dome and gradually move to a more sunny locale. The moss and algae didn't seem to take off until after I moved the seedling trays into the sun, which is plentiful albeit low-in-the-sky winter sun. Anybody? Thanks, Ken Blackford San Diego, CA USDA zone 10 --- On Wed, 2/1/12, MARK MAZER AND FREDRIKA MAZER Your problem is that the top surface is not drying out because the Dry-stall absorbs so much moisture. Topdress with something like granite grit. Rainwater is probably the source of the moss.