Thank you Tim, Nhu and James for your feedback, I generally add no organic material in my seedling mix, which is generally 1/3 perlite, pumice and DG, topped with DG to hold down the perlite and pumice ... although there may be a minute amount in the DG I collect. Perhaps my pumice source ('Dry-stall' from the feed store) is the culprit as that is new this year .... or .... the rain water collected from the roof that I use for irrigation? With the deep rooted seedlings, it is relatively easy, albeit tedious, to pull off the layer of moss w/o damaging the seedling in each pot, but I was concerned the moss would get too thick ... like that posted by you, Nhu, wow! I may try the kitty-litter top dressing, James ... anybody else use this? Thanks again, Ken Blackford San Diego, CA USDA zone 10 --- On Tue, 1/31/12, Ken <kjblack@pacbell.net> wrote: I've had problems this year with algae and moss infesting the top layer of my outdoor seedling pots and trays ... http://flickr.com/photos/amarguy/…