The advantage of using open flats is that you can grow much more in less space (1 flat vs. 2-3 plug trays) vs. using cell packs or plugs. If space is no consideration then either way works.

Yes, thanks--the difference in space makes sense and does matter. When using flats to grow grasses, do you know if people prick out and pot up individual grass seedlings one by one or do people take 3-4 together to move up? When I grew the Deschampsia, I took individual seedlings and moved them to plugs one at a time, which felt kind of ridiculous.

I do it in clusters - groups about 1/2 the size of the cell it is going into if that makes sense. If you let it root-mat well first you can just cut it into small square/plugs for transplanting.

Thanks--I'll give this a shot too.