I don't use a 72 cell insert for anything.

What size do you use?

I start snapdragons every year. I do have a greenhouse but I start them indoors under lights to give then a head start before I move them to the greenhouse. DO NOT use a heating mat to start them. Snaps are a cool weather germinator and a heat mat will kill the seeds. I wouldn't leave them in the small 9 packs the entire 10 weeks I'd transplant them into larger containers half way through. There is no where for the roots to spread out and the plant won't bush out and with the roots squashed they can't take up water or nutrients. The good news is that snaps once they are a viable plant do not freeze easily and can be set out a bit earlier than other plants. If you don't have room under lights you can set them outside during the day and bring them in at night once the weather warms or put them in a cold frame.. Snaps do well with partial shade.

I use the 6 cell packs--48 per tray for most flower seeds I plant and only use the 9 packs for small plants like alyssum and lobelia

I start snaps for farmer's market cutting sales around the second week of March. I surface sow in plug trays with more than one seed per cell and remove the weakest one. Snaps germinate, as others have stated, in cooler temps. After I seed them I cover the flat with plastic to keep the humidity up. Only bottom water or use a mister, anything else can wash the tiny seeds away.

I plant them out here in north/central Sask. around the second week of May, it is early as we get our last frost June 6 but snaps can take it.

I like the plug trays to start flower seedlings. They are very small but for the first 2 weeks or so, until a good solid plug is formed, I can have 1000 plants in a tiny space. I know easily when to pot up, when I can grasp the plant by it's leaves and pull it gently out of the tray and all the roots remain intact ( in the shape of the plug, no dirt drops off). The plug trays need to be up of the table not directly on the surface, this allows the plant's roots to be air pruned.

For cosmos, I have not done them in a while but my notes show that the year I had the best results I germinated them over a heat source around April 1st, and at about 3 weeks I pinched out the growing tips, and pinched again when I planted them to the beds. I sowed them directly in the 72's with no potting up. I found cosmos get lanky if kept too long in pots . Some plants just don't benefit from an earlier starting date, other's do.

Thank you all very much, such great and detailed info! :)