I sometimes use 2 liter pop bottles too. Only problem is they blow over, blow away pretty easily. I usually keep them in plastic bins weighed down with bricks to prevent blowing.
Lots of seeds can be direct sown in fall. Also recommended for a very few seeds for wintersowing which need warm-cold-warm periods. Don't know what seeds, though I think it's mostly some shrubs and trees that are tough to germinate. I've never done it. I don't usually start WSing until January.
Karen
