Hi, Jennifer:
I've "wintersowed" them as annual (i.e. I start them in late March/early April for transplant in mid-May). I think that is what you are considering "spring sowing". I've not found them to be particularly fussy about transplanting, and they seem to bloom earlier than those direct-sown.
I love zinnias, and grow several varieties in my cutting garden. What type(s) are you growing?
Linda
