I have a lot of seeds that I never got planted when I was wintersowing, if I planted them now in a pot, would they be large enough to take to a plant swap by Sept. 10?
It really is cutting it short. Anything that pops up will be very small. Some seeds will not sprout or will be spotty sprouting because they might like it a bit cooler to germinate, and some may not germinate at all if they need a cold period.
So make sure they are fast warm temperature germinators.
I just planted kale and collards.
Even something that sprouts in 7 days or less will still only have a couple/three weeks' growth by September 10. I don't know of any seed that will grow fast enough to have much top growth in that amount of time. Even my balloon flowers are only an inch tall and I summer sowed them the third week of July.