Can a seedling grow in a 12 hour light period?

That's a good point, MadSativa, that I didn't make clear. I plant to start and veg these plants under 18/6 indoor lighting for a month or so....THEN move them into the sunroom for the flowering stage. If 9 or 10 hours of direct sunshine isn't enough, I can augment the lighting as the sun goes down to make sure they get a full 12 hours if that is critical. On heavily overcast or rainy days, I could simply leave the curtains pulled and the lights on all day.

I'm thinking stealthy and economical at the same time.