Are you sowing now to plant out next spring? In theory, this gives stronger plants that get off to an earlier start, but I usually just sow early spring and do OK.
If you are planning to grow them in there next year, I am not sure how it will work. They need a lot of watering so that might be even worse under cover, plus you would have to watch for mildew. Exhibition growers do often grow under cover to keep the blooms pristine. It might be worth googling that
