It depends where you are. You can sow perennials late, but it can be too late for some annuals.

I'm in zone 7A

In Richmond VA

I think it's still okay to plant perennials in your zone. What is your aster seed? I'm in zone 8, Oregon and I sowed some rudbeckias and perennial herbs last week. The herbs are already sprouting and one kind of rudbeckia is about to sprout. The temperature outside is perfect for germination. The plants will have enough time to mature; and in case if they will not be ready, you can protect them for their first winter. I would say just go for it. I'm going to sow more rudbeckia this week again.