Posted: 19/03/2017 at 08:59
You have to look at the conditions the plant likes to grow in first. Whether you have a dry climate, or a wet one, a nice, friable medium is best for most annual flowers.
I'm on clay too, but if I want to sow direct, which I tend to do, although I don't use a lot of annuals, I make sure the ground the seeds are going in is suitably prepped, and that usually means creating some reasonably free draining soil when you're on clay.
Get the ground worked over a bit with some compost, and some grit worked in as well, and then you'll have a better medium for the seeds, whatever you're sowing