I thought dahlias were grown from tubers which you plant in the spring. I could be wrong and you might be able to use seed.

I grew daliah last year from seed, they flowered the same year. sow them in spring in seed compost novicesteve, i started them off on the windowsill...

It depends whether you want bedding dahlia,s, or the larger border dahlia,s, if the former you can sow seed in a greenhouse in february or march at 61 deg f grow them on and plant out when danger of frost is past.
if the latter, these are best done by division of the tubers or from cuttings, as they do not come true from seed.
derekm

I sucessfully grew dahlias from seed a few years ago just in normal seed compost.