Garden Clinics has this to say:
Showy daisy-like heads flower late summer and autumn in a wide range of colours including blue, violet, purple, pink, red and white all with central disc of yellow or purple. Michaelmas daisy is a loose term which is often used to cover most of the taller species, although it strictly refers to Aster novi-belgii. The 'China asters' grown as bedding plants are now classified in the genus 'Callistephus'. Clouds of lilac-blue flowers has a long flowering season, summer right through to late autumn. Black arching stems. Grows to 90cm and spreads well. Vigorous and spreading.