I am in Zone 5a so colder than you. I found that Asters and perennial mums are better planted earlier in the season to allow the root to establish. Fall is my typical time to plant, transplant and divide. I have never killed a plant except asters and mums. If planted in fall starting from early September, they do not come back due to the shallow roots. Your zone is warmer and with the Atlantic ocea effect so will be a little more forgiving.