Brassicas stated time to maturity is 45 days is it not, but that is likely in Ideal conditions.
Depending on what else they have to eat they may not touch them til they get that frost nip...in say mid October, that is why many plant them in early-mid August as a Fall/winter plot, cause even then you got 60-75 days to get them mature before they get the frost nip and turn into big lollipops.
You can plant brassicas now, but then you got to worry about the hot dry dog days of summer stretch. They like water and come frost your plants may be past prime.
I planted secret spot and agree with Bonzy...looks great and deer eat <u>young </u>ryegrass, but you will be spraying that plot in future years to kill it as it takes over.