I think the recommendation is to wait until the soil has warmed and night temps are above 60 degrees F if you're planting rhizomes. After the first season you can indeed leave Canna in ground yearround in your zone 8.

Some people (in colder zones where nonhardy) start them in a pot inside early in a warm place, watering very lightly until the first leaves appear, then placing in sunny window, to get an early start. But in Zone 8 they grow so quickly when planted outside it doesn't seem necessary to start inside early...just your choice. Good luck. josh

Cannas are hard to kill here in Texas. If you're worried just mulch well. Robin