In warm summer regions, fall can be an ideal time to plant a brand new perennial garden or to rejig an established one. The moisture is back (or hopefully will be shortly), the days and nights are cooler and as gardeners we are more comfortable being outdoors than high summer, so we will likely put some extra time and effort into the task.
The only cautionary thing is how late to push things. My old rule of thumb is always to try and get things planted 4 to 6 weeks before the ground freezes hard in your region. I can’t tell you what that date is, but your local weather office likely knows, and a quick Google search will lead you to the information or ask the staff at your local garden centre.
If you’re planting things that are in bloom such as Japanese Anemone or Hibiscus, tall grasses like Miscanthus and other late-season perennials it doesn’t hurt to give them a good mulching AFTER the soil is frozen. A few buckets of leaves are about all it takes to help these plants through their first winter. There is no need to do this extra step for most perennials that flower in spring or summer, however.
Sometimes the nurseries and garden centres offer great discounts in the autumn on perennials they’ve had on the benches all season, so keep a watch out for bargains, and don’t be afraid to buy a perennial just because it’s not in flower. You’re buying a promise for next season!