We are often asked if it is ok to put our plants in the ground in autumn. It is! Traditionally, autumn was the main time for planting hardy perennials, because they would be bare-rooted. The advent of cheap plastic pots made container growing an economic option and plants became available all year-round.
The RHS recommend planting hardy perennials in either spring or autumn these days. Autumn planting has two main advantages. First, the soil is still warm from the summer and this promotes fast root establishment, leading to plants having a really good head-start for next the next year. Second, after an initial watering-in, you don't need to continuously water them to keep them alive, as you often do with spring and summer planting.
In short, plants put in now will establish quickly and give you a good display next year.