They say fall is for planting and that's true. In fall, plants can focus on settling in and putting down roots, rather than struggling to set flowers. But it takes several weeks for them to become established. If you're like me and are expecting a frost any day now, with more perennials to plant, it's time to make some decisions. Do you put plant them anyway or do you put them in the ground, pot and all, the way I do with divisions headed for my spring plant swap?
Both options will likely be fine. While its optimal to give your perennials at least 4 weeks before a frost, they are better off in the ground than out of it. If you decide to plant, plant a little deeper than normal, to prevent heaving as the ground freezes and thaws. And plan on mulching them either way for protection, once the ground freezes.