Question: Is it OK to plant or divide perennial plants this fall?
Late summer and early fall are a great time to add plants, move plants, replace plants that succumbed to the heat, or even divide some of the plants you have. We finally had a good soaking rain! Just wait for the soil to drain before you get busy. Walking on water-logged clay soil compacts it, which is bad for the soil and for plant roots.
Local nurseries still have a good variety of perennial plants, and most will be featuring sales to lure gardeners back to shop. If the potted plants are rootbound, it's essential to loosen up the roots to some extent before you plant, either with your fingers, or by making some vertical cuts along the sides of the rootball. If the perennial is extremely leafy, you might want to cut the foliage back by a third or so. Plant at the same depth as it was in the pot, water it in (this helps to settle the soil even if it is moist), and mulch with a couple inches of woodchips or bark (don't let it touch the stems). If it turns extremely hot and sunny, you might want to consider providing temporary shade, with a milk crate, a laundry basket, or even an old umbrella!
How late can you plant? At my cold country garden, I try not to plant container perennials after the end of September, because of the risk of frost heaving (repeated freezing and thawing pop shallow-rooted plants right up out of the ground, and then the roots dry out and die). Some plants are more prone to this problem, such as Heuchera and bearded iris. A tip from a member of the Southern Tier Iris Society, when planting bearded iris after Labor Day: place a brick or a stone on the rhizome to prevent heaving.
As for dividing your own plants, bearded irises should be done as soon as possible (see tip above). Ideally, peonies are divided or moved in October, but September works too. Most other spring and summer-blooming hardy perennials can be divided and replanted in September. Fall-blooming perennials are best done in early spring.
Of course, if you live in the warmer sections of the city of Ithaca, or near the lake, you have a longer, warmer fall, and can stretch these guidelines a little farther.