Q. When in the fall does it become too late to plant perennials?
Perhaps you have saved your money to buy your favorite perennial on sale at the end of the season. There is nothing more frustrating than planting it in the late fall to find it dead in the spring. Perhaps it heaved from the ground during the winter exposing its roots to the harsh winter elements. Or perhaps the harsh temperatures or sunless skies contributed to its demise.
To help insure your perennial's survival during the winter, its best that its roots get established before the first frost. Find out when your first frost is historically expected. Information can be found on the Farmer's Almanac or Cornell.edu websites. Make sure your perennial is in the ground a full month before that date with a heavy layer of mulch for further protection.
If for some reason you have not been able to get your perennial in the ground before a frost is imminent, you can try some other planting techniques that are a bit riskier but just might save your plant. First, water the plant in it's pot and give it some time to drain. Normally you would water the plant after putting it in the ground, but if you are expect freezing temperatures, then adding water might create frost heave. Plant the perennial deeper than usual with heavy mulch to prevent frost heaving - it can be replanted correctly in the spring after the last frost. Or the perennial can be planted in its plastic pot in the ground just before the soil freezes. The rim of the pot should be right above or at the soil line and heavy mulch should be added.
Sometimes, the winter will only affect the growth above the ground but the roots survive, so make sure you give the plant some time to put out its new spring growth before you declare it ready for the mulch pile.
You and your perennials have hopefully survived the winter intact!