Today is the first day of autumn if you garden in a cold climate in the Northern Hemisphere, that is. Just as spring comes much later than the supposed first day of spring (the vernal equinox), so the first day of autumn comes much earlier than the first day of autumn (the autumnal equinox). You have to go by what the weather and the plants tell you, not by what the calendar says.
It took me years to figure this out. All that good garden advice about planting perennials, shrubs, and trees in the fall just didn’t make sense, because I was going by what I was taught in elementary school about the seasons. In the milder climate of Long Island, autumn didn’t really get started until October, but that’s too late to plant most plants (bulbs being a big exception) here and expect them to get in enough root growth before winter. I had concluded, and some garden advice explicitly states this, that fall planting was not for cold climate gardeners.
But really, you just have to wrap your mind around the idea that autumn starts somewhere in the middle of August. You will know by the change in the night-time temperatures and by that first flash of red on the tree that always turns color before the others. (Around here, it’s a certain maple in the neighbor’s yard.) We have already been having night-time temperatures in the 40s F. The goldenrod is blooming, and the first asters. (I had a debate with my daughter Cadence over whether goldenrod was a late summer plant or an early autumn plant. I finally concluded that in this climate, late summer and early fall overlap so much they might as well be the same thing.)
It starts to feel–and look–like autumn. As long as you quit looking at the calendar, you will know. Pay attention to the weather reports, and plant your new acquisitions, and divide and relocate your old favorites, right before a rain storm. I believe most plants will continue root growth after the tops get killed by frost, but the longer they have to settle in before this, the better.