I got a call yesterday from an acquaintance desiring to know if this was the time to cut back perennials and whether it should be done at all. I told her it's optional: you can do trimming back now or later. But I recommended later. As in springtime.
For me, when temperature plunge, it's rather easy to choose later for any and all outdoor tasks. In Alaska, later also has a practical value. Perennials that are not cut back protect the crown and roots from exposure and frost heaving. Sounds a bit like what happens after a plant "all night-er." The point is, you want the temperature to remain steady around the crown. Usually the snow makes a great insulating blanket for us but occasionally we get a warm snap in the middle of winter and it melts away. Sunlight beating down on a beautiful 6 degree February day will warm up any exposed soil. Bad for plants. Avoid exposed soil in winter around perennials if at all possible. Leaving the dead herbaceous part of the plant intact increases the liklihood of survival, in my experience.
I do have a few exceptions to this "leave it" rule. If the plant was a diseased mess over the summer, cut it back. If the plant seeds around like it's going out of style (hello, Alchemilla mollis!), at least cut the seed heads back. And if the plant turns into a gooey mush at the first hard frost, well, I cut those unfortunates back, too (hello again, Alchemilla mollis, old friend!).
In the "cut back no matter what" category: peonies, to the ground, if you please. If you happen to mention to a peony grower that you did not cut your peonies back, prepare for some hyperventilating. They are frightened of that scourge of all things peony: Botrytis.
Next post: what is still blooming. Should be a short one.
Nepeta (catmint) before and after the shears, just for demonstration purposes. I usually don't cut this one back.