While it's tempting to want to cut back the entire garden in the fall and hunker down over the winter dreaming of spring, it can be a nice addition to the winter garden if you leave just a few perennials up over the winter.
Although our clients have a wide variety of plants and conditions across CT, and what we do for a client in Durham is different than what we do for a clients in Branford, these general rules should get you through the season.
The seeds of Echinacea (coneflower) and Rudbeckia (Black Eyed Susan) will attract and feed birds. Not to mention the seed heads, even when empty, can be attractive on a frosty morning.
I think the seed heads of Sedum are particularly attractive on the off season - not to mention, when it snows they hold onto the snow and look like mini frosted cakes!
Heuchera (Coral Bells) is a favorite of mine in all gardens! Although not as vibrant as they area during the growing season, they do come in a virtual rainbow of colors and add a splash of color to the winter garden. Chrysanthemums (aka hardy mums) are hardy in our zone just as their name suggests, I find them to be slightly invasive so I use them as annuals. If you decide to keep them in your garden and weed out their little minions, they do prefer to have their foliage left up over the winter.
Some of these are preference based. We have a client on the water in Westbrook that doesn't care about winter interest so we cut most of her garden back in the fall, including most of her ornaments grasses.
Here is my basic list of perennials I usually cut back in the fall:
Bachelor's Button (Centaurea montana) Is a beautiful spring flowering perennial. They tend to become black and unsightly after the first frost so these can get cut back to the crown.
Bearded Iris. By the fall, the foliage in these Iris becomes floppy. Besides being unsightly, it becomes cover for Iris borer and fungal disease. This is one plant I recommend disposing of the foliage rather than composting it.
Beebalm (Monarda). Beautiful in many of our gardens from Old Saybrook to Branford, this beautiful perennial is susceptible to powdery mildew even with the most resistance varieties. Again, dispose of this foliage rather than composting it.
Blackberry Lily (Belamcanda) A nice, delicate looking perennial. Although I do like this plant, one of the only gardens that I have it in is my personal garden in Killingworth. I cut this back completely as soon as it gets cold. This keeps the foliage from flopping and becoming unsightly. It also prevents crown rot and is less encouraging to borers.
Blanket Flower (Gaillardia x grandiflora). I LOVE this plant! Long blooming and easy to care for, cutting this plant back enhances it's already hardiness and can give it additional vigor. In the spring, you can divide it and share the wealth with the rest of your garden.
Catmint (Nepeta). As it's name suggests, this perennial is in the mint family and benefits from severe pruning throughout the season. It can get large and lanky, and it's foliage will suffer from winter cold, so it's a good idea to cut it back in the fall.
Columbine (Aquilegia). This is a lovely spring blooming perennial with dainty flowers held above clover-like foliage. I cut this back and I try to remove any foliage as soon as I see any leaf miner damage. These little insects leave squiggly lines all thought the foliage and feed within the vascular tissue of the plant. Because this is an early bloomer, they send out new foliage in early spring. It's much easier to remove the foliage in fall and eliminate risking damage to new growth in the spring.
Crocosmia (Crocosmia) - I'm A HUGE fan!! If a plant can be seductive, this one definitely is! I find the seed heads interesting and as it can bloom late into the season, I leave them up throughout the fall. Soon after a frost, the foliage looks unsightly and therefore I cut it back - eagerly awaiting it's seductive flowers next season.
Daylily (Hemerocallis). The foliage on these sheds thought the season. We have a client in Madison with a large bed that has only daylilies in them and I'm constantly dead leafing them, Fall is just dead leafing the entire plant rather than just the dead leaves - because all the leaves are dead. These usually just pull right up, no need to sheer them back. If you're going to pull the leaves out in the fall, be sure the ground isn't super soft after a rain or you could potentially pull the plant right out.
False Sunflower (Heliopsis) Another great perennial for the garden. These flowers usually fade in late summer and the foliage starts to look less than spectacular. You can cut these back as soon as they're done blooming, or you can wait for fall. Sturdy stems will start to flop over the winter if not cut back.
Iris (Iris Germanica, Sweet Iris, Iris Sibirica) These lovely perennials can be found in most gardens, including my own. Iris are confusing to many gardeners. In the late fall the foliage will become brown and mushy, so I usually cut it back to about an inch or so above the crown. Sometimes you'll see basal foliage peeking out of the ground, but you can leave this alone. The Iris are hardy throughout all of Connecticut so they know what they're doing. Cutting back the basal foliage could harm next year's growth, so just cut back this season's growth and you should be all set come spring.
Japanese Anemone (Anemone x hybrida) Japanese Anemone are a garden staple of mine. The foliage tends to get very unsightly after a frost so I just cut these back to the ground. Whorls of flowers will come back to greet you next season.
Ligularia A fantastic plant for the shade. I prefer the variety 'Othello' with it's mahogany colored leaves. Ligularia often turns to mush after a hard frost, so this is one I usually cut way back to the ground.
Penstemon (Penstemon barbarous) Foliage fades in late summer so these can be cut back any time you like.
Peony (Paeonia) Who doesn't love peony?? Peonies need a bit of cold in order to set bud for the following spring. Their foliage is extremely susceptible to powdery mildew so definitely remove the foliage in the fall. Again, this is foliage I generally don't compost unless you're absolutely certain there's no disease present.
Perennial Sunflower (Helianthus) Just like it's cousin, False Sunflower (Heliopsis) foliage fades in the early autumn garden. When the foliage starts to fade feel free to cut these back to the crown and place them in the compost bin.
Phlox (Phlox panniculata) Most garden phlox is prone to powdery mildew, although there are a few resistant varieties such as the volcano series. Many of these can bloom late in the Autumn, so I leave these up until I'm certain they're done blooming for the season. Once the frost hits, I cut the foliage back to the crown and discard it unless I have a disease resistant variety.
Salvia (Salvia nemerosa) Fabulous perennial! I don't think I have many gardens without some variety of salvia in it. From Branford to Durham, Madison shoreline to Killingworth inland, most of my gardens contain some form of saliva. This perennial is a strong performer and will bloom until a hard frost knocks it out. At that point the foliage looks grey and sad, so cut it back to the crown and eagerly await another strong performance next season.
Yarrow (Achillea) Like me, Yarrow don't like cold, wet conditions. By fall, the foliage becomes brown and brittle. Go ahead and cut this all the way down to the crown and the basal foliage will fill in for next spring.