I've heard it said:

"If it's brown, cut it down,
If it's green, leave it be."

I use this rule of thumb, but this time of year, I give anything that's looking 'unruly' a major haircut, and nothing is the worse for the wear come spring.

It depends on the perennial. Some I leave up for winter interest, like sedum 'Autumn Joy' or ones with seedheads, for the birds, like coneflowers.

This year, for the first time, I had widespread damage from the four lined plant bug. The damage was only cosmetic, but the leaves did look ugly for the rest of the season. I read up on these bugs, and it said that the larva (?) overwinter in the stems, so cut them down in the fall to diminish the numbers next season. Those I will cut down.

I also cut down the peony foliage, which can harbor disease like botrytis. (sp) Alot of perennials need some stems left up for winter protection. I wait until spring to cut down aruncus, astilbe, astrantia and asclepias, among others.

I also keep in mind the look of the garden, which I like to keep tidy. So cutting down the brown and messy looking stems keeps things looking neat. You were fine in cutting down the phlox...no problem there.

There are plants that overwinter better with the stems and foliage left for insulation. The only one I can think of right now is chrysanthemums, but there are others. Every fall I start with the ones that really should be cut down - peonies and phlox, then move on to things that can be cut down and look bad. Since I never make it through that list, I'm never tempted to get to the things that should be left standing during the winter.

Cutting down the foliage of 'spent' perennials in the fall extends the gardening season for me. I am out there in Oct. and Nov. cleaning up any perennials that I don't want to leave standing for winter interest. I am also getting the shrubs ready for winter with cages for protection from rabbit damage, and continuing to water trees and shrubs until the ground freezes. I can also plant bulbs for next season, and add compost to the beds before winter sets in. I don't have to try to get into the beds when all those spring bulbs are coming up. Spring is also a busy time, so anything I can accomplish in the fall makes me feel like I don't have so much to do come spring.

Like others, I do a good fall clean up since it really does extend the gardening season. And then I have lots of material to add to the compost pile to begin the cycle for next season's batch. I like to start out with a clean slate in the spring so I can hit the ground running - and not have to spend time being careful about cutting slimey dead material away from around the emerging new growth.

I rely on snow cover for insulation rather than spent foliage - and for the most part, we do get adequate snowfalls. Can remember only once in recent times when winter brought extended periods of -30 with windchills to -50 and no snow cover at all. No amount of leftover foliage could have saved some things that year.

We don't know what perennials you have, but balloon flowers, hostas, tall bearded iris, Siberian iris, daylilies, Oriental & Asiatic lilies, nepetas, Veronicas, salvias, sedums (but they're nice to leave for winter interest), phlox, peonies, baby's breath ... and the list goes on, can all be cut back/down in the fall without harm.

The only perennials I cut down for sure are my phlox. They get pm too and I like to clean up that area. I do get rid of some of the dead leaves from my daylilies and irises, but that's it. I prefer to do a good clean-up in Spring. I like leaving some dead leaves for overwinter protection. My annuals I leave in the garden over winter. I turn the soil in Spring and turn them in then. I'm lucky, the only disease problem I have is pm on my phlox.

Kat

Thanks so much everyone! This was EXTREMELY helpful. And duluthinbloomz4, I have many of those you listed. I can now chop away with abandon, for the most part anyway.

If you have salvias, Perovskia, artemisia, or lantana, you should NOT cut them back until spring. You will have a better chance of them returning if you leave the stems standing. Prune them in spring after you see new growth coming up from the base.

I'm sure you already know the importance of putting down a good thick mulch? :)

Things NOT to cut back are lavender, russian sage, butterfly bush, and most of my clematis. Cutting these back may prompt new growth which will be frosted and it will set the plants back and possibly kill them.

You shouldn't cut down Buddleia or Ornamental grasses as this opens up the crown to possible damage through the winter. My Buddleia stays semi-evergreen for a long time through the winter and looks pretty and shrub-like. The Ornamental grasses look nice in the winter too. Lavender is another one that I've heard is not supposed to be cut down until Spring, although I tried for 3 years and couldn't get it to overwinter at this house anyway!

I leave most of the perennials and some of the annuals standing through the winter. The stalks provide winter interest, and they catch the leaves that blow through the garden, creating a natural mulch. They also provide a little wind-break and privacy screen (most important in the front where it's very exposed to harsh winter winds). I do cut down a few, such as Peonies, some Phlox, and anything that was diseased or is real scraggly looking.

The birds also like to forage through the gardens in the winter, and it gives them some cover, and I figure it gives the good bugs places to overwinter as well.

I know most of you cut down your phlox. I no longer do, and haven't the past few years.

I don't care to look at them, but have found that I no longer get PM since leaving them till spring!

Early, petite bulbs look best if stuff has been cleared away in the fall.

Alia

Cutting down is one thing. Deadheading is another.

I agree with Donnabaskets and Gottagarden on not cutting back (to the ground) those perennials. I'm not sure I'll cut back agastache, but I will deadhead the blue fortune once the goldfinch are finished with the seeds.

Lavender should never be cut back more than 1/3 at a time.

I just (September 5th) severely deadheaded a butterfly bush to save it (blown over by wind and rain) and had to remove about 2 feet to lighten the top enough to reduce the weight to keep it from coming out of the ground. It's blooming and happy. However, I wouldn't cut it OFF like I do in late winter/early spring. In my zone, butterfly bushes grow like weeds.

I have a vitex that dropped too many seeds last winter, so I've got to take off enough of the spent blooms to prevent the reseeding. I'm on the verge of calling it invasive. It's usually treated like a buddleia.