Putting the perennial garden to bed means getting the perennial plants cleaned up, divided and ready to survive unharmed through the winter season when only the roots are living on most. There are a few perennials that retain evergreen leaves through the winter. Perennials are defined as herbaceous plants that live at least 3 years. Putting the annual garden to bed is another node.
The further north one lives the more important these preparations become. The growing season is short and winters may be severe. Perennial plants must store enough food and water to survive under the ground through a long and dry (if water is frozen it is not available to a plant anymore) season.
Part of "putting the garden to bed” is giving it a good dinner and a cup of tea before hand, but not too soon before hand. If a plant goes into winter without sufficient stored nutrients it will more likely die but if it is fed just before freezing time with high nitrogen fertilizers new growth will be encouraged and that new growth (and perhaps the entire plant) will likely be killed by winter's forces.
In the fall dead leaves and stalks should be removed. It is usually best to cut them rather than pulling them. That way the roots are not disturbed. Throw them in the trash if diseased or full of really persistent seeds (like mint or Eupatorium) and compost the rest. Sometimes this step of removing dead growth is not done with plants that are especially valuable as winter shelter or food for wildlife. Their dry stalks can be left standing. Also, some dead/dry plant material is still attractive and can be left to provide an interesting structure to the garden in the winter. Even these interesting or valuable parts should be removed when the first sign of new growth occurs in the spring.
Fall is the best time to divide crowded or overgrown perennials. Always amend the soil with organic matter when you replant the divisions.
The books say to remove mulch and then replace it after the first hard freeze. This is supposed to help prevent the ground from "heaving" with repeated thaws and freezing. “Heaving” is supposed to be bad for the roots. I've never done this, way too much work for me and I don't live THAT far north (zone 7 in North America). What I do is add a top layer of compost as new mulch every fall. This keeps the soil loose without me having to dig it in. The worms do all the work. Mulch will also help keep weed seeds from germinating.
Sometimes I have certain plants I want to spread by seed and I will sprinkle those about and allow time for them to germinate and grow a bit before mulching - usually in August or early September. We have a lot of winter weeds that germinate once it gets cold here and I like to use a chemical germination retardant called “Preen” in November or December to prevent their germination. I could weed them but that is not a job I like in February. The active ingredients only work for about 3 months so spring seeds can still be planted when the time comes OR if Preen is applied too late the desirable seeds can be germinated in a pot and then transplanted. It doesn't hurt the growth, just prevents germination.
Many perennials that are planted maybe just a touch further north than they should be require some protection; a little mound of evergreen branches, a special little teepee all their own, tucking in next to a warm wall or stone in order to make it through the winter. These less hardy plants will sometime succumb despite efforts to protect them but they are often so worth having that they can be treated as if they were just an annual and if they live more than one year, that is a bonus.
Other perennials are so obviously too far north of their comfort zone that one must either acknowledge premeditated plant murder or bring them indoors for the winter.
Fall is also the time to plant bulbs. It is the time to mark where your soon to be hidden underground precious babies are located. It is also the time to bring in garden ornaments that may be broken by freezing.
Taking care of perennials in the fall ensures a spring that is like a gift. Plants just seem to pop up from nowhere, all on their own. Even when you did the fall work yourself, by spring it all seems like an unexpected gift.
SOURCES:
reading, talking and experience