Answer: In northern regions, Mother Nature has a way of protecting plants with an insulating blanket -- snow. It's one of the best winter mulches around -- it's in cold areas without snow cover that plant losses are greatest, even plants rated hardy in northern reaches.
Wait until after frost to trim things back, and then trim only what is actually frost-killed. Cut peonies close to the ground, and remove all debris as a precaution against disease. Take a close look at your other plants; sage will stay nearly evergreen, and you can trim it back to live wood next spring. Oregano is less woody -- trim it off at the ground.
Mulch the ground around but not over your plants with several inches of an organic mulch such as shredded bark or chopped leaves. If you cover the crowns of the plants with a heavy moisture-retentive mulch, they may rot. Once the ground has frozen solid, some gardeners will cover their perennials with a layer of evergreen boughs (leftover holiday decorations)or more straw. If the snow falls before the ground freezes, you don't have to use these extra layers unless your area gets serious winter thaws that melt all the snow -- that sounds unlikely.
Apart from mulch to prevent heaving and thawing as the ground freezes and thaws during temperature fluctuations, the only thing to worry about is excessive wind. Wind can make the garden far colder than the thermometer reads. If your gardenis very windy you might consider erecting some type of a wind break.
Finally, in the spring keep an eye out for frost-heaved plants. If there are any, push them gently back into the ground with your foot and replenish the mulch around them. When the perennials begin to make early new growth, gradually pull away any mulch that has fallen over them so they can reach the light, but be ready to replace it should a hard freeze threaten. Once frost is a thing of the past, tidy up any stray winter kill and enjoy the new growth!
Most perennials will appreciate a bit of balanced fertilizer and a top dressing of compost every spring, along with having their mulch topped up to help keep the soil cool and moist during the summer. Over the summer, remove spent flowers to help plants direct their strength to maintenance of their perennial parts rather than seed production.
You might want to look at a book or two about perennials to get some more detailed pointers. Enjoy!
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