Answer: Keep in mind that perennials have developed the ability to survive winter, often in the wild and unattended. Assuming the plants are rated hardy in your zone, and assuming the drainage in your garden is good, there should not be undue problems. Woody plants and evergreens in particular survive winter better when they are fully hydrated when the cold weather comes. For perennials, it is also important to see that the soil is not bone dry going into winter but you do not want to keep them sopping wet as this can cause them to rot. Usually the fall rains are sufficient to keep the soil moist; it stays moist longer during cooler weather than it does during the summer.
Generally it is a good idea to maintain a mulch of two or three inches during the summer and to top this up to about four to possibly six inches in the fall. Place the mulch around but not over the plants. (Mulching atop the crowns creates a risk of rot, especially during a wet spring.) In cold weather the mulch helps stop the soil from freezing and thawing when the temperatures fluctuate and thus limits "heaving" which is when plants are popped out of the ground as the temperature changes. Many books will tell you to mulch "after the ground is frozen" but in some areas it may never freeze solid. In this case, simply wait until there has been a good killing frost or two and the rodents have all found homes elsewhere, then top up your mulch. Once the weather is cold and stays cold, or if there is an insulating blanket of snow, the danger to plants is minimal.
By early spring you will probably find that stray bits of leaves and twigs have lightly covered the crowns of your plants. This bit of protection helps the emerging shoots survive the occasional freezes that follow warm days. To mimic this, you may wish to toss a bit of light mulch such as straw or oak leaves or very fine shreds of bark over newly emerging shoots if a hard freeze threatens that night -- but be sure to remove it promptly so the shoots can reach the light. Eventually it will all be moved aside and added to the mulch layer as part of the general spring cleanup.
With a little observation and experience, you will begin to see how the plants accommodate the seasonal changes. Enjoy your garden!
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