As this is written in early November, it’s still too early to apply winter protection to newly-planted perennials, but it’s not too early to plan for it, if you garden in a cold-winter area (USDA Zone 6 [-10°F] or colder).
Although you might think a winter mulch keeps plants warm, it’s intended to do the opposite—to keep the ground frozen, instead of repeatedly thawing and refreezing. That freeze-thaw seesaw can heave lightly-rooted plants right out of the ground, leaving their roots vulnerable to freezing or drying out fatally. Perennials planted or transplanted in the fall are especially susceptible during their first winter.
To protect plants from heaving during their first winter, put a 4-6in layer of loose organic material such as straw, Oak leaves, pine needles, or evergreen boughs (cut into 1-2ft lengths) over the crowns after the ground freezes (generally in December here in Litchfield, Connecticut). Fortunately, after Dec. 25, there is a ready supply of Christmas trees to cut up for this purpose. Do not use bark mulch or other types of leaves, because these materials mat down and hold too much moisture over the crowns. Take care to avoid covering the evergreen foliage of plants such as Digitalis (Foxgloves) and Dianthus. Remove this winter cover gradually in spring when frosts become infrequent, usually at about the time Daffodils and Forsythias are in bloom.For these colder zones, we also recommend that you protect bulbs planted less than six inches deep. Again, after the ground freezes, apply a 4-6in covering of the same loose organic material over the bulbs. Because many of these smaller bulbs tend to bloom in very early spring, begin to remove the cover gradually in late winter or early spring—a bit earlier than you might for perennials.