February 3, 2015
Perennials, by definition, come back year after year. Without proper care, however, some perennials may not flower after a rough winter. Others may technically be hardy to Ann Arbor’s 5b-6a zone hardiness, but not survive a particularly wet, cold or windy winter. Fortunately, a little preparation in the fall can increase your perennials chances of survival over the winter. Freezing temperatures are damaging to plants because, when the temperature drops below freezing, water inside the plant cells can freeze, causing the cells to burst, which damages the plant. The damage may be minor, such as the plant not flowering the following year, but harsh weather may also kill the plant, even one that should technically survive in this zone.
There are two basic types of perennials, root hardy and fully hardy. Root hardy perennials may experience damage to their foliage from cold temperatures, but the roots will remain healthy, in a dormant state, over the winter months. Fully hardy perennials have foliage as well as roots that will survive the winter weather. They enter a state of dormancy. In this dormant state, the sap content in the plant decreases and the plant conserves water. These actions make the plant less vulnerable to freezing temperatures. While fully hardy perennials may experience some superficial damage to their early growth from spring frosts, they typically recover well, with no lasting damage.
Thoroughly watering your perennials late in the fall, before the ground freezes, gives them the nourishment they need to make it through winter. Once the ground starts to freeze, water will not be able to penetrate effectively, which can leave the soil overly moist around the crown of the plant. This can cause the plant to rot and die. A deep watering before the ground freezes, and whatever moisture the plant picks up naturally over the course of the winter, will be enough to keep it in good condition over the winter dormancy period. It is important to note that a plant in dormancy has little needs for nutrition or water.
Trim and Mulch
Once the ground freezes hard, the perennials should be cut back, leaving about three inches of growth. Follow up with two to four inches of mulch. Ground bark and shredded leaves are excellent mulches. Pine boughs also work well. This keeps the ground a consistent temperature through the winter, preventing the damaging freeze-thaw cycle that can push the plants out of the ground.
Some perennials, while hardy to the Ann Arbor area, will do much better if they are provided with extra protection. Perennials that flower on the previous years growth may survive the winter, but not bloom, due to winter damage to their buds and stems. There are a variety of ways to protect these plants. Covering them with overturned plastic pots, piles of shredded leaves, Styrofoam plant protectors or burlap fabric should provide all the protection these plants need to bloom the following year. These protections should be put in place before the first cold snap of the season.
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