A perennial plant lives through two or more years, growing again from the roots, bulb or rhizome in spring, after going dormant in the fall. Some perennials are better at surviving winter cold than others, so the gardener has to give the more sensitive plants special care in the fall. These chores include cutting them back and mulching them to prepare them for the harsh weather ahead.
Not all perennials need the same care. Keep the labels of the plants you buy and plant. Make a map of your garden so you can identify individual perennials. If you know what perennial is growing where, you can tailor your fall chores to the specific needs of each plant.
The roots of perennials grow until the ground temperature lowers to about 40 degrees Fahrenheit. This is the time to cut back your perennials. If your area gets a little snow but not a lot of rain, you should leave a couple of inches of stems to help trap the snow.
You might not want to cut back if your perennials are ornamentals or provide seeds for birds. Remember that ornamental grasses sometimes provide cover for rodents. If you want to drive the rodents away, cut back ornamental grass.
Perennials need to adjust to some cold, so don't mulch until mid-November. Straw, wood chips and shredded leaves work well. Whole leaves and grass clippings can compact around the plant, preventing water from draining, an invitation to disease.Many hardy perennials can survive the winter with a two- or three-inch mulch, which will provide protection at 5 to 10 degrees Fahrenheit and prevent wide fluctuations in temperature.
Alternate freezing and thawing can result in "frost heaving" that damages dormant roots. Mulch borderline hardy perennials with a six- to eight-inch layer of mulch. If your area has extremely harsh winters, you can use up to a foot or more of mulch to protect your plants.
Remember, mulches make good homes for rodents. Hay has weed seeds, so it does not make good mulch. Remove mulches as early as possible in the spring.
Make sure the graft union of roses is a couple of inches below ground or covered with soil. Cut back to the top of the mulch.
You can add compost in the fall. If you cut back your perennials, it will be easier for you to apply it. Test your soil. Lime is slow acting. If your soil needs lime and you apply it in the fall, it will have time to change the soil pH by the spring when the plants begin to grow.
Caging Tall Perennials
The fall is a good time to cage tall perennials with wide-mesh wire fencing or concrete reinforcing wire. If wind blows in your area, put a stake in the cage to help hold the plants in place. When the plants begin growing the following spring, the branches and leaves will grow through the spaces, hiding the wire.
Dig up tender summer bulbs and store them in a place that is cool but does not freeze. These include caladiums, cannas, dahlias, gladiolus and tuberoses. Plant spring bulbs before November so they can get established. These include daffodils, crocus and tulips. If you plant them too early, they will start growing above ground.
If your area has heavy snow and high winds, the fall is a good time to install windbreaks. The accumulation of snow will help moderate soil temperature and protect your perennials from drying winter wind.