Temperate region plants are adapted to seasonal temperature changes. They begin hardening off to protect themselves from the cold as soon as the days start getting shorter and cooler. Trees and shrubs stop growing and accumulate soluble sugars, and young shoots turn woody. Plants with deciduous foliage drop their leaves and go dormant. Conifers also start to harden off, although photosynthesis does continue over the winter, at a much slower rate. Before going into dormancy, herbaceous perennials accumulate the nutrients in their underground parts that they will need to produce new shoots and leaves in the spring.
As a rule, it is best to add winter protection to recent plantings and any plants that aren’t cold tolerant. It’s also important to protect plants from salt spray and deicing salt, snow and ice falling off roofs, and any plants in the line of snow shot from a snowblower.
You may also wish to protect less hardy plants from the sun and wind, both of which tend to dry out foliage and cause bark to crack.