In general, the better established a perennial is, the less winter protection it requires. However, even though plants would winter over safely if uncovered, they should be protected from the heaving which follows alternate thawing and freezing. In the case of perennials planted in the autumn this is exceedingly important for the first winter. Snow is a good protective covering, but it is rarely that plants will be so covered during the entire winter. It is accordingly advisable to apply a mulch.
Mulching in the fall provides an opportunity for fertilizing as well as protection, especially if good straw manure is used. Straw or cornstalks provide excellent mulch for perennials but both encourage mice. These pests may be killed by poisoned wheat or by pouring carbon bisulphide down their burrows. Where mice are especially troublesome, a compost of leaves, sawdust, lawn rakings, etc., should be used, inasmuch as heavy litter or one containing grain is ideal for attracting animals. Perennials such as lilies, whose crowns are completely underground, require the greatest protection. Perennials such as primroses and foxgloves, which carry over some fleshy foliage, must be covered lightly, if at all, to prevent rotting of the crowns. It is desirable to leave dead tops, stalks, etc., on the plants until spring. The tops will protect the plants to some extent over winter and there is also less danger to the plant from premature removal of tops in the fall before the latter are entirely dead. Cultivation should cease in the fall after danger from weed seeds maturing is past. This will discourage too late growth, and any weeds will serve as a protective covering.
It is important that mulch for perennial gardens should not be applied too early. There is a great danger in applying a litter of straw manure or leaves before the warm weather is fully over, and thus encouraging top growth which either rots during the winter or is frozen. Mulch should not be applied until after the first heavy frosts, and preferably not until the ground is slightly frozen in the early winter. The tufted pansy, primrose, and Shasta daisy are very susceptible to injury from a heavy mulch.
Mulch should not be removed until danger from extreme weather is past. The time for removal is governed by local conditions. The mulch should be removed gradually - not all at once - and extreme care should be used in removal in order not to injure or destroy smaller plants or plants which appear late in the spring. Any mulch has a tendency to delay the spring development of the roots. It should be removed, however, before the ground has become so warm that root growth has become definitely encouraged and the sprouts which are starting to grow are becoming weak and spindly.