Fall and Winter Garden Cleanup
Annuals: Clear all annuals, pulling them up by the roots. Many insects, adults or eggs, spend the winter in debris. Cultivate the area exposing it to the sun allowing larvae to be eaten by birds and other predators. After ground freezes, mulch or compost over site.
Perennials: Prune perennials you are not leaving for winter interest. There may be reasons not to cut them: they provide wildlife shelter and their own mulch to protect their roots over the winter and they also add winter color and textural interest.
Prune trees and shrubs after they are dormant (or do this in early spring on plants that are likely to have winter injury). Spring flowering shrubs can be pruned in the fall but they will not blossom plentifully the next spring. Pick up any fallen leaves of trees that had fungal leaf problems this past year. Fungi can last over winter in debris. Discarding the affected leaves reduces the amount of fungi available to cause problems next spring. If rainfall is inadequate, water plants until ground freezes. If there is a micro-winter thaw, water plants again.
Here is a useful rule of thumb for dividing perennials: Plants that bloom in spring and summer, divide in fall; Fall blooming plants, divide in spring.
After the ground is frozen, apply three to four inches of compost or mulch around trees, perennials, and shrubs.