Trees in fall are in a state of serious change and reorganization. The tree is becoming dormant. A tree heading toward winter will sense the changing temperature and light and obey the dormancy controls built into the leaf. The mechanisms, called "senescence", tells a tree to close down for coming winter.
Trees may look inactive going into winter but the fact is they continue to regulate their metabolism and only slow down some physiological activities. This decrease in photosynthesis and transpiration begins a tree's dormant phase. Trees still continue to slowly grow roots, respire and take in water and nutrients.
Winter is a difficult time for a tree. A dormant tree still needs to be protected (winterized) to remain healthy and free from diseases and insects. Bad news is, winter weather encourages destructive pests to snuggle in and wait for spring to revive their destructive lifecycles. Small investments in your time can pay off big come spring.
Prune dead, diseased and overlapping branches in late fall. This will form and strengthen the tree, encourages new strong growth in the spring, minimizes future storm damage and protects against overwintering disease and insects. Remember that dormant pruning has another benefit - it is easier to do during winter dormancy than in spring.
Correct structurally weak branches and limbs. Remove all deadwood that is clearly visible. Properly prune branches that can touch the ground when loaded with rain and snow. Foliage and branches that are in contact with soil invite undesirable pests and other problems. Remove damaged and declining twigs, branches, and bark or any new sprouts that have grown at the tree base, or along stems and branches.
Mulch and Aerate-
Young trees are especially vulnerable to fluctuations of temperature and moisture and need mulching protection. Mulch is good insurance that both conditions will be evenly managed during cold and drought. Mulching is a good practice for both dormant and full-growing, vegetative trees.
Spread a thin layer of composted organic mulch to cover the soil several inches deep. Cover an area at least as large as the branch spread. In addition to protecting feeder roots, mulch also recycles nutrients directly to these roots.
Aerate soils and compacted mulch if they are water-logged or poorly drained. Saturated and dense soil can suffocate roots. It is critical not to damage tree roots in the soil as you do this so work only on those few inches at the surface crust.
Fertilize and Water -
Fertilize by top dressing over the mulch with a balanced fertilizer if the essential elements are in short supply within the soil. Be sure to use nitrogen lightly, especially under large, mature trees and around newly planted trees. You do not want a vegetative "flush" of growth during late fall periods of warming. Large applications of nitrogen cause this growth.
Dry spells in winter or hot daytime temperatures will desiccate a tree very quickly. Watering may be needed where soils are cool but not frozen, and there has been little precipitation. Winter droughts need treatment with water the same as summer droughts, except it is much easier to over-water in winter.
Dormant Spray -
A dormant spray may be a good idea for deciduous trees, ornamentals, fruit trees and shrubs. But remember not to spray until after you prune. Obviously, you will lose much of your effort and expense if you cut off treated limbs.
Choice of chemicals is important. Dormant sprays include lime, copper and sulfur combinations to kill overwintering microorganisms. Dormant oil controls insects and their eggs. You just may need several types of sprays and oils to be effective.
Avoid spraying any of this material in the hot sun as it can damage dormant buds. Get specific chemical recommendations from you local county extension agent.