From The OSU Extension Service
The late dormant season is best for most pruning. Pruning in late winter, just before spring growth starts, leaves fresh wounds exposed for only a short length of time before new growth begins the wound-sealing process. Another advantage of dormant pruning is that it’s easier to make pruning decisions without leaves obscuring plant branch structure.
Pruning at the proper time can avoid certain diseases and physiological problems:
- DO NOT prune oaks from April to October to avoid oak wilt disease. If oaks are wounded or must be pruned during these months, apply wound dressing or latex paint to mask the odor of freshly cut wood so the beetles that spread oak wilt will not be attracted to the trees.
- To avoid the increased likelihood of stem cankers, prune honeylocusts when they are still dormant in late winter. Avoid rainy or humid weather conditions if they must be pruned in summer.
- Prune apple trees, including flowering crabapples, mountain ash, hawthorn, and shrub cotoneasters in late winter (February-early April). Spring or summer pruning increases chances for infection and spread of the bacterial disease fireblight. Autumn or early winter pruning is more likely to result in drying and die-back at pruning sites.
- Some trees have free-flowing sap that “bleeds” after late winter or early spring pruning. Though this bleeding causes little harm, it may still be a source of concern. To prevent bleeding, you could prune the following trees after their leaves are fully expanded in late spring or early summer. Never remove more than 1/4 of the live foliage.
· All maples, including box elder
· Butternut and walnut
· Birch and its relatives, ironwood and blue beech
- Trees and shrubs that bloom early in the growing season on last year’s growth should be pruned immediately after they finish blooming; otherwise, you will be cutting off next year’s blooms. Never remove more than 1/3 of the live foliage.
Lilacs, Azaleas, Rhododendrons, Pieris, Weeping Cherries, Forsythia, Fothergilla, Deutzia, Dogwoods, Magnolias, and Redbuds.
With few exceptions, evergreens (conifers) require little pruning. Different types of evergreens should be pruned according to their varied growth habits.
- Spruces, firs, and Douglas-firs don’t grow continuously but can be pruned at any time because they have lateral (side) buds that will sprout if the terminal (tip) buds are removed. It’s probably best to prune them in late winter before growth begins. Some spring pruning, however, is not harmful.
- Pines only put on a single flush of tip growth each spring and then stop growing. Prune before these “candles” of new needles become mature. Pines do not have lateral buds, so removing terminal buds will take away new growing points for that branch. Eventually, this will leave dead stubs. Pines seldom need pruning, but if you want to promote more dense growth, remove up to two-thirds of the length of newly expanded candles. Don’t prune further back than the current year’s growth.
- Arborvitae, junipers, yews, and hemlocks grow continuously throughout the growing season. They can be pruned any time through the middle of summer. Even though these plants will tolerate heavy shearing, their natural form is usually most desirable, so prune only to correct growth defects.