Proper pruning time is one of the hardest landscape tasks to pin down. February into March is ideal because it is the late dormant season, just before the onset of spring growth when woody plant energy is at its highest for wound repair and there are very few insects and disease-causing pathogens.
Maples, elm and birch have early spring sap flow that will “bleed” from pruning cuts. It doesn’t hurt the tree but can be aesthetically unpleasant. If you have dead, broken or diseased branches, prune those out as soon as possible, but avoid pruning in full swing spring when leaves are forming because healing energy is low. Avoid autumn pruning which disturbs tree root set and invites winter damage. Only prune oaks in complete dormancy before the end of February or first week of March to avoid any introduction of Oak Wilt Fungus. Wound dressings on trees is not recommended.
For shrubs, the plant’s growth habit, bloom time and health determine proper prune time. Spring-flowering shrubs like lilac and forsythia bloom in spring on last year’s growth, so wait until after flowering to prune. If you have huge, neglected, overgrown shrubs that need to be rejuvenated, March into early April is the time. Keep in mind heavy spring pruning will eliminate or reduce flowers for two to three years.
ISU extension suggests two ways to prune for rejuvenating overgrown shrubs. The first is to prune over a three-year period, cutting one-third of the biggest, oldest stems to ground level in late dormant season. Repeat cutting out half of the oldest stems the following year, thinning out some of the new growth too, keeping only the most vigorous shoots. The third year, take out the rest of the old wood, keeping only the most vigorous shoots. If you don’t want to follow a three-year plan, prune heavily by totally cutting back to about 4 inches to 6 inches above the ground in late dormant season. You will get lots of new shoots, but loose this year’s blooms. The following year keep only the strongest shoots and remove the rest. Head (cut) back the retained shoots to encourage branching.
Summer bloomers like Potentilla and Spirea bloom in summer on current year growth. So now is a good time for pruning since new growth produced will still bloom. Deciduous shrubs that are used for foliage, colorful bark or fruit should also be pruned in late dormant season before growth begins, a three-year-plan is best for those. Prune evergreen shrubs, such as juniper and yew, in early to mid-April before new growth begins. Light pruning may also be done in midsummer. For more information, visit Extension.iastate.edu/yardgarden and download publication #SUL 0005 Pruning Trees: Shade, Flowering, and Conifer — Sustainable Urban Landscapes.Lisa Slattery is a Linn County Master Gardener.