Extension Educator, Horticulture
A common question around the Extension office this time of year is "Can I prune now?" The old adage is "prune when your pruners are sharp." Light pruning, just removing small or broken or diseased branches, can be done anytime. However major pruning requires a little thought to keep plants healthy and blooming. One key to pruning is to do a little pruning every year that way you won't have shrubs eating the garage or knocking on the front door.
Timing is especially important when pruning spring blooming shrubs. Early blooming shrubs develop their flower buds during the summer and fall of the previous year. It is sometimes referred to as blooming on old wood. Therefore as a general rule, shrubs that flower before June 15 should be pruned immediately after flowering. Pruning these shrubs in late summer, now or early spring will remove the flower buds for the season. Generally the shrubs don't die, they just don't flower.
Spring flowering shrubs are generally pruned by the renewal method. Each spring after flowering, prune out the largest stems to the ground to stimulate new growth from the crown and remaining stems.
Shrubs that should be pruned immediately after flowering include lilac, deutzia, kerria, mockorange, weigela, forsythia, arrowwood viburnum, St. johnswort, and redtwig and yellowtwig dogwood.
Shrubs that bloom after June 15 can be pruned in early spring, generally February and March. These generally bloom on new wood. Many of these shrubs can be pruned by the rejuvenation method. Rejuvenation is the complete cutting of all stems down to 4 to 6 inch stubs.
Rejuvenation is used when multistemmed plants become too large with too many stems to justify saving any one to two year old growth. In other words, the shrub is a tangled mess of stems.
The following respond well to rejuvenation pruning: Anthony Waterer sprirea, abelia, honeysuckle, beauty bush, snowberry, slender deutzia and privet.
Evergreens can be pruned lightly in winter to provide holiday decoration. However the majority of pruning should be delayed until late winter, early spring or summer depending on the species. For arborvitae, wait until mid-March and early April. Junipers can be trimmed in April. For yews, it is best to prune between late March and mid-June. Yews can also be pruned again in September. If pruned now the cut stubs may turn brown.
Pines such as Scotch, white, or mugho, are best trimmed in mid-to-late June just as the candles have elongated. Spruce, including blue, can be done in early July.
Most deciduous trees should be pruned while they are dormant, especially oak and elm. Pruning wounds on oak and elms can attract borers and beetles that are carriers of diseases such as Dutch elm disease and oak wilt. Winter also allows a good view of the branch structure. Dormant pruning of maples and birches is also recommended, since these species will often "bleed" if they are pruned in the spring. Ideally deciduous trees should be pruned in February or March once the severe cold weather has passed.
Pruning paints or wound dressings are not recommended. These products make us feel better but really do not help the plant. The bottom line is we are entering a stressful time for plants so pruning may be best left for later in the season or next spring.
For some great videos on pruning, check out our website http://www.urbanext.uiuc.edu/video/