Kathy Zuzek, UMN Extension Educator, Horticulture
Many shrubs are planted in landscapes for their wonderful floral display. One of the few shrubs that have bloomed to date during this slow-to-arrive spring is forsythia. With masses of yellow flowers, forsythias are wonderful plants in the spring landscape. But cultivars with appropriate flower bud hardiness for use in Minnesota are large shrubs with rapid growth rates. Their ability to quickly produce large quantities of upright and arching stems that tangle together to produce an unkempt-looking plant turns them into aesthetic liabilities in the landscape after blooming. Consistent pruning is the solution. Enjoy the floral display but remember that if you have reason to prune your forsythia, pruning should happen immediately after bloom.
Like many of our shrubs that bloom in early spring, forsythias produce flower buds on previous years' stems. To enjoy maximum bloom on these shrubs both this year and next year, it is important to prune these plants immediately after flowering stops this spring. This allows enough time for plants to produce stems during the 2011 growing season that will house next year's flower buds. A list of shrubs that bloom on previous years' wood can be found at http://www.extension.umn.edu/b-assets/efans/ygnews/2010/03/dont-prune-the-bloom-when-to-p.html.
You may also need to prune shrubs this spring because of damage that occurred during winter thanks to rodents, rabbits, and low winter temperatures.
Mice and voles were busy under the snow line this winter stripping bark at the base of shrubs and young or thin-barked trees (Photo 1). Rabbits can create the same kind of injury by stripping bark above the snowline. If this feeding removed bark completely around stems and trunks, plants are effectively girdled and stems and trunks will die above the point of girdling.
Most deciduous shrubs have the ability to produce new shoots from their crowns or from roots. Prune off the girdled stems this spring and new shoots will replace the damaged stems over the next few years. On the other hand, the best course of action for a completely girdled tree is removal and replacement with a new tree. Many trees sucker below girdling damage. Many trees are also produced through grafting or budding onto rootstocks. Suckers from below the graft union that are trained into a new tree will give you a different (and probably undesirable) tree than the one you started with.
Rabbits will often clip entire stems off of deciduous shrubs as they feed during winter. If this browsing injury left you with irregularly shaped shrubs and stems of widely different heights, you may want to prune these plants to recreate their natural plant habit. Entire stems can be removed at the base to allow growth of new replacement stems from the crown or roots. Or stems can be pruned back to ¼" above a bud that will direct growth in a desirable direction as the bud expands and grows. Make this pruning cut at a 45° angle with the high point of the cut directly above the bud.
Winter injury should also be removed from woody plants this spring. Shrub roses are a prime example of this. Many repeat flowering shrub roses experience extensive cane mortality due to low winter temperatures. The injury is very evident in spring. Dead canes will be brown or black and there will be no actively expanding buds. Healthy canes can be green, red, or brown (depending on cane age and cultivar) and you will see buds along these canes actively expanding and growing in spring (Photo 2). Often low winter
temperatures kill canes protruding above snow while the bases of canes covered by snow are not injured. Find an outwardly facing bud in the live portion of canes and prune canes back to ¼" above this bud using the 45° angle cut mentioned above. If canes are killed to the base, remove them completely and new canes will develop from the crown and roots of the plant.
As you prune woody plants in spring, you may see sap oozing from pruning wounds (Photo 3). This may continue for several days and happens to varying degrees on many trees and shrubs. It is most noticeable on trees because pruning wounds are larger and are at or above eye level. This oozing or "bleeding" occurs when increasing temperatures in spring activate enzymes that convert the starches that store energy in plants into sugars that can be easily transported throughout the plant. As starches are converted into sugar, water movement is initiated upwards in stem and root xylem tissue due to differences in sugar concentrations throughout the plant. Pressure increases in the plant and this pressure results in the "bleeding" from new pruning wounds. Bleeding can be unsightly but is not harmful to the plant. It actually signals active cell activity and a rapid pace of wound healing.
Remember that oak trees should not be pruned in April, May, or June. Sap feeding beetles of the Nitidulidae family are the insects commonly responsible for long-distance movement of fungal spores that lead to new infection centers of oak wilt. The beetles are attracted to both fresh wounds (like a pruning cut) on oak trees and to the fruity smell of the fungal spore mats produced in April, May, and June on oaks infected with oak wilt (Photo 4). As they visit the mats, they pick up spores that can be transferred to a fresh wound on a healthy tree. As these spores infect the tree, oak wilt occurs.