Pruning flowering shrubs
Larimer County CSU Extension Master Gardener
With the tease of spring in the air, many gardeners are itching to start work outdoors. People often wonder when the best time is to prune flowering shrubs. Regular, proper pruning influences flowering, size, shape and resistance to disease and pests of shrubs. Spring-flowering shrubs produce blooms on wood that grew the previous summer.
Pruning shrubs like lilac and forsythia in the early spring will remove flower buds – so prune after the plant blooms, unless you are willing to sacrifice the flowers. Summer-flowering shrubs bloom on new wood developed this growing season, and these shrubs, like dogwoods and blue mist spirea, can be pruned in early spring.
Since pruning encourages new growth, one of the most visible advantages is improved flower production. In the case of shrubs with unusual bark color, such as red- and yellow-twig dogwoods, the new growth is a more dazzling color than older stems.
Pruning is also a way to guide size and shape of shrubs. Prune back to a side bud or branch that is growing in a pleasing direction to influence the shape of the shrub. Pruning is a contributing factor in maintaining size of the plant, although it will not be permanently successful in helping overcome problems of an overly large shrub that was planted in too small a space.
Removing old wood opens the center of the plant to receive more sunlight and better air circulation, which increases overall health. It also reduces the shrub’s susceptibility to diseases and pests such as powdery mildew, oystershell scale and borers.
Since the main goal of pruning flowering shrubs is to promote flower growth, it is important to use good pruning techniques such as thinning or rejuvenation. Unfortunately, it is common to see shrubs that have been sheared (simply chopping off all the ends) because it is quick and easy. While this creates a uniform rounded ball shape, it actually can reduce the number of flowers and weaken the overall health of the shrub by creating bushy exterior growth rather than new growth from the base of the plant. Ultimately, the shrub becomes a thin shell of foliage and flowers with a woody spindly interior and base, making it more susceptible to wind and weather damage, as well as insects and disease.
Two techniques that encourage plant health are annual thinning and rejuvenation pruning. Annual thinning strives to remove the oldest third of the wood back to the ground each spring before the shrub leafs out. This stimulates new, flowering growth from the base of the shrub and keeps the shrub attractive and vigorous. Rejuvenation pruning, best done every three to five years in early spring before growth begins, involves cutting the entire shrub back to the ground. The shrub simply reacts by re-growing from its roots, resulting in a youthful, compact plant with maximum bloom. Shrubs that lend themselves to rejuvenation include hydrangeas, spirea and red- and yellow-twig dogwoods.
There are several caveats associated with rejuvenation pruning including the fact that spring-flowering shrubs will not bloom in the year they are renewed. Extremely overgrown shrubs with large woody bases or shrubs with a lot of dead branches may not be healthy enough to respond well to renewal pruning. In addition, landscape fabric and some mulches around the base of a rejuvenated shrub may interfere with new growth. Finally, lilac cultivars that were budded onto common lilac root stock should not be rejuvenated because new growth will be the common lilac rather than the graft.
As warm days beckon gardeners outside, fight the most common gardening temptations: performing spring cleanup of leaves, which still add value as insulation; planting perennials before the average last frost date in mid-May; or pruning roses, whose tender new growth can be damaged by our variable spring weather. Instead, take the opportunity to look closely at the form of the shrub now, before it leafs out, so once the blooms fade you will be ready to prune. Using thinning or rejuvenation pruning, you will be able to give them a head-start to a healthier, bloom-filled future.
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