The optimal time of year to prune deciduous shrubs is in late winter and early spring before bud break. Healing of pruning cuts is rapid as spring growth starts and pests that could infect or invade open pruning wounds are dormant in winter. But if maximizing bloom on deciduous shrubs and trees is important, pruning times may change.
Generally, shrubs flowering later in the season (hydrangeas, summer-blooming spiraea, and potentilla), flower on current season's stems, commonly referred to as new wood. These shrubs can be pruned in late-winter to early spring without reducing bloom.
Spring flowering shrubs (forsythia, bridal wreath spiriaea, and rhododendrons) generally flower on the last year's stems, commonly referred to as old wood. These shrubs are best pruned immediately after flowering to conserve next year's bloom.
To more fully understand how and when spring blooming shrubs form buds on new and old wood, enlarge the annotated images in Figures 1 through 5 by clicking on the thumbnail image. These figures supplement the following pruning examples for Emerald Triumph viburnum and Summer Wine ninebark.
Figure 1 shows a branch of Viburnum 'Emerald Triumph'. The round flower buds at the tips of the 2009 growth will bloom this spring so this plant blooms on "old" wood. The vegetative buds that will produce new stems and leaves during the 2010 growing season are the tall thin buds below the flower buds. Leave this branch unpruned and in spring of 2010, you will have the bloom and new stems and leaves shown in Figure 2. The stems will continue to grow and lengthen through mid-summer. Along these stems, new vegetative buds will form and a flower bud will be produced at the tip. These will be the buds that produce stems and flowers in 2011.Now imagine pruning this viburnum sometime in the next 2 months. As you remove or shorten stems, you are also removing any flower buds at the stem tip that would have bloomed in spring of 2010. Pruning in late winter will not affect the plant's health, but it will decrease the number of flowers and fruit produced in 2010.
What if you waited to prune until late this summer? Bloom would be maximized this spring. But by late this summer, new flower buds for the 2011 bloom will have already developed at the tips of this year's stem growth. By waiting to prune until late summer some of next year's flower buds will be removed. Pruning also stimulates new growth and late summer pruning can result in a flush of new growth that will not harden off properly, resulting in winter injury. So pruning in late summer maximizes bloom in spring of 2010, reduces bloom in 2011, and creates the risk of a late season growth flush that would be susceptible to winter injury.
If maximum floral display is important, the best time to prune this shrub is immediately after bloom. The shrub will bloom this spring. After pruning, growth will occur until mid-summer. Next year's flower buds will be found at the tip of this growth by late summer. The plant will harden off properly and will not be injured during next winter. By pruning immediately after bloom ends, a gardener will maximize bloom in both 2010 and 2011.Figure 3 shows the normal plant habit of 'Summer Wine' ninebark. Ninebarks are often pruned to control the rampant growth that leads to long stems and the open and loose plant form you can see in the photo. In Figure 4, you can see a specimen of 'Summer Wine' that was pruned into a dense symmetrical plant habit. This plant was pruned the previous spring immediately after it finished blooming, which gave the plant time to set floral buds that led to the next season's heavy bloom you see in the photo.
How do you tell where "old" wood ends and "new" wood starts? The vast majority of plants form a vegetative bud at the end of a year's stem growth. As that bud opens and expands the following spring, the bud scales fall off and leave a distinct scar around the stem. Figure 5 shows an example of this terminal bud scale scar on an ash branch. This scar shows where one year's growth ended and the next year's growth started. Oftentimes there is also some difference in stem color between the new stem growth and last year's stems.
Table 1 lists major shrubs, vines, and small trees that grow in Minnesota along with information on whether they bloom on previous year's growth, current year's growth, or both. All of these plants can be pruned in late winter or early spring without affecting plant health. Plants that bloom on "new" wood can be pruned in late winter or early spring without diminishing this year's bloom. Wait to prune plants whose flower buds are produced on "old" wood until immediately after flowering if you want to enjoy the full amount of bloom this spring.