Spirea is a widely appreciated, hardy shrub that thrives across the Midwest, including the variable climate of Wisconsin. It is popular for its abundant bloom and generally low-maintenance nature. Proper pruning is necessary to maintain a tidy shape, prevent it from becoming overly woody, and maximize the spectacular floral display. Successful blooming depends heavily on timing your cuts correctly.
Classifying Your Spirea by Bloom Time
The first step in planning your pruning schedule is to correctly identify the type of Spirea you have, as this dictates the proper timing. Spirea shrubs are categorized into two main groups based on the age of the wood on which they set their flower buds. Pruning at the wrong time will remove the buds needed for flowering.
The two groups are defined by whether they bloom on “old wood” or “new wood.” Old wood refers to stems that grew during the previous season, where flower buds were formed the summer or fall before they bloom. New wood is the growth produced during the current season, with buds forming and blooming later in the same year.
Spring-flowering Spirea, such as Spiraea vanhouttei (Bridal Wreath) and ‘Snowmound’, bloom on old wood. Summer-flowering varieties, including Spiraea japonica cultivars like ‘Goldflame’, ‘Little Princess’, and the ‘Double Play’ series, bloom on new wood. This difference determines the correct time to prune your plant.
Pruning Schedule for Spring-Flowering Spirea
For spring-flowering Spirea, prune immediately after the flowers fade. This optimal window is typically late spring or very early summer in Wisconsin. Make cuts right after the bloom cycle is complete, generally within two weeks, before the shrub begins forming next year’s buds.
Pruning at this time allows the plant maximum time to produce strong new growth that will host the flower buds for the following spring. Pruning too early eliminates the current year’s bloom. Pruning too late, such as in late summer or fall, removes the newly formed buds, significantly reducing the flower show next season.
This immediate post-bloom timing directs energy into new vegetative growth, which quickly matures during Wisconsin’s growing period. Maintaining this schedule prevents the loss of the characteristic white, arching cascade of blooms. By late summer, the new stems will have hardened off and set their buds, preparing for winter dormancy.
Pruning Schedule for Summer-Flowering Spirea
Summer-flowering Spirea varieties must be pruned during the dormant season, which is late winter or very early spring in Wisconsin. This timing, usually from March into early April, should occur just before the new leaf buds begin to swell and break dormancy. These plants bloom on new wood, meaning flowers are produced on the growth that emerges during the current season.
Pruning the shrub while dormant encourages a vigorous flush of strong growth from the base once the weather warms. A hard cut stimulates the plant to produce many new stems, and summer flowers will appear on the tips of these stems. This process leads to a denser, more floriferous shrub compared to one left unpruned.
This group of Spirea, including S. japonica types, is highly tolerant of aggressive pruning. Cutting them back by as much as one-third of their height, or even more severely for rejuvenation, results in a healthier, more compact shrub with abundant summer color. Pruning during dormancy also avoids stimulating tender new growth late in the season that could be damaged by an early fall frost.
Essential Pruning Techniques
Regardless of whether your Spirea blooms on old or new wood, the method of cutting should employ three main techniques to ensure shrub health and proper shape. The most impactful method is renewal pruning, which involves selectively removing the oldest, thickest canes right down to the ground. This action promotes new, youthful growth from the base, which is more vigorous and produces better flowers.
This process is typically done by removing about one-third of the oldest stems annually or every few years, rather than shearing the entire plant. Thinning involves taking out weak, crossing, or damaged branches throughout the canopy to improve air circulation and sunlight penetration. Better air flow helps prevent fungal diseases common in dense shrubs.
For summer-blooming varieties, deadheading is often employed after the first flush of flowers fades. Deadheading is the removal of spent flower clusters, which encourages a second, smaller wave of blooms later in the summer. Targeted cuts are always preferable to general shearing to maintain the plant’s natural, rounded form.