Spring-blooming woody plants like lilacs (especially the old, grafted varieties), ornamental cherries, forsythia, crabapples, azaleas and some magnolias set their flower buds for the following year in early summer shortly after flowering.
Usually, flower buds are triggered to bloom by environmental conditions which normally occur after an extended fall and winter cold period, followed by longer days and warming temperatures in spring. It is not typical for these plants to have a second bloom in the fall, but environmental conditions sometimes trigger premature flowering in the fall. Some plants may have only a few flowers rebloom, while other plants may have more flowers open in the fall.
Some of the reasons for this out of season bloom are extended summer heat and drought conditions where supplemental water is lacking. Severe early defoliation, especially from certain fungal pathogens, can also contribute to reblooming. The past two springs have been very wet and diseases such as anthracnose and Pseudocercospora spp. leaf spot may have caused leaves to brown, shrivel and drop early. This stresses the shrub and contributes to out of sync rebloom if other conditions are right. Good sanitation practices such as cleaning up infected leaves will be helpful in reducing fungal infections the following year.
If a plant is healthy and relatively unstressed, the normal seasonal move to cooler weather triggers dormancy. Plants that are deciduous will drop leaves as daylight length and temperature both decrease. Next year’s leaf and flower buds will also remain in a dormant state. Flowering and leafing out will be triggered by increasing daylight and air temperatures the next spring.
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If certain woody plants have been stressed during the growing season, however, the change to cooler weather followed by some warmer weather can trigger some of the flower buds to open prematurely. This false dormancy especially affects flower buds near the tops of old-style lilacs where it is sunnier and warmer. Ornamental cherries may show sporadic flowering all over the tree where there is a southern exposure.
While fall reblooming of ornamental trees and shrubs can lead to a disappointing floral display the following spring, it is not harmful to the plant. After a less showy spring bloom period, flower buds will be produced normally. If stressful conditions caused by environmental conditions, insect pests or fungal pathogens are minimal, then a second bloom in autumn is unlikely to occur. Gardeners and landscapers can only do so much, and while the weather is out of our control, making sure plants are maintained properly to avoid stress during the summer will go a long way in helping them remain as healthy as possible.
The UConn Home & Garden Education Center supports UConn Extension’s mission by providing answers you can trust with research-based information and resources. For gardening questions, contact us toll-free at (877) 486-6271, visit our website at homegarden.cahnr.uconn.edu, or reach out to your local UConn Extension center at extension.uconn.edu/locations.
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