Although there are notable exceptions, the majority of flowering plants in the Arnold Arboretum’s Living Collection bloom over the course of spring and early summer. While it is normal for some plants like the Franklin tree (Franklinia alatamaha) to flower in early fall, the autumn or winter occurence of flowers on plants that normally bloom in spring can be a surprising sight in the landscape. Why do some plants flower again during the off season? Is it a sign of climate change?
For Iñaki Hormaza, a Research Associate of the Arnold Arboretum and Professor at the Mayora Research Station of the Spanish Council for Scientific Research (CSIC), this phenomenon provides an opportunity to study how plants regulate their biological functions. “As temperatures start to drop after summer,” he explains, “flower buds enter a dormant stage to help them to survive harsh winter conditions.” The duration of this dormancy is specific for each plant, but usually lasts the entire winter. “Once the plant has accumulated its required number of cold hours,” he adds, “warmer temperatures induce flowering.”
However, in some cases, such as the Fuji cherry (Prunus incisa f. serrata) near Faxon Pond pictured here, the required dormancy may be quite brief, and warm days in November or December may trigger early flowering. Atypical flowering behavior may be related to flower developmental arrest resulting from environmental stresses like low temperatures or drought in early fall followed by warm and rainy weather. It is likely that this situation may be observed more often in the future as global climate change delivers more frequent irregularity in weather patterns. The Arnold Arboretum’s extensive collections provide ample opportunity to study dormancy to understand how crucial biological functions like flowering may be impacted by incremental shifts in our environment.