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 occurrence of flowers on plants that normally bloom in spring can be a surprising sight in the landscape. Why do some spring-blooming trees come back into flower in 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.”

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