How Trees ‘Know’ That It Is Time to Bud in Spring

US National Arboretum horticulturalist Scott Aker explains what signals budding of trees in spring in a video by The Atlantic. Aker explains how trees don’t start the process of budding just because temperatures get warmer, but instead that the process is triggered by the changing levels of the photoreceptor phytochrome as nights shorten. The warmth of spring then encourages the already-started bud growth.