How Plants Sense Winter: Dormancy Signals

By Nina Walsh · October 3, 2026

The Two Signals: Photoperiod and Temperature

Plants use two primary environmental cues to time dormancy: day length (photoperiod) and temperature. These signals interact in complex ways that vary by species.

Photoperiodism in Dormancy

Shorter days trigger hormonal changes. As daylight drops below a species-specific threshold, the plant produces abscisic acid (ABA), which slows growth and initiates leaf senescence. This is why warm Octobers do not prevent color change — trees respond to light, not temperature.

Chilling Hours and Vernalization

Many plants require a minimum number of hours between 32F and 45F (chilling hours) to break dormancy properly in spring. Without sufficient chilling, buds open unevenly or produce fewer flowers.

Chilling Requirements by Species

Apple trees: 500-1000 chilling hours
Blueberries: 400-800 chilling hours
Tulips and daffodils: 12-16 weeks of cold
Peonies: 400-600 chilling hours

Internal Clocks: Circadian and Circannual Rhythms

Plants have internal biological clocks that track daily and seasonal cycles. These persist even under constant light, suggesting a genetic program that anticipates seasonal changes.