Winter wheat (usually Triticum aestivum) are strains of wheat that are planted in the autumn to germinate and develop into young plants that remain in the vegetative phase during the winter and resume growth in early spring. Classification into spring or winter wheat is common and traditionally refers to the season during which the crop is grown. For winter wheat, the physiological stage of heading (when the ear first emerges) is delayed until the plant experiences vernalization, a period of 30 to 60 days of cold winter temperatures (0° to 5 °C; 32–41 °F).