What are chill hours and why do fruit and nut trees need them?
Nut and fruit trees (except citrus) need a specific number of chill hours each winter to regulate their growth.
Trees listen for the signal that winter has arrived. The necessary signal strength varies between species, but is officially referred to as “chill hours, when the temperature stays between 32°-45°F. The hormone responsible for dormancy breaks down in this range, allowing buds to develop into flowers or foliage when the weather warms up in late winter. Interestingly, temperatures below 32°F are ineffective and do not count and hours when temperatures exceed 60°F are actually subtracted from the accumulated chill hours.
If a tree does not experience enough chill hours in a winter, flower buds may not open or may open unevenly, resulting in a lack of fruit production or deformed fruit.
Figs, olives, and quince have the lowest natural chill requirements, followed by persimmons, pomegranates, almonds, and chestnuts. The more commonly grown fruits such as cherries, apples, peaches, and plums have much higher chill hours. Selective breeding of specific trees has resulted in low-chill varieties for southern residing states.