Annual Chill Unit Accumulation in the U.S.

Abstract

The effectiveness of different temperatures in overcoming the winter-time physiologic state of dormancy (rest) of temperate zone fruit plants has been described (2, 3, 6). Through controlled refrigeration experiments, temperatures between 3°C and 10°C were determined to be the most chill efficient, temperatures above 15°C reversed the chilling process, and freezing or near freezing temperatures were ineffectual. The term, chill unit, expresses this time-effective temperature relationship and is defined as: 1 hour × a coefficient of the chilling efficiency of the mean hourly temperature. The optimum temperature has a coefficient of 1.0, ineffective temperatures have a zero coefficient, chill reversing temperatures have a —1.0 coefficient and intermediate temperatures are assigned intermediate, fractional, coefficients. Subsequently, the rest requirement of many deciduous fruit species were defined in terms of chilling units (4, 5).