Adapted from the articles “Chill model comparison – Why two models judged last winter very differently” by Katherine Jarvis-Shean, UCCE Farm Advisor Sacramento, Solano and Yolo Counties and “Low Chill Winter?” by Franz Niederholzer, UC Farm Advisor, Colusa/Sutter/Yuba Counties in the Sacramento Valley Prune News, June and April 2014, respectively.

Dormancy is a process by which fruit trees protect cold sensitive tissue — flowers, shoots, and leaves — from winter cold injury. Field observations and lab research show that dormancy is a two-step process that requires a certain amount of cold temperatures (chilling) followed by a certain amount of heat before bud break can begin. Differences in chilling from year to year influence the timing of bud break and, in some cases, the quality of flowers. Low chilling can cause non-uniform flowering and/or leaf out.

There are currently two popular ways to count chill – Chill Hours or Chill Portions. Depending on how you count chill accumulation, the winters prior to 2014 and 2015 bloom were either some of the warmest on record or just about normal. Based on the behavior of most prune trees at bloom in those years and numerous research studies, the Chill Portions model is probably more representative of how flower buds count chill than the traditional Chill Hours model. But how does the Chill Portions model work, and what makes it different from other chill models?

The math behind the Chill Portions model is more complex than the Chill Hours model. But the new model is based on some fairly simple components – different temperatures having different chill values, and warm hours being able to subtract from previously accumulated chill.

In the Chill Portions model, hours at different temperatures have different chill values. In the Chill Hours model, any hour between 32-45° F has the same value, one chill hour. In the Chill Portions mode, hours at 43-47° F having the maximum chill value, and chill value drops at lower and higher temperatures, down to no chill value at 32° F and 54° F. Rather than saying, ‘We had X chill hours but they were warm chill hours’, the Chill Portions model puts a value on that difference. It also expands the range of temperatures considered effective for chill accumulation.

Warm hours can subtract from chill accumulation in the Chill Portions model, but not in the Chill Hours model. Accumulation of chill according to the Chill Portions model is a two-step process. In the first step, a ‘chill intermediate’ is accumulated, but this accumulation can be subtracted from given subsequent warm temperatures. In the second step, once the ‘chill intermediate’ accumulates to the certain threshold, it is converted into a permanent ‘chill portion’, which can’t be lost. Accumulation of a new chill intermediate starts again from zero.

There are a number of ways in which warm temperatures can subtract from chill accumulation in the Chill Portions model. Chill subtraction is highest if the warm temperatures quickly follow cool temperatures, for example when a warm day follows a cool night. The longer the duration of warmth in that daily cycle, the more subtractions. However, very short exposure to warm temperatures, say an hour at 68° F on a January afternoon, has no subtracting effect.

Chill accumulation calculators for both models using CIMIS weather station data can be found at UC Fruit and Nut Center. There you can also find additional information on the models, how to calculate accumulation with your own weather data, and estimates of chilling requirements for some crops in Chill Portions.