Business Situation

Though it is common knowledge that many crops are negatively impacted by cold temperatures, many of the same crops can benefit and thrive from cold temperatures in moderation.   For flowers and leaf buds to develop normally on stone and pome fruit trees, they rely on having enough chill hours to start the process of going from winter dormancy to spring bloom.  This can have the effect of delaying foliation, and reducing fruit quantities and quality.  Producers and processors track the number chilling hours during the Fall/Winter season to have an estimate of the yield and potentially identify needed changes in crop maintenance activities. 

"We actually need more chilling hours for the fruit and nut trees," said Roger Duncan, a University of California farm adviser. "Almond trees are looking better, but the fruit trees really need another 150 hours of cold."  (complete article)

Chilling Degree Hours Models

David Byrne (Dept. Of Horticultural Sciences, Texas A&M University) has researched the potential impacts chilling has on various crops and has developed a model to estimate and count Chilling Degree Hours.

• Chilling Accumulation: its Importance and Estimation

There are three widely accepted models used to estimated the number of Chilling Degree Hours that have occurred -- each requires hourly weather observations for the region being measured.  For this scenario, we have chosen to use the Utah Model since it includes relative chilling effectiveness and negative chilling accumulation.

Utah Model

1 hour below 34 degrees F =0.0 chill unit

1 hour 35-36 degrees F =0.5 chill units

1 hour 37-48 degrees F =1.0 chill units

1 hour 49-54 degrees F =0.5 chill units

1 hour 55-60 degrees F =0.0 chill units

1 hour 61-65 degrees F =-0.5 chill units

1 hour >65 degrees F =-1.0 chill units

Solution

To manage the negative impact that could result in a winter that did not have enough Chilling Degree Hours, we structured a solution that uses the Utah Model to protect a crop of Yellow April Glo Nectarines.  The optimal Chilling Degree Hour range for the chilling season for the Yellow April Glo Nectarine is 200-300 hours.