Winter Chilling Hour Accumulations for Arkansas| Chill Hours and Heat Units for Fruit Production

Fruit crops vary in the amount of cold winter temperatures they require during dormancy in order to break bud and flower in the spring. The number of hours temperatures are below 45 °F and above 32° F are considered 'chilling hours'. Generally chilling hours are calculated starting after the first frost. We will report chilling hours from Nov 1st through the end of February for several locations across AR. 

What are chilling hours and why are they important?

Chilling hours are the number of hours a plant has spent below a certain temperature during the dormant period. 

Chilling hours are important because they regulate the plant's ability to 'wake-up' after their dormant period in winter. During the cold dormant period the plant essentially accumulates signals within their tissues that indicate when it is time to 'wake up' based on how many hours the plant has spent below a certain temperature. More hours spent at a certain range of col temperatures, equals more signals.

Different crops require a different amount of these signals before they are ready to break dormancy. Once the required number of signals is reached and temperatures warm, an alarm goes off within the plant telling it to wake itself up!

We monitor chilling hours because it is a means to know when plants will be ready to break dormancy in the spring.

The rate of accumulation of these signals within the plant varies based on temperature. The ideal temperature for plants to accumulate chilling hours is 45°F, but it is generally assumed that temperatures between 35-45°F provide good chilling hour accumulation. At temperatures below 35°F, generally no chilling is accumulated.

There are several ways to calculate chilling hours.

Some models assign different rates of chilling hour accumulation to different temperatures. Some models deduct chilling hours when temperatures go over 60°F. The Utah model is the standard model used for fruit crops.

Once the adequate number of chilling hours has been reached and temperatures warm the plant will be ready to break dormancy and buds will begin to grow and the plant will flower. For this reason it is important to choose varieties that have similar chilling requirements as what is received at your location. This will ensure in most years adequate chilling is achieved and that plants do not come out of dormancy before the winter is over.

Cold damage to emerged buds is common when low chill varieties are planted in an area that receives medium to high chilling hours on average.

For example: A peach variety with a 200 hour chilling requirement planted in a place with 700 average chilling hours is likely to break bud once the 200 hours have been met and if an un-seasonal warm spell occurs. However there is still likely to be 500 more hours of cold temperatures for that location! This peach variety is likely to experience cold damage to blooms in most years.

Serious impacts to plant growth occur when insufficient chilling hours are accumulated during the dormant period.

 Symptoms of Lack of Chilling Hours: 

• delayed bloom
• reduced fruit set
• reduced fruit quality 

Average Regional Chilling Hour Accumulation in Arkansas

Average chilling hour accumulation has fluctuated drastically in recent years from previous standards. For this reason we plan to post the number of chilling hours accumulated here for each year. More locations will be added as we are able to access that data.

Average Chilling Hours Accumulated by March 1^st for Major Locations in AR from 1990-2000*:

U of A Campus, Fayetteville- 1,024

Fruit Research Station, Clarksville- 1,081

Southwest Research Station, Hope- 901

Wynne, AR- 1,069

*Source: Vance, L and C. Rom. Chill and Heat Accumulation at Four Sites in AR, 1990-2000. Horticultural Studies. AAES Research Series 494. (Calculations were based on the Utah model)

Chilling Requirements for Fruit Crops