UACES Facebook Fruit tree temperatures for Arkansas | Chill days and heat units for horticulture

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.

As you will see below 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 1st 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

Crop

Avg Chilling Hours Required

Apple

 800-1,000

Blackberry

200-600 

Bluberry, Northern Highbush

900-1,000

Blueberry, Southern Highbush

150-500 

Blueberry, Rabbiteye

400-700

Cherry

700-1,000+ 

Fig

100-200 

Grape- table, wine

100-600 

Grape- Muscadine

200-600 

Nectarine

 400-900+

Peach

300-800 

Pear

400-900

Plum

400-700

Strawberry

200-300

 

2017-2018 Arkansas Chilling Hours Report

Clarksville, Fruit Research Station (zone 7a)

  For Selected Reporting Period
Date Average Low Temp Average High Temp Chill Model Utah Model
November 1st-21st, 2017 46.9 63.2 76.5 105.1
November 22nd, 2017-January 18th, 2018  30.8 48.5  410.0  452.4 
 Januarry 19th - February 20th, 2018  35.4 52.2  245.0  233.6 
February 21st - March 20th, 2018*   38.6 56.3   64.6 90.1 
March 21th - April 15th, 2018 43.0 63.4 138.7 102.6
 Cumulative   38.9  56.7  934.8 983.8 

 *A technical malfuction with the weather station between February 21st and March 7th resulted in the loss of recorded chill hours in both the Chill Model and Utah Model. The temperative did drop between 34-45°F for nine days and therefore the cumulative chill hours are greater than what is represented in this table. 

Hope, Southwest Research Station​ (zone 8a)

  For Selected Reporting Period
Date Average Low Temp Average High Temp Chill Model Utah Model
 December 4th-20th, 2017  33.7  64.3 121.0  115..0
 December 21st, 2017- January 24th, 2018  28.3 55.8  235.0 215.0
 January 25th - February 27th, 2018  37.2  62.7 242.5 205.0
 February 28th - March 22nd, 2018  41.5 74.4  106.0  53.0
March 23rd - April 15th, 2018 81.8 47.2 66.0 27.0
 Cumulative  44.5  60.8  770.5 615.0

 

Explanations of Chilling Hour Calculations for the Models Used

Chill Model=Calculated using max temp of 45F and min temp of 32F, 1 unit is accumulated per each 1 hour between these two temperatures 

Utah Model

1 hour below 34°F = 0.0 chill unit

1 hour 35 - 36°F = 0.5 chill units

1 hour 37 - 48°F = 1.0 chill units

1 hour 49 - 54°F = 0.5 chill units

1 hour 55 - 60°F = 0.0 chill units

1 hour 61 - 65°F = -0.5 chill units

1 hour >65°F = -1.0 chill units

Source: Richardson, E. A., Seeley, S. D., Walker, D. R., Anderson, G. L. And Ashcroft, G. L. (1975) Pheno-climatography of spring peach bud development. HortSci. 10: 236-237.