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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 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
Blueberry, 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

 

2021-2022 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 1-16, 2021 39.3 62.9 206.6 145.0
November 17-30, 2021 36.5 61.4 162.0 127.0
December 1-15, 2021 41.4 64.9 129.1 46.3
December 16-31, 2021 41.5 62.3 117.7 68.8
January 1-15, 2022 27.1 50.1 176.6 135.9
January 16-31, 2022 25.7 58.0 159.2 109.4
February 1-15, 2022 29.7 67.1 145.7 75.6
February 16-28, 2022 30.3 62.9 134.8 72.5
March 1-15, 2022 37.2 66.8 146.3 71.6
March 16-31, 2022 41.4 68.5 152.8 80.0
April 1-15, 2022        
Average*/Cumulative**
35.0* 62.5* 1530.6** 935.4**

 

Kibler, Vegetable Research Station (zone 7b)

For Selected Reporting Period

Date Average Low Temp Average High Temp Chill Model Utah Model
November 1-16, 2021 39.9 65.9 212.0 139.3
November 17-30, 2021 34.4 66.0 153.3 84.1
December 1-15, 2021 39.8 68.4 117.9 32.1
December 16-31, 2021 41.4 63.5 147.2 71.2
January 1-15, 2022 26.4 52.1 166.5 113.0
January 16-31, 2022 23.9 54.2 149.6 95.9
February 1-15, 2022 26.7 66.1 136.2 63.8
February 16-28, 2022 29.2 55.6 98.1 40.3
March 1-15, 2022 34.6 71.6 151.9 51.4
March 16-31, 2022 40.4 71.5 168.7 76.3
April 1-15, 2022        
Average*/Cumulative** 33.7* 63.5* 1500.4** 766.1**

 

Hope, Southwest Research Station​ (zone 8a)

For Selected Reporting Period

Date Average Low Temp Average High Temp Chill Model Utah Model
November 1-16, 2021 37.1 64.9 57 57
November 17-30, 2021 35.1 65.5 59 58
December 1-15, 2021 40.0 66.1 54 54
December 16-31, 2021 48.1 71.0 67 67
January 1-15, 2022 30.8 57.0 173 129
January 16-31, 2022 26.9 53.1 207 145
February 1-15, 2022 30.3 58.6 152 109
February 16-28, 2022 35.3 57.5 156 144
March 1-15, 2022 37.0 64.6 94 86
March 16-31, 2022 38.7 70.8 133 133
April 1-15, 2022        
Average*/Cumulative**
35.9* 62.9* 1152.0** 982.0**
*Average high and low temperature data were collected from: https://www.weather.gov/wrh/climate?wfo=shv
**Chilling hours for Hope, AR were calculated using the chill hours app developed by Mississippi State University Extension. The chilling hours app can be found here.

 

 

2020-2021 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 1-16, 2020 47.8 67.8   78.7 -22.2
November 17-30, 2020 42.3 59.1   166.1 132.7
December 1-15, 2020 34.6 51.6   232.1 203.1
December 16-31, 2020 32.9 51.3   240.8 213.8
January 1-15, 2021 31.0 45.2   279.8 214.3
January 16-31, 2021 34.3 50.4   288.3 253.2
February 1-15, 2021 24.7 37.4   173.8 126
February 16-28, 2021 29.0 49.0   131.5 98.6
March 1-15, 2021 46.1 63.5   109.4 41.3
March 16-31, 2021 46.0 64.9   97.7 27.0
April 1-15, 2021 48.5 69.1   65.0 -55.4
Average 37.9 55.4 Cumulative 1863 1232

 

Hope, Southwest Research Station​ (zone 8a)

For Selected Reporting Period

Date Average Low Temp Average High Temp   Chill Model Utah Model
November 1-16, 2020 49.0 74.1   30 30
November 17-30, 2020 45.1 64.5   43 43
December 1-15, 2020 39.2 57.5   195 182
December 16-31, 2020 38.8 56.3   139 123
January 1-15, 2021 33.9 51.7   235 209
January 16-31, 2021 41.1 57.1   99 97
February 1-15, 2021 32.7 46.1   243 165
February 16-28, 2021 27.5 57.8   145 66
March 1-15, 2021 50.3 70.2   33 33
March 16-31, 2021 50.8 71.4   133 131
April 1-15, 2021 53.5 74.3   157 156
Average 42.0 61.9 Cumulative 1452 1235

*Chilling hours for Hope, AR were calculated using the chill hours app developed by Mississippi State University Extension. The chilling hours app can be found here.

**Average high and low temperature data were collected from: https://www.wunderground.com/history/monthly/us/ar/texarkana/KTXK

 

Check the Arkansas Chilling Hour Reports Archive to view past years reports.

 

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.

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