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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
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!
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.
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:
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
For Selected Reporting Period
*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.
Check the Arkansas Chilling Hour Reports Archive to view past years reports (2017-2021).
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
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.