Application of a Chilling Hour Climatology to Predict Fruit Crop growth in Illinois
or
How to Tell When Your Fruit Crops are Ready to Start Growing in the Spring
R.M. Skirvin1, A.G. Otterbacher1, K. Kunkel2, P. Czubak1 and S. A. Yiesla3
1 University of Illinois, Department of Natural Resources and Environmental Sciences, 258 ERML, 1201 Gregory Dr., Urbana IL 61801
2 Director, Midwestern Climate Center, Illinois State Water Survey, 2204 Griffith Dr.
Champaign, IL 61820
3 University of Illinois Extension Service, 117 Mumford Hall, Urbana IL 61801
If a plant has the genetic potential to do so, when it is dormant, it can survive winter (Skirvin and Otterbacher, 1995). The amount and severity of cold that a plant can withstand is a function of its genetic makeup, the type of environment that preceded the plant into dormancy, the amount of environmental protection that the plant receives (e.g. whether it is growing among trees and shrubs at the edge of a forest or standing alone in an open field), and the severity of the winter. The type of dormancy that a plant can develop also influences hardiness; plants are most hardy when they enter "rest" or true dormancy. Once a plant has entered rest, it will not grow again until it has received enough cold to overcome the dormancy.
To overcome true dormancy, deciduous fruit trees and small fruits require a certain number of hours of chilling temperatures in the range of 0° to 6-7°C (32 to 45°F). In high latitudes much of the winter may be so cold that native plants (boreal species) do not receive true chilling temperatures until late winter, when the intensity of the winter is reduced. Therefore some boreal species may have such low chilling requirements that they are unsuitable for culture in mid-latitude regions where they could easily receive enough chilling to grow in the autumn, long before winter begins. Until recently the status of chilling was determined by totaling all hours below 7°C (45°F) and using that value to estimate the status of dormancy. However, the best temperature for chilling is not extreme cold, but is probably in a range of about 0° to 6-7°C (32 to 45°F), For instance in apple, Thompson et al. (1975 [as discussed in Saure, 1985]) found that chilling at 2°C was more effective at breaking dormancy than 6°C; 6°C was more effective at breaking dormancy than 10°C. Thus, a better way to estimate dormancy is to maintain a record of the actual number of hours that have passed between 0° to 7°C.
Not all buds of a plant have equal chilling requirements. Generally flower buds require less chilling than lateral buds. This is obvious when you remember that flower buds often appear several days earlier than vegetative buds. Similarly terminal buds have a lower chilling requirement than lateral buds. Therefore, in moderate climates (with cool (not severe)) winters terminal buds can begin to grow soon enough to establish apical dominance over laterals. In areas of severe winters, by the time the growing season begins both the apical and axillary buds may have all of the chilling requirements met. Therefore when spring finally arrives, the plant will begin to grow from both lateral and terminal buds simultaneously. The resulting plant may be more bushy than an identical plant grown in milder conditions.
The amount of chilling required to break dormancy varies widely among species and cultivars. A chart of comparative chilling requirements is given in Table 1.
Table 1. Approximate chilling requirements of various fruit crops to break their winter rest. The differences between numbers represent differences between low chilling and high chilling cultivars within the species (adapted from Westwood, 1993)
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Plant Type Approximate Hours of chilling required to break
break dormancy (<7 degrees C)
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papaw 1000 to 1800
plum (American) 700 to 1800
plum (domestic) 700 to 1750
apple 800 to 1750
raspberry 800 to 1700
filbert (hazelnut) 850 to 1700
pear 600 to 1500
currant and gooseberry 800 to 1650
walnut 400 to 1550
plum (Japanese) 600 to 1600
pecan 650 to 1550
cherry (sour) 600 to 1500
cherry (sweet) 500 to 1450
blueberry (highbush) 800 to 1250
blueberry (rabbiteye) 200 to 400
peach 375 to 1200
apricot 300 to 1000
blackberry 350 to 600
quince 50 to 450
almond 50 to 450
persimmon (oriental) 50 to 450
grape 50 to 400 (adequate growth)
400 to 1650 (much better and faster
growth after adequate
chilling)
strawberry 50 to 300
fig 50 to 300
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These differences are most obvious in an area that has little or no frost. For instance, springs are very long and lovely in Portland, Oregon. The reason is that frosts are rare and most of the winter hovers with temperatures between freezing (32°F) and about 50°F. Therefore, as soon as a plant receives its requisite number of hours of chilling (for example, 300 hours) it will begin to flower and grow. The 400-hour plants will flower a week or so later; the 500-hour plants a week after that; etc. In contrast, Midwestern springs are beautiful but more compacted in time because the winters are so cold and persistent, that even non-dormant plants will not grow until the outside weather is warm enough. Therefore, Midwestern fruit plants flower about the same time regardless of when their chilling needs had been satisfied.
Once a plant's chilling requirement has been met, it is ready to grow, regardless of time of year. For this reason spring thaws and warm March weather can be disastrous to fruit growers. Warm weather can induce premature growth which can be damaged by subsequent cold and frosts. Remember, the growing season in Central Illinois does not begin until about May 12-15th, when the likelihood of frost has passed.
Although the amount of chilling required to break dormancy has been investigated for many species, it can be difficult to find specific chilling requirements for some cultivars. Even when the chilling requirement is known, weather data is seldom presented in a way that growers can determine whether their crops have received sufficient chilling to commence growth. In addition, data is seldom available to predict a date when a particular crop should have received enough chilling hours to begin growing. Such information would allow growers to make better decisions about frost protection, pruning, and other cultural practices that could encourage flower development or succulent growth which are susceptible to cold damage.
To predict chilling, growers need reliable data regarding the number of hours of chilling temperatures that can be expected at any date in a given location in the state. To obtain such information hourly temperature data were obtained from the National Climatic Data Center (the Hourly Airways Surface Observations dataset). These data were obtained at airports. The period of analysis and the latitude of the station is given in the following table. For most of the period, data were available for each hour. However, during the period 1965-1972, data were digitized only every 3 hours. For this period, temperatures for the missing hours were estimated by interpolating between the 3-hour values. Since temperature usually varies rather smoothly, this approximation is not expected to introduce significant error.
Table 2. Sites used obtain hours of chilling
Station Latitude Period of Analysis Data presented in
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Chicago-O'Hare IL 42 deg, 00 min 1959-1995 Table 3
Evansville IN 38 deg, 02 min 1951-1995 Table 4
Paducah KY 37 deg 04 min 1951-1995 Table 5
Peoria IL 40 deg, 40 min 1951-1995 Table 6
Rockford IL 42 deg, 12 min 1951-1995 Table 7
Springfield IL 39 deg, 50 min 1951-1995 Table 8
St. Louis MO 38 deg, 45 min 1951-1995 Table 9
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One of the authors (Kunkel) developed a computer program that totaled chilling hours and presented the data in terms of how soon one could expect to obtain a particular number of hours of chilling temperatures. For instance, in Chicago (Table 3) over a 45 year period 500 hours of chilling has not been observed earlier than November 13, but 500 hours of chilling has always occurred before December 12; half (50%) of the time 500 hours is obtained by November 26. For the grower this means that after December 12, a 500 hour plant (e.g. apricot) has had enough chilling that it could begin to grow during a February thaw and its blossoms appear long before frost free dates. In the case of apricot, the chilling requirement is so low (300 to 1000 hours) and they flower so early that their flowers seldom escape frost.
To use the data, growers should determine the approximate chilling requirement for their crop in Table 1. Then use the cities and latitude in Table 2 to locate the city closest to your farm. Then use the appropriate table to determine the date when a particular number of chilling hours can be expected. With this date in mind, growers can take necessary actions to protect their crop from cold injury, especially spring frost damage that is so common in Illinois and the Midwest.
References
Saure, M.C. 1985. Dormancy release in deciduous fruit trees. Horticultural Reviews. 7:239-300
Skirvin, R.M. and A.G. Otterbacher. 1995. How plants survive cold winters. Proc. 1995 Illinois Small Fruit & Strawberry Schools. University of Illinois, Cooperative Extension Service Horticulture Series 99:31-39
Westwood, M.N. 1993. Temperate Zone Pomology. Timber Press, Inc., Portland OR.
Table 3. Chicago, Illinois, chilling data (1959-95)
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Frequency that chilling requirement is met by this date
Chilling
hours 0%z 10% 20% 30% 50% 70% 80% 90% 100%y
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100 9-29 10-7 10-11 10-14 10-18 10-24 10-31 NA NA
200 10-16 10-22 10-25 10-27 10-31 11-5 11-11 NA NA
300 10-27 10-31 11-2 11-5 11-10 11-16 11-22 NA NA
400 11-6 11-9 11-10 11-13 11-17 11-26 12-1 NA NA
500 11-13 11-15 11-20 11-23 11-26 12-5 12-12 NA NA
600 11-17 11-21 11-27 12-1 12-4 12-14 12-22 NA NA
700 11-22 11-30 12-4 12-10 12-17 12-29 1-7 NA NA
800 12-1 12-6 12-15 12-20 12-29 1-17 2-23 NA NA
900 12-8 12-16 12-25 1-1 1-22 2-14 3-7 NA NA
1000 12-12 12-28 1-5 1-19 2-7 2-28 3-17 NA NA
1100 12-20 1-4 1-15 2-1 2-20 3-6 3-25 NA NA
1200 1-10 1-23 1-28 2-18 3-4 3-13 3-31 NA NA
1300 1-26 2-2 2-8 2-25 3-12 3-22 4-9 NA NA
1400 2- 1 2-11 2-27 3-7 3-21 4-2 4-19 NA NA
1500 2-17 2-25 3-9 3-14 3-30 4-9 5-3 NA NA
1600 2-22 3-8 3-19 3-23 4-5 4-18 NA NA NA
1700 2-27 3-16 3-27 3-30 4-13 5-10 NA NA NA
1800 3-9 3-25 4-5 4-9 4-22 NA NA NA NA
1900 3-18 4-2 4-15 4-19 5-7 NA NA NA NA
2000 3-27 4-11 4-28 5-2 NA NA NA NA NA
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zhas never occurred before this date
yhas never occurred later than this date
Table 4. Evansville, Indiana, chilling data (1951-95)
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Frequency that chilling requirement is met by this date
Chilling
hours 0%z 10% 20% 30% 50% 70% 80% 90% 100%y
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100 10-12 10-15 10-19 10-21 10-24 10-27 10-31 11-3 11-8
200 10-26 10-31 11-1 11-3 11-6 11-10 11-12 11-17 11-25
300 11-7 11-10 11-12 11-15 11-17 11-23 11-24 11-27 12-7
400 11-17 11-20 11-23 11-25 11-28 12-1 12-3 12-7 12-15
500 11-25 11-28 11-30 12-3 12-6 12-11 12-14 12-22 12-24
600 11-30 12-7 12-10 12-11 12-14 12-19 12-25 12-31 1-9
700 12-8 12-15 12-17 12-20 12-26 12-31 1-2 1-14 NA
800 12-14 12-24 12-26 12-29 1-3 1-9 1-12 1-22 NA
900 12-22 1-1 1-5 1-12 1-17 1-24 1-27 2-6 NA
1000 12-30 1-9 1-13 1-19 1-28 2-5 2-10 2-19 NA
1100 1-8 1-17 1-22 1-27 2-7 2-18 2-21 2-28 NA
1200 1-16 1-24 1-30 2-4 2-16 2-28 3-4 3-7 NA
1300 1-29 2-2 2-10 2-17 2-25 3-9 3-15 3-25 NA
1400 2-2 2-9 2-18 2-24 3-5 3-20 3-25 4-17 NA
1500 2-11 2-20 2-26 3-5 3-16 4-2 4-9 NA NA
1600 2-17 2-28 3-9 3-13 3-22 4-11 NA NA NA
1700 2-24 3-14 3-18 3-23 4-5 NA NA NA NA
1800 3-3 3-23 3-31 4-4 5-2 NA NA NA NA
1900 3-10 4-2 4-11 4-22 NA NA NA NA NA
2000 3-17 4-18 NA NA NA NA NA NA NA
_______________________________________________________________________________
zhas never occurred before this date
yhas never occurred later than this date
Table 5. Paducah, Kentucky, chilling data (1951-95)
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Frequency that chilling requirement is met by this date
Chilling
hours 0%z 10% 20% 30% 50% 70% 80% 90% 100%y
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100 10-12 10-15 10-19 10-21 10-24 10-27 10-31 11-3 11-8
200 10-26 10-31 11-1 11-3 11-6 11-10 11-12 11-17 11-25
300 11-7 11-10 11-12 11-15 11-17 11-23 11-24 11-21 12-7
400 11-17 11-20 11-23 11-25 11-28 12-1 12-3 12-7 12-15
500 11-25 11-28 11-30 12-3 12-6 12-11 12-14 12-22 12-24
600 11-30 12-7 12-10 12-11 12-14 12-19 12-25 12-31 1-9
700 12-8 12-15 12-17 12-20 12-26 12-31 1-2 1-14 NA
800 12-14 12-24 12-26 12-29 1-3 1-9 1-12 1-22 NA
900 12-22 1-1 1-5 1-12 1-17 1-24 1-27 2-6 NA
1000 12-30 1-9 1-13 1-19 1-28 2-5 2-10 2-19 NA
1100 1-8 1-17 1-22 1-27 2-7 2-18 2-21 2-28 NA
1200 1-16 1-24 1-30 2-4 2-16 2-28 3-4 3-7 NA
1300 1-29 2-2 2-10 2-17 2-25 3-9 3-15 3-25 NA
1400 2-2 2-9 2-18 2-24 3-5 3-20 3-25 4-17 NA
1500 2-11 2-20 2-26 3-5 3-16 4-2 4-9 NA NA
1600 2-17 2-28 3-9 3-13 3-22 4-11 NA NA NA
1700 2-24 3-14 3-18 3-23 4-5 NA NA NA NA
1800 3- 3 3-23 3-31 4-4 5-2 NA NA NA NA
1900 3-10 4-2 4-11 4-22 NA NA NA NA NA
2000 3-17 4-18 NA NA NA NA NA NA NA
_______________________________________________________________________________
zhas never occurred before this date
yhas never occurred later than this date
Table 6. Peoria, Illinois, chilling data (1951-95)
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Frequency that chilling requirement is met by this date
Chilling
hours 0%z 10% 20% 30% 50% 70% 80% 90% 100%y
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100 10-3 10-8 10-11 10-12 10-16 10-20 10-24 10-26 11-6
200 10-19 10-21 10-24 10-26 10-29 11-2 11-3 11-9 11-13
300 10-25 11-2 11-3 11-5 11-8 11-11 11-15 11-18 11-25
400 11-1 11-10 11-12 11-13 11-17 11-21 11-24 11-28 12-6
500 11-9 11-18 11-19 11-21 11-24 11-30 12-4 12-10 1-4
600 11-20 11-25 11-27 11-29 12-4 12-12 12-18 12-23 1-19
700 11-30 12-3 12-5 12-8 12-14 12-25 1-4 1-14 2-8
800 12-10 12-13 12-15 12-19 12-27 1-7 1-19 2-5 NA
900 12-16 12-22 12-30 1-6 1-16 2-3 2-10 2-22 NA
1000 12-26 1-6 1-13 1-22 2-3 2-17 2-25 3-4 NA
1100 1-7 1-22 1-29 2-2 2-18 3-2 3-6 3-13 NA
1200 1-19 1-28 2-6 2-12 2-27 3-11 3-16 3-22 NA
1300 2- 3 2-13 2-19 2-26 3-11 3-19 3-25 4-1 NA
1400 2-18 2-28 3-4 3-11 3-18 3-25 4-3 4-16 NA
1500 2-23 3-7 3-12 3-19 3-26 4-4 4-10 5-14 NA
1600 3- 3 3-16 3-22 3-30 4-5 4-14 4-30 NA NA
1700 3-14 3-22 4-2 4-8 4-18 5-3 NA NA NA
1800 3-24 3-31 4-9 4-21 5-10 NA NA NA NA
1900 3-31 4-10 4-28 5-25 NA NA NA NA NA
2000 4-11 4-21 NA NA NA NA NA NA NA
_______________________________________________________________________________
zhas never occurred before this date
yhas never occurred later than this date
Table 7. Rockford, Illinois, chilling data (1951-95)
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Frequency that chilling requirement is met by this date
Chilling
hours 0%z 10% 20% 30% 50% 70% 80% 90% 100%y
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100 9-30 10-5 10-8 10-9 10-12 10-16 10-22 11-2 NA
200 10-13 10-17 10-20 10-21 10-24 10-28 10-30 11-12 NA
300 10-23 10-27 10-28 10-30 11-4 11-8 11-9 11-24 NA
400 10-30 11-5 11-6 11-8 11-12 11-16 11-20 12-5 NA
500 11-6 11-11 11-14 11-17 11-21 11-26 11-28 1-3 NA
600 11-15 11-20 11-23 11-24 11-30 12-5 12-12 1-18 NA
700 11-21 11-28 12-1 12-4 12-12 12-21 1-2 1-23 NA
800 11-30 12-6 12-11 12-16 12-26 1-24 2-12 3-12 NA
900 12-9 12-19 12-25 1-1 1-19 2-13 2-28 NA NA
1000 12-17 12-30 1-8 1-20 2-9 2-26 3-7 NA NA
1100 1-13 1-18 1-23 2-2 2-23 3-8 3-15 NA NA
1200 1-24 1-31 2-12 2-18 3-8 3-15 3-24 NA NA
1300 1-29 2-14 2-25 3-1 3-15 3-27 4-3 NA NA
1400 2-13 2-28 3-6 3-12 3-23 4-4 4-10 NA NA
1500 2-25 3-10 3-13 3-21 3-30 4-11 4-22 NA NA
1600 3-3 3-18 3-24 3-28 4-7 4-24 NA NA NA
1700 3-9 3-26 4-2 4-5 4-18 5-12 NA NA NA
1800 3-19 4-1 4-10 4-16 4-30 NA NA NA NA
1900 3-25 4-8 4-22 4-29 NA NA NA NA NA
2000 4-1 4-21 5-8 5-12 NA NA NA NA NA
_______________________________________________________________________________
zhas never occurred before this date
yhas never occurred later than this date
Table 8. Springfield, Illinois, chilling data (1951-95)
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Frequency that chilling requirement is met by this date
Chilling
hours 0%z 10% 20% 30% 50% 70% 80% 90% 100%y
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100 10-7 10-14 10-16 10-18 10-20 10-23 10-27 10-31 11-8
200 10-21 10-26 10-28 10-30 11-4 11- 7 11-9 11-12 11-18
300 10-30 11-4 11-7 11-9 11-15 11-17 11-21 11-24 12-1
400 11-6 11-14 11-16 11-18 11-22 11-27 11-29 12-3 12-14
500 11-15 11-22 11-25 11-26 11-30 12-7 12-13 12-20 1-2
600 11-24 11-29 12-2 12-3 12-10 12-19 12-26 12-30 1-16
700 12-4 12-8 12-10 12-13 12-19 12-31 1-4 1-18 NA
800 12-9 12-15 12-17 12-23 12-30 1-9 1-18 2-1 NA
900 12-16 12-24 12-29 1-2 1-18 1-29 2-9 2-13 NA
1000 12-29 1-6 1-12 1-18 1-29 2-11 2-19 2-27 NA
1100 1-4 1-19 1-22 1-28 2-12 2-22 2-28 3-7 NA
1200 1-17 1-31 2-2 2-5 2-22 3-3 3-7 3-17 NA
1300 1-26 2-8 2-14 2-18 3-2 3-13 3-18 3-22 NA
1400 1-31 2-22 2-26 3-1 3-14 3-22 3-26 4- 5 NA
1500 2-13 3-2 3-7 3-10 3-24 4-1 4-6 4-21 NA
1600 2-18 3-9 3-15 3-20 3-31 4-10 4-30 NA NA
1700 2-24 3-19 3-26 4-1 4-8 5-3 NA NA NA
1800 3-6 3-25 4-4 4-12 5-4 NA NA NA NA
1900 3-15 4-5 4-24 5-7 NA NA NA NA NA
2000 3-26 4-26 NA NA NA NA NA NA NA
_______________________________________________________________________________
zhas never occurred before this date
yhas never occurred later than this date
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