Do you know your Chill Hours?

Chill hours are an important tool for not only orchardists, but also for a wide range of horticulturists. Chill hours enable these operators to grow more ‘climate specific’ varieties, with improved results.

Why do you need to know your Chill hours? Well it could save you some money; by enabling you to plant ‘climate appropriate’ varieties in the right spots. It could also help you to extend your picking season(s), by planting a wider variety for harvesting during a longer period. All these gains and you also will not be having so much trouble with fruit not setting, and buds not forming, as they should.

The standard measurement for a calculated value, called Chill hours, is the number of hours where the air temperature is below 7 degrees Centigrade at a given location. Stone and pome fruit trees rely on enough chill hours for flowers and leaf buds to develope normally. If the buds do not receive sufficient chilling temperatures during winter to completely release dormancy, trees may develop physiological symptoms such as;

  • delayed and extended bloom,
  • delayed foliation
  • reduced fruit set and
  • reduced fruit quality.

Growers and industry traditionally keep close track of chill hours to decide on the orchard management practices required and for a comparison of past years’ weather and crop loads. The approximate number of chill hours/units/portions needed for normal development varies depending on the variety and the species.

Chill Accumulation - Importance and Estimation

(David H. Byrne and Terry Bacon, of the Dept. Of Horticultural Sciences, Texas A&M University, published the following information.)

Stone fruit trees such as peaches develop their vegetative and fruiting buds in the summer and, as winter approaches, the already developed buds go dormant in response to both shorter day lengths and cooler temperatures. This dormancy or sleeping stage protects these buds from oncoming cold weather. Once buds have entered dormancy, they will be tolerant to temperatures much below freezing and will not grow in response to mid-winter warm spells. These buds remain dormant until they have accumulated sufficient chilling units (CU) of cold weather. When enough chilling accumulates, the buds are ready to grow in response to warm temperatures. As long as there have been enough CUs the flower and leaf buds develop normally. If the buds do not receive sufficient chilling temperatures during winter to completely release dormancy, trees will develop one or more of the physiological symptoms associated with insufficient chilling.

Chemical sprays such as Dormex that help break dormancy are being researched. These chemicals can be used in late January or early February if insufficient chilling is suspected. On the other hand, the expense of a dormancy-breaking chemical can be avoided if the grower knows that trees have received sufficient chill accumulation. Hopefully the mean temperature chill model will prove to be a tool that can help us select cultivars better and manage our present cultivars to minimize the detrimental effects of a mild winter.

Insufficient Chill hours symptoms.

A classic symptom of insufficient chilling is delayed foliation. A tree may have a small tuft of leaves near the tips of the stems and be devoid of leaves for 12 to 20 inches below the tips. Lower buds will break eventually but full foliation is significantly delayed, fruit set is reduced, and the tree is weakened. Furthermore, heavy suckering from lower parts of the tree causes management problems, and the normal development of next year’s fruit buds can be impaired.

Flowering, in response to insufficient chilling, often follows the pattern seen with leaf development. Bloom is delayed, extended, and due to abnormalities in pistil and pollen development, fruit set is reduced. In many peach cultivars, flowers drop before or around shuck split, but in others such as ‘Jersey Queen’ and ‘Harvester’, buttons form. Buttons result from flowers which apparently have set but never develop into full-size fruit. The fruit remains small and misshapen as they ripen. If you cut these fruit open, the seed is dead. Because buttoning is not apparent early in the season, growers can not thin off the abnormal fruit and the developing buttons serve as a food source and overwintering site for insects and diseases.

The effects of insufficient chilling on fruit quality are probably the least discussed but appear to be very common especially in central and south Texas. The effects on leaf growth and fruit set are dramatic but the effects of insufficient chill on fruit quality are subtle, and can occur when other symptoms do not. Insufficient chilling will cause many cultivars to have an enlarged tip and reduced firmness. Furthermore, fruit ground coloration may be greener than usual, possibly due to the fruit losing firmness before the ground color can fully change from green to yellow. The extent of these quality problems depends on the cultivar and the degree of chilling deficiency.

Chill Accumulation Models

The question of what is dormancy and what constitutes ‘chilling temperatures’ has yet to be clearly defined. Most people agree that temperatures below freezing or above 60 degrees F are not effective for chilling unit accumulation. There are various models used to calculate chill, each one defining what a chill unit is. The three most common models are: the number of hours below 45 degrees F model, the number of hours between 32 and 45 degrees F model, and the Utah model (Table 1.). The first two models are simple and define a chilling unit as one hour below or between certain temperatures. The Utah method is more complex because it introduces the concept of relative chill effectiveness and negative chill accumulation, or chill negation.

Common chill accumulation models

450 F and Under Model

1 hour <= 450 F =1.0 chill unit, NB; 45F = 7.2C

32-450 F Model

1 hour between 32 and 450 F =1.0 chill unit, NB; 32F = 0C

Utah Model

1 hour below 34 degrees F =0.0 chill unit

1 hour 35-36 degrees F =0.5 chill units

1 hour 37-48 degrees F =1.0 chill units

1 hour 49-54 degrees F =0.5 chill units

1 hour 55-60 degrees F =0.0 chill units, NB; 60F = 15.5C

1 hour 61-65 degrees F =-0.5 chill units

1 hour >65 degrees F =-1.0 chill units

All of these models require hourly temperatures to be recorded for calculation, and the point in time to begin recording chilling temperatures is not well defined. In addition, since these models were developed in states where high chill-requirement peach cultivars are grown, their usefulness under medium and low chill accumulation conditions has been limited.

Another approach, the Mean Temperature Model, uses mean winter (December and/or January) monthly temperatures to estimate accumulated chilling units. Researchers in Georgia and Florida independently developed a relationship between the mean monthly temperature of their coldest month(s) and total chill unit accumulation. Combining data from both studies, the Stone Fruit Breeding Program at Texas A&M University developed a method to estimate chill accumulation which has demonstrated to be accurate for estimating chill accumulation in Texas from the Lower Rio Grande Valley up to the Red River, and should work well throughout the southeastern U.S..

Chilling accumulation, determined with this model, has been tested and compared to peach tree behavior at Stephenville, Fredricksburg, College Station, Yoakum, and Weslaco, TX. The coldest month or months are used for the calculation. In low chill regions (regions where average January temperature is 59-63 degrees F) where January represents the dormancy season, January mean temperature is most accurate for estimation. In high chill regions (regions where average January temperature is below 48 degrees F) a mean December- January temperature is recommended. In medium chill regions (regions where average January temperature is 48-58 degrees F) January mean temperature has been best for calculating chill accumulation except in years when mean temperatures between December and January differed by more than 6 degrees F. In this case, the December-January mean was more accurate.

Mean temperature chill accumulation model calculations.

  • For January mean: Total chill accumulation= 3547-54(January mean temperature)
  • For December-January mean: Total chill accumulation= 4280-68.8(December January mean temperature/2)

The accuracy and the simplicity of calculating chill accumulation with mean temperatures will aid fruit researchers and growers in several ways. Mean temperature data is routinely kept by cities and state climatologists and is usually easily accessible for tracking chill accumulation for a specific area over long periods of time. This will make it easy for a researcher, extension agent, or grower to more accurately match cultivars to a given locale. Also, this method will make it possible for the grower to know, before fruit thinning time, if chill accumulation was sufficient for a given cultivar. If insufficient chilling is suspected for a cultivar, the grower can implement management and marketing strategies to reduce the impact on costs and labor.

It is only possible to calculate Chill Hours for the actual year of data recording as most temperature records available from a LTWS only include a daily maximum, minimum and mean air temperature. It is not possible to interpolate this level of data to calculate a long-term pattern of chill hours with any accuracy. However, an educated guess as to the chill hours range for any site can be estimated by comparison of the annual heat accumulation for the year of recording against the 30 year range.

This diagram was produced as the result of a microclimate assessment undertaken by Topoclimate utilising temperature dataloggers, refer to the article What are Microclimates? This graph was produced for Geoff Nutt, of Kangaroo Island, who was looking to expand into growing apples for cider. As these results demonstrate, there are, on the whole, insufficient Chill hours across his property, for successful pome fruit set. He has reduced his proposed operation and will now utilise the one site on his property that does have the required Chill hours for apples. The range of chill hours recorded for all Topoclimate farm plans are shown against the optimal chill hour requirements of a range of stone and pome fruit. This gives some guidance as to areas suitable for various crops across the property when mapped spatially.

Apples are a very good example of the range of minimum chill hour requirements for fruit set initiation. In this diagram, you can see the differences in Chill hour requirements between Red Delicious apples and Pink Lady apples, both major Australian commercial varieties. Pink Lady Apples only require 400 chill hours below 7ºC for fruit set but Red Delicious apples require 700 Chill hours or 75% more chill.

The Chill requirements, Chill hours, being the hours below 7C, for a range of commercial stone fruit, Apricots, Peaches and Nectarines, are shown in this diagram and compared to the measured chill hours 2004/5 on a Stanthorpe vegetable growing property. This shows, for example that Bonanza low chill peaches only require 200 Chill hours but that the Canadian harmony peaches grown at much higher latitude require nearly 1000 chill hours.

The cooler pockets of cultivatable soil in the higher altitude Granite belt of far northern NSW and Southeast Queensland have a long history of apple and stone fruit production based on the early settler’s identification of specific microclimates in these areas with sufficient chill hours to set fruit.

Many of the large orchards have undertaken microclimate mapping and produced Chill hour maps of their properties. The detail of information now allows these orchardists to more accurately match different apple and stone fruit varieties to the measured chill hours for each block.

This is an example of a Chill hours map from one of the larger Apple Growers in the Stanthorpe area. Notice how the Chill hour iso-contours tend to follow the physical contours of the landscape. This is because cold air tends to sink and flow slowly towards low spots in the landscape during the night, creating areas with higher chill hour readings. This apple grower can now utilse this information to plant high chill varieties of apples or other stone fruit in the darker blue areas and low chill varieties in the light blue and tan coloured areas.

This diagram shows Chill requirements being hours below 7C, for plums, cherries and other associated fruit, compared to measured chill hours 2004/5 at the above site near Stanthorpe.

The range of Chill hour requirements of different varieties of fruits is again illustrated by the differences between low chill Satsuma plums (325) and the Blue damson plums at 800 Chill hours.

Temperature data at the microclimate scale is crucial for sorting the optimal crops suitable for a site and illustrates the value of detailed air temperature recording for technically savvy horticulturalists.

There are quite a range of other fruits which have the same chill requirements as apples and stone fruit. These include Sweet Fuyu persimmons, jojobas, pears, figs, quince and many others.

This diagram shows the Chill requirements, being hours below 7C, for pears, figs and other fruit compared to measured chill hours 2004/5, at Stanthorpe. You can see from this graph that for instance Bartlett pears, a mainstay of the New Zealand pear industry, require almost twice as much Chill as Nashi pears, a mainstay pear variety in Asia but generally grown at warmer latitudes.

For an accurate assessment of Chill hours available on your property, a microclimate survey involving the use of temperature dataloggers, across your landscape, is required.