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 develop 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.

Calculating Chill Hours

It is only possible to calculate Chill Hours for the actual year of data recording as most temperature records available 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.

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.

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.