Stone fruit trees 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: 1) delayed foliation, 2) reduced fruit set and increased buttoning and, 3) reduced fruit quality.
Insufficient Chilling Symptoms
Delayed Foliation. 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 normal development of next year’s fruit buds can be impaired.
Reduced Fruit Set and Buttoning. 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.
Reduced Fruit Quality. 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.
There are various models used to calculate chilling, each one defining what a chilling unit is. The three most common models are: the number of hours below 45 degrees F (7°C) model, the number of hours between 32 and 45 degrees F (2 and 7°C) model, and the Utah model. 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 chilling effectiveness and negative chilling accumulation (or chilling negation).
In fieldClimate.com we use the model for calculation of chill portions (CP). Chilling accumulations are calculated as chill portions, using a temperature range from 2 to 7°C. Calculations of chill proportions end after 96 hours of equal or more then >15°C '(it holds between7 and 15°C)
Calculations are based on the work of Erez A, Fishman S, Linsley- Noakes GC, Allan P (1990) The dnamic model for rest completion in peach buds. Acta Hortic 276: 165-174.