The buds of many plant species enter a dormant stage over the course of their life cycle. In temperate regions, the buds of most tree and shrub species enter their dormant stage at the end of summer and emerge from it the following spring.
This period of dormancy (winter hibernation) is part of the yearly cycle of deciduous trees. There are two main factors that contribute to dormancy: cooler temperatures in autumn and fewer hours of sunlight. During this hibernation, fruit trees need to accumulate a certain number of “chill hours”, which will later enable them to bud correctly. We can therefore define chill hours as the total number of hours spent at temperatures below 7.2ºC.
Record of temperature and humidity at the Trecoop Fruites weather station in Sudanell in 2019.
Generally speaking, the species cultivated by Trecoop Fruites growers require the following number of chill hours:
- Peach trees: 100-1,100 chill hours.
- Pear trees: 500-1,500 chill hours.
- Almond trees: 100-500 chill hours.
- Apricot trees: 200-900 chill hours..
- Apple trees: 500-1,500 chill hours.
In our region, the months that provide the highest number of chill hours are usually December and January, when there are many days in which the maximum temperature does not exceed 7ºC. It is important to note that the most vigorous trees need more chill hours than their less-vigorous counterparts on the same farm.
What happens if the tree doesn’t get enough chill hours?
The main consequence is that tree budding is delayed. Later in the growth cycle, it may also cause fruit to fall from the tree when the lack of leaf area provides the plant with insufficient nutrients. Fortunately, this is not often the case in our region. Elsewhere, however, the number of chill hours is a key factor when deciding on the types of tree to cultivate. There are even chemical products that can interrupt bud dormancy, whereas other plants can be treated with hot water.
Pérez, F. y Martínez-Laborde, J.B, 1994. Introducción a la fisiología vegetal. Ediciones Mundi-Prensa. Madrid. pp. 2018.