This blog carries detailed information about the concept of chilling hours in apple and why is it so important.

If you plant apple trees, you probably already know the term “chilling hours” in apples. What precisely are apple chilling hours for those of us new to apple cultivation? Why and for how long do apple trees require chilling? All this seems a bit unclear, right? This blog carries all the information about apple chilling you will likely need.

“Chilling hours are the total amount of cold hours or days that a deciduous fruit tree (or nut tree) needs each year to produce flowers and fruit. Every fruit tree species has a unique requirement for the number of chilling hours needed to produce fruit. Some fruit trees only require 100 cool hours, while others require 1,000 or more”. 

Apple is different from other fruit trees in terms of chilling hours. The number of chilling hours needed for various apple varieties ranges from 100 to 1,700.

The impact of lower chilling hours on apple trees

• The flower buds on an apple tree may not open at all or open later in the spring if they don’t receive enough chill hours. 
• Delays in leaf emergence are also possible. Additionally, flowers may bloom infrequently. 
• The longer the bloom period, the higher the risk that the tree will be susceptible to pests. So, as you might anticipate, a lack of chill hours will definitely impact fruit yield.

Role of temperature on the chilling hours of apple

Temperature is an important factor in determining the plant developmental events of perennial plants. The chilling requirement is commonly expressed in the number of hours when the temperature is at or below 7°C and more than the freezing temperature i.e., 0°C during the winter. In temperate locations, the presence of winter chill is a requirement for deciduous fruits so that dormant buds can emerge from endodormancy (ED) and start growing in the spring.

Varying fruiting trees and kinds have different cool hours for fruit production. Some fruit trees need 100 chill hours, while others need 1000. Apples have the highest chilling requirements of any fruit tree, followed by apricots and peaches.

Are you aware? The Fasal system can detect the exact temperature requirement for apple trees. With the help of the sensors, you can get accurate chilling hours for apple trees. Fill out this form to take a demo.

Impact of climate change on chilling hours

• Fruit trees exhibit many irregularities, such as bud break absence, flowering delay, or both, due to insufficient chill hours or during mild winter. 
• The tree still exhibits paralysis of shoot growth following bud break, the formation of tiny leaves, a slow rate of fruitification, and a shorter flowering-maturation cycle. 
• The quality and productivity of apple fruit are positively correlated with the wintertime accumulation of adequate CUs.
• Dormancy becomes irreversible once there has been a sufficient accumulation of CUs (a chilling accumulation necessary for the level of precursors, such as ABA and auxin, to reach a threshold), and it is not affected by short-term warm temperature peaks.
• However, if the warm temperature remained for a longer period, the floral buds would be shifted from endodormant (real dormancy) to ecodormant state (dormancy owing to external forces), resulting in the irregular breaking of bud dormancy, and it affects the fruit yield and quality.
• Depending on the variety, apples need between 100 and 1700 chilling hours.

Delayed chilling symptoms

1. Delayed Leaf Development, or “little tuft of leaves near the tips of the stems [but] no leaves for 12 to 20 inches below the tips,” results in poor fruit set, suckering from the rootstock, and poor development of the next year’s fruit buds.

2. Poor Fruit Quality and Reduced Fruit Set. The result is small, poorly formed fruit (called “buttons”) that develop late, are not worth harvesting, and can harbor pests and diseases, or larger fruits that are misshaped and poorly coloured, making them unsellable. “Flowering is delayed, extended, and due to abnormalities in pistil and pollen development, fruit set is reduced

Models of chilling hours

The chill range and the chill accumulation during the chill period can be calculated using various models (usually December and January). David H. Byrne and Terry Bacon described three models in their 1992 study, “Chilling Accumulation: its Importance and Estimation.” Temperatures must be recorded hourly for every model.

1. Model of chilling hours: The Chilling Hours Model is the most traditional approach to measuring winter chill. This model assumes that temperatures between 0°C and 7.2°C have a chilling effect, with each hour at these temperatures contributing one chilling hour. Thus, during the dormant season, chilling hours accumulate and are summed up (Luedeling, 2012).
2. Dynamic Model: Israel was the country that created the Dynamic model. Based on hourly temperatures, it estimates chill in what are known as “chill portions.” The Dynamic model predicts that effective winter chill temperatures have a bell-shaped distribution, with the best chilling temperature at 6 °C and the lowest temperatures at -2 °C and 14 °C. High temperatures counteract previously acquired chill, while moderate levels can increase it.

An intermediate product is created by exposing a product to actual winter chill temperatures. Subsequent exposure to high temperatures has the potential to ruin this intermediate product. This intermediate product is permanently banked as a chill portion once a certain amount of it has been accumulated. The cumulative winter chill is estimated by adding the chill components over the fall and winter seasons.

This intricate model, which also incorporates the timing of exposure to temperatures over a cycle, seems far more accurate in mild winter climates.

3. Utah Model: The Utah Model was created in Utah, USA. It has a weight function that categorizes different temperature ranges according to their chilling efficiency, including the negative contributions of high temperatures. This model of chill units (CU) defines a CU as the persistence of the buds for an hour at a temperature deemed ideal for chill accumulation (2.5-12.5°C).

Due to the introduction of relative chilling effectiveness and negative chilling accumulation concepts, the Utah model is more complicated (or chilling negation).

Temperatures between 0 and 16 C encourage the breaking of rest, whereas temperatures above 16 C neutralize such effects, according to Richardson et al. (1974). The optimum promoting temperature is 7 °C (1 hour at 7 °C = 1 chill unit); temperatures between 0 and 16 °C are less effective.

The model is described as follows:

The model assumes that chill accumulation happens between 2.5 and 12.5°C, outside of which the accumulation is either zero or negative (Richardson et al., 1974). Although this model produces good results in cool and cold temperate regions, it produces many negative chill values in sub-tropical climates, which limits its utility and applicability (Dennis, 2003). This model has been modified by leaving out the negative values from the Utah model; as a result, it is now known as the Positive Chill Units (PCU) model, and using it in these subtropical regions has improved the results.

How can Fasal assist?

• Fasal is an IoT-based intelligence platform for horticulture crops. It collects real-time data of the crops with the use of on-farm sensors.
• Sensors installed in the Fasal system inform farmers about weather uncertainties and temperatures needed for apple production. 
• With the Fasal system, farmers can know how many chillings hours are required for apple production.
• Farmers can view their apple trees, soil, and microclimate conditions anytime, anywhere, on any device using the Fasal app. It informs them of any crop-threatening circumstances.

An illustration of how to calculate chilling hours quickly

The data was collected from a farmer’s field in Himachal between 12.17 am on December 4 and 12.31 am on December 5, using the Fasal app. The chilling hour was calculated using the Chilling Hours Model, which tallied up 6 chill hours, and the Utah Model, which tallied up 10 chill hours, between 12.17 am and 12.31 am on the following day. Similar calculations can be made to determine whether an apple tree’s chill requirement would be met for a given season by calculating the chill hour for that time period.