The concept of chilling hours is fundamental in many woody crops, especially in deciduous crops (almond, apple, cherry, pistachio, etc.), although also in some evergreen crops, such as the olive tree, which require a minimum of winter chill to ensure proper flowering in the following season.
In simplified terms, chilling hours measure the tree's winter dormancy period. During this dormancy, the plant stops its external vegetative activity, but key physiological processes occur internally that will determine subsequent budding, flowering, and fruit set.
Why is the winter shutdown necessary?
Winter rest allows:
- Correct floral differentiation.
- A homogeneous sprouting.
- A balanced flowering.
- Better fruit set and final fruit size.
Although leaf fall is visible in some crops, the important thing is not so much the defoliation as the physiological fulfillment of dormancy.
What exactly do cold hours measure?
Traditionally, chilling hours are calculated as the number of hours with temperatures below 7 °C accumulated during the winter. Each crop (and variety) has minimum requirements.
For example:
- Almond: around 400 chilling hours (indicative average value)
These hours usually accumulate between November and February, with the ideal being that the requirement is completed continuously and steadily before the end of winter.
The current problem: irregular winters
With climate change and the expansion of crops into limit zones, a key problem arises:
- Cold periods interrupted by episodes of high temperatures
This causes:
- The accumulation of cold is slowing down.
- Or even that some of the accumulated cold is lost (according to the calculation model).
- The tree “believes” that it has emerged from rest without having completed it correctly.
The result is usually:
- Irregular flowering.
- Low curd.
- Size inequality.
- Loss of profitability.
Importance of measuring chilling hours
Measuring chill hours allows:
- Detect breaks in rest.
- To know if the crop will meet its minimum needs.
- Make technical decisions in advance.
In these cases, the technician can opt for chemical interventions (dormancy or budding inducers) to simulate or complete the process that the climate has not provided naturally.
Practical application of the Plantae team
Before planting a sensitive crop (e.g., pistachio):
- A weather station is installed.
- Several winters are recorded.
- It is assessed whether the area meets the chilling hour requirements.
- A decision is made as to whether the crop will be viable or will require additional interventions.
Today there are plantations that are not profitable precisely because it was not taken into account whether the winter was cold enough for the chosen crop.
Summary
- Las horas frío miden el reposo invernal necesario para una buena floración.
- It is not enough to accumulate cold: it must be continuous and stable.
- Warm, irregular winters disrupt this process.
- Measuring chill hours with sensors allows for anticipation and correction with technical management.
- Ignoring this factor can lead to productive and economic failures.
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