Chilling Hours Accumulation Calculator
Enter daily chill hours, number of days, and the cultivar's chilling requirement; the tool computes accumulated chilling = avg daily chill hours × days, and fulfillment = accumulated ÷ requirement × 100%, judging whether a deciduous tree can break dormancy normally.
Input Data
Results
At a glance:Enter daily chill hours, number of days, and the cultivar's chilling requirement; the tool computes accumulated chilling = avg daily chill hours × days, and fulfillment = accumulated ÷ requirement × 100%, judging whether a deciduous tree can break dormancy normally.
Formula
Accumulated chilling (h) = daily chill hours × days.
Fulfillment (%) = accumulated ÷ requirement × 100.
$$CH = h_{daily} \times days$$$$Fulfillment\% = \dfrac{CH}{CR} \times 100$$How to Use
- Enter the daily chill hours, accumulation days, and the cultivar requirement.
- The calculator returns the accumulated chilling and its fulfillment %.
Chilling requirement reference for common deciduous fruit trees (chill hours < 7.2°C)
| Requirement level | Accumulated hours (h) | Representative trees/cultivars |
|---|---|---|
| Very low | 100–300 | Tropicalized peach, some blueberries, fig |
| Low | 300–500 | Low-chill peach/nectarine, some plums |
| Medium | 500–800 | Most pears, some apples and cherries |
| High | 800–1200+ | Most apples, sweet cherries, some European plums |
| This example 480h vs 400h required | 120% fulfillment | 8h/day×60days=480h, satisfied |
Values are approximate; in warm winters use the Utah/Dynamic model and rely on local measurements.
FAQ
What is chilling requirement and why do deciduous trees need cold?
After leaf fall in winter, the flower and leaf buds of deciduous trees enter endodormancy and must accumulate enough cold to release dormancy and regain temperature responsiveness, so they bud and flower evenly in spring. That required cold accumulation is the cultivar's 'chilling requirement'. Insufficient chilling fails to release dormancy normally, causing delayed and uneven budding, flower-bud drop, sporadic flowering, and poor fruit set.
Why use 7.2°C as the threshold?
7.2°C (45°F) is the most common threshold of the chilling-hours model — every hour below this temperature counts as one chill hour. It comes from early observations of chill effectiveness on fruit trees; simple and widely used. But in reality different temperatures have different effectiveness, and high temperatures can cancel chilling, so precise assessment uses the temperature-weighted Utah model or the dynamic model.
Is fulfillment over 100% a problem?
No. ≥100% means the local cold meets or exceeds the cultivar's need, dormancy releases normally, and budding/flowering is usually more even — a good thing. This tool sets no upper limit on fulfillment; the higher the value, the more abundant the cold. The real concern is fulfillment well below 100%, meaning insufficient chilling.
Can deciduous trees be grown in warm-winter Hong Kong?
Yes, but choose low-requirement cultivars. Hong Kong winters are mild with limited hours below 7.2°C, so high-requirement apples and cherries cannot meet their chilling; choose cultivars needing about 100–400 hours (low-chill peach, nectarine, some plums, or tropicalized varieties), or pair with dormancy-breaking agents. Assess cultivar suitability by the local actual chilling before planting.
Is the chill-hour model accurate? What to watch for?
The chill-hour model is intuitive and good for conceptual understanding and cool-climate estimates, but it assumes any temperature below the threshold counts equally as 1 hour and ignores high-temperature cancellation. In warm, fluctuating winters it can over- or under-estimate; better use the Utah chill-unit model or the dynamic model (chill portions), and estimate daily chill hours from local hourly temperature measurements for reliable results.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.