Chilling Hours & Enough Winter Chill?
Wakes apples
“One accepted answer here saves a hundred people an hour each.
Enter daily hours below 7°C and the number of cold days to get your accumulated chill hours, the percent of the requirement met and whether dormancy will break evenly.
Enter your winter
Next: if chill is short, choose low-chill varieties, use rest-breaking sprays (e.g. hydrogen cyanamide) where permitted, and site orchards in cooler pockets.
Simple chilling-hours model (hours <7.2°C); Utah chill units and dynamic (chill portions) models refine this — match the variety to your climate's chill.
Chilling hours — key facts
- Chill hours
- hours/day < 7°C × cold days
- Apple
- ≈ 1000 hours
- Pear
- ≈ 800 hours
- Peach
- ≈ 700 hours
- Too little chill
- delayed, erratic budbreak
- Low-chill types
- ≈ 200–400 hours
- Short on chill?
- low-chill variety or rest-breaking spray
- Privacy
- Runs in your browser; nothing uploaded
Cold is the alarm clock that wakes a fruit tree
Temperate fruit trees survive winter by going dormant, and they refuse to wake up until they have banked enough cold — hours spent below about 7°C. Apples want roughly 1000 of these chilling hours, pears about 800 and peaches around 700. Get enough and the tree breaks dormancy cleanly, flowering and leafing out together in spring. Fall short and budbreak is delayed, drawn-out and patchy, with sparse bloom and poor fruit set.
This tool turns your daily cold hours and number of cold days into accumulated chill, the percent of the requirement met, the shortfall and a met-or-not flag. Use it to check whether your winters suit a variety before you plant, and where chill is short, lean on low-chill cultivars or rest-breaking sprays. Pair it with the Frost Date, Growing Degree Days and Orchard Tree Spacing tools to plan the whole orchard.
Match variety to climate
Check chill needs before you plant a tree.
Avoid erratic budbreak
Ensure enough cold for even flowering.
See the shortfall
Know how many chill hours you are missing.
Plan for warm winters
Switch to low-chill types or rest-breaking sprays.
Frequently Asked Questions
What are chilling hours?+
Chilling hours are the number of hours a temperate fruit tree spends below about 7°C during winter dormancy. Trees count this cold as a signal that winter has passed, and only break dormancy and flower evenly once they have accumulated enough. Total chill is roughly the hours per day below 7°C multiplied by the number of cold days.
How are chilling hours calculated?+
The simple model counts every hour the temperature is between roughly 0°C and 7°C. In this tool, accumulated chill = hours per day below 7°C × number of cold days. For example 10 cold hours a day across 100 days gives about 1000 chill hours — enough for many apple varieties to break dormancy properly.
How many chilling hours do fruit trees need?+
It depends on the species and variety: apples need roughly 1000 hours, pears about 800, and peaches around 700, though there are high-chill and low-chill cultivars on either side. Cherries, plums and apricots all have their own ranges. Always match the variety's stated chilling requirement to your local winter.
What happens if a tree gets too little chill?+
Insufficient chilling gives delayed, erratic and prolonged budbreak: flowers and leaves emerge unevenly over weeks, bloom is sparse, and fruit set and quality suffer. In warm-winter regions this is the main reason a fine variety underperforms — it simply never received the cold signal to wake up properly.
What are low-chill varieties?+
Low-chill varieties are cultivars bred to break dormancy after far fewer chilling hours — some peaches and apples need only 200–400 hours. They let growers in mild-winter or subtropical areas produce temperate fruit that high-chill varieties could never crop reliably. Choosing the right chill class for your climate is the single biggest decision.
Can I make up for missing chill?+
Where winters are marginal, rest-breaking sprays such as hydrogen cyanamide, plus oils, can partly substitute for missing chill and force more even budbreak. They must be timed and dosed carefully and used within local regulations. They help — but choosing a variety whose chill requirement fits your climate is far more reliable.
Does warmth cancel out accumulated chill?+
In more advanced models, yes — warm spells (especially above ~16°C) can negate chill banked earlier, which is why models like chill portions exist. This calculator uses the simpler hours-below-7°C count, which is a good planning estimate for steadily cold winters but can over-count where mild and cold days alternate.
When is chill accumulated?+
Chill accumulates through the dormant season, typically from leaf fall in autumn through to budbreak in late winter or early spring. Most of it is banked during the coldest months. Count only the genuinely cold dormant-season hours; chill received outside dormancy or during warm interruptions counts for little.
Are these figures exact?+
They are solid planning figures. Real chill depends on hour-by-hour temperatures, microclimate, the chill model used and the specific variety. Use logged or forecast temperature data where you can, treat the result as a guide to whether your site suits a variety, and pick cultivars with a comfortable margin.
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