Chill Hours: What They Are and How Many Your Yard Gets
A fruit tree can survive every winter in your yard and still refuse to fruit. The missing piece is usually chill: most temperate fruit trees need a set amount of winter cold before they will break dormancy, bloom evenly, and set a crop. This page explains what a chill hour is, why the experts count it four different ways, and what happens when the count and the tree do not match. The estimator below then puts a typical winter's chill next to the requirements of common fruit, for any U.S. ZIP code.
Estimate your ZIP code's chill
Enter a ZIP code and we will estimate its typical chill accumulation from the NOAA weather station that serves it. Treat the answer as a planning estimate, not a measurement: it is built from 30-year monthly averages, and any single winter can land well above or below it.
What a chill hour is
When days shorten and temperatures fall, a temperate fruit tree enters dormancy: growth stops, leaves drop, and the buds sit out the winter in a protected resting state. The tree will not resume normal growth just because a warm week arrives. It first has to log a required amount of cold, its chilling requirement, and only then does warmth count toward bud break. Researchers measure that cold in chill hours, the accumulated hours a dormant tree spends in the chilling range. The requirement is a property of the variety, not the species alone: one apple variety may need 200 hours and another 1,000, which is why two trees of the same fruit can behave completely differently in the same yard.
The most efficient chilling temperatures sit between about 32 and 45 °F. Colder than freezing, a tree receives very little effective chill; warmer than about 60 °F for extended stretches, some models say previously banked chill can be partly undone. Chilling starts accumulating in fall, roughly from October 1 in most counting schemes, and the requirement is usually met sometime in mid to late winter, after which the tree is simply waiting for warmth.
Four ways to count the same winter
There is no single chill number, because there are several models and they answer slightly different questions:
Hours below 45 °F is the original, simple model: every hour under 45 °F from October 1 counts as one. It is easy to compute from a thermometer record, and extension services in the Southeast publish station totals on this basis. Mississippi State Extension notes it works best where temperatures below freezing are rare, because it credits deep-freeze hours the tree barely uses.
Hours between 32 and 45 °F is the band model: only hours inside the most efficient range count, and sub-freezing hours count for nothing. In cold climates it returns far lower totals than the below-45 model for the same winter, which is why a northern grower and a southern grower can quote wildly different chill figures for similar orchards.
The Utah model weights each hour by temperature instead of using a hard threshold: hours in the mid 30s to high 40s earn a full chill unit, hours at the edges earn half, chilly-but-not-cold hours earn nothing, and warm hours above about 60 °F subtract units. It captures the way a January thaw can set a tree back, at the cost of needing an hourly temperature record to compute.
The Dynamic model tracks chill portions, a two-step chemistry metaphor: cold first builds an intermediate compound that heat can still destroy, and once enough of it converts, the portion is locked in permanently. It was developed for warm-winter fruit regions, where the older models performed worst, and it is the standard in much of California and Israel's research and advisory work.
The models disagree because dormancy is not a thermometer reading; it is physiology approximated by arithmetic. A variety's published requirement is only meaningful under the model that produced it, and most nursery labels never say which one that was. The practical defense is to buy varieties whose requirements were tested in your own region, which is what your state extension's variety lists are for.
When a tree gets too little chill
An under-chilled tree does not die. It wakes up badly. Buds open late, unevenly, or not at all; bloom straggles over weeks instead of arriving in one fertile flush, so pollination and fruit set suffer; leaves may emerge after the flowers, or fail on some branches entirely; and the crop that does form is often small and poor quality. Mississippi State Extension's blueberry photo of an under-chilled bush, leafless while its properly chilled neighbors leafed out, is the textbook picture. The failure is easy to misread as disease, winter kill, or a bad tree, and it repeats every warm winter, which is why matching the requirement to your typical chill, not your best winter, matters.
The other mismatch: a low-chill tree in a high-chill place
Surplus chill does not harm a high-chill variety; it simply meets its requirement early and waits for heat like everything else. The real mismatch runs the other way: a low-chill variety planted where winters are long and cold satisfies its small requirement by mid-winter, so the first February warm spell can start deacclimation and bud swell, and the returning cold then damages buds that a higher-chill variety would have kept safely shut. Extension dormancy research flags exactly this pattern: plants with low chilling and heat requirements are the most exposed to false-spring damage. When you move a southern variety north, its winter survival rating is only half the question; its eagerness to wake up is the other half.
Warming winters are squeezing the map
Chill is a shrinking resource in many traditional fruit regions. The University of Georgia's peach program, tracking its Byron research station, reports historical chill averaging about 1,044 hours with a steady downward trend, roughly a 12 percent decline across the record it examined, and Georgia's breeding priorities now chase varieties that fruit on less chill. The same pattern drives California's shift toward chill portions and low-chill breeding: as winters warm, the varieties that worked for a century creep out of reach of the places that made them famous, and the workable band for each variety slides north and uphill. For a home grower the lesson is simple: plant for the winters your tree will see in ten or twenty years, not the winters your grandfather counted, and lean toward the lower end of a variety's range when your estimate sits near its requirement.
Zone tells you survival; chill tells you fruiting
Hardiness zones and chill hours both describe winter, but they measure opposite ends of it. A zone is built from the single coldest night of a typical year: it answers whether the wood and buds live through winter at all. Chill is built from the accumulated mild cold of the whole season: it answers whether the tree's internal calendar lets it bloom and fruit properly in spring. A peach rated hardy to zone 6 will survive a Georgia-to-Minnesota range of winters, but it fruits well only where winter delivers roughly its 200 to 800 chill hours, and a high-chill apple can survive in zone 8 while sulking through winters too warm to satisfy it. Check both numbers for any fruit tree you plant: the hardiness zones guide covers survival, ZIP by ZIP, and this page covers the fruiting side.
How much chill common fruit needs
Typical ranges, in hours below 45 °F, from Mississippi State University Extension Publication 3067. Variety matters more than species, so use the ranges to shortlist, then check the named variety against your estimate and your state extension's tested list.
| Fruit | Chill hours required |
|---|---|
| Apples | 200 to 1000 |
| Blackberries | 200 to 600 |
| Blueberries | 150 to 700 |
| Citrus | 0 to 100 |
| Figs | 100 to 200 |
| Grapes | 100 to 600 |
| Peaches | 200 to 800 |
| Pears | 400 to 900 |
| Pecans | 300 to 500 |
| Persimmons | 200 to 400 |
| Plums | 400 to 700 |
| Pomegranates | 100 to 200 |
| Strawberries | 200 to 400 |
Sources: fruit requirements and model definitions, Mississippi State University Extension Publication 3067, "Chilling-Hour Requirements of Fruit Crops" (Eric T. Stafne). Utah model: Richardson and co-authors, Utah State University, 1974. Dynamic model: Fishman, Erez, and Couvillon, 1987, used by University of California advisors. Warming trend: University of Georgia Peach Blog, "Historical chill hours at Byron and production data in Georgia" (Chen, 2023), and the University of Georgia Weather Network's published station chill hours, used for the spot check above. Estimates: NOAA NCEI 1991 to 2020 U.S. Climate Normals, monthly normals for the station serving each ZIP.