Pollination & Climate Threshold comparison • User supplies local chill

Fruit Tree Chill Requirement Checker

Compare a recorded or externally estimated local chill total with a cultivar requirement; this tool does not calculate chill from weather data.

Enter your details

1. Target Fruit Cultivar:

2. Recorded or externally estimated chill hours (<45°F model):

Use a nearby weather station or a trusted regional chill tool. USDA hardiness zone is not a chill-hours measurement.

Compare chill using a matching modelChill Hours, Utah Chill Units and Dynamic Chill Portions use different models. Match the cultivar requirement and weather calculation to the same model; the quantities are not interchangeable. This illustration is not a weather record.Original illustration · Not a field-test photographView full size Download SVG

How this tool works

Deciduous fruit cultivars differ in chilling requirement. The simple chill-hours model counts hours below a threshold, while chill-units and dynamic models handle temperature differently. Cultivar values are approximate and can vary by source, strain, site, and model.

Assumptions & Physical Constraints:

The entered value and cultivar requirement must use a comparable model. This checker does not import weather data, reverse warm-hour effects, predict frost, or guarantee synchronized bloom.

Example: If a trusted local source reports 850 chill hours and the selected cultivar is commonly listed at 800–1,000 hours, the tool marks the value as within the published range—not as proof of fruit set.

Rule-set status: Interface/model revision 4.2.0 • September 6, 2026. Sources and limitations below; this date is not a field-trial date.
Sources and model basis: University of Georgia Extension C 1063 (Home Garden Peaches) • UC Fruit & Nut Research Information Center Chill Calculator.

Found a questionable result? Report the inputs and expected result. Calculator feedback is reviewed against the cited rule set.

Frequently Asked Questions

Why shouldn’t I rely solely on USDA Hardiness Zones for chilling hours?

USDA zones measure only the 30-year average extreme minimum winter temperature (the coldest night of the year), NOT the total cumulative hours between 32°F and 45°F. Winter cloud cover, elevation, and warm afternoon spikes cause huge year-to-year chill variations in the same zone.

What happens if a tree does not receive enough chill hours?

The tree suffers from delayed foliation. Leaf buds open erratically over several months, flowers abort without pollinating, and branches may develop bare "blind wood" without fruit.