timing
Chill hours
Last reviewed 2026-10-04
Chill hours describe the amount of cool-season exposure that some fruit plants need before they can resume normal flowering and growth. The exact temperature range and calculation method vary by crop and source. A chill requirement is a seasonal biological measurement, not a simple hardiness rating.
What it means
Deciduous fruit trees and some small fruits may remain dormant until enough cool conditions have accumulated. If a plant receives too little suitable chill, flowering can be delayed or uneven. Warm spells can complicate the calculation, and different models count temperature differently.
How it affects the garden
Choose varieties whose chill requirement fits the local climate. A tree that needs more winter chill than the site provides may grow leaves but set fruit poorly. A low-chill variety in a cold location can also flower early and face spring frost. Check regional extension guidance and the cultivar description before planting.
The planting calendar helps with seasonal tasks, but it does not calculate chill accumulation for every cultivar. A hardiness zone alone cannot answer the question.
Example
A peach cultivar selected for a mild winter may flower reliably where a high-chill cultivar would remain uneven. If the low-chill tree flowers very early, a later frost can still damage the blossoms.
Common confusion
Chill hours are not the same as frost-free days or hardiness zones. They describe a cool exposure requirement, not minimum winter survival.
Sources
Utah State University Extension explains chill requirements and fruit tree selection.
