Chill hours calculator
Most fruit trees need a set number of cold hours over winter before they will flower and fruit properly. Enter your ZIP or zone to estimate yours and see what fits.
Chill hours = time between 32 and 45 °F over winter. This is a broad estimate from your zone — coastal, inland and elevation differences are large, so a local station figure (enter it above) is better if you have one.
How this is calculated
“Chill hours” here are the classic model: the number of hours between 32 and 45 °F from roughly November to February. Buds need this cold period to break dormancy evenly in spring — too little and they open late, straggly and unfruitful.
The estimate is derived from your USDA zone, so it is a wide range. Chill varies a lot with elevation and with distance from the coast, so if your county extension or a nearby weather station publishes a local figure, enter it directly.
Chill is necessary but not sufficient — the plant also has to survive your winter low and your summer heat. Always check the variety’s hardiness rating too.
Common questions
What happens if a fruit tree doesn't get enough chill hours?
It flowers late, unevenly, and over a longer stretch than normal, which usually means poor fruit set and a smaller crop - the tree isn't damaged, it's just off its normal rhythm for that year.
Can I grow a high-chill fruit tree in a low-chill climate?
Not reliably - a tree that needs 800+ chill hours planted somewhere that only reaches 200-300 will struggle to break dormancy properly most years. Look for low-chill cultivars bred for warm-winter climates instead.
Does a warm winter reset chill hour accumulation?
No - chill hours accumulate cumulatively through the whole dormant season; a few warm days don't erase hours already banked, though the classic model does discount hours above about 60 °F slightly.