Question 1

What's a chill hour, exactly?

Accepted Answer

A chill hour is one hour spent below 45°F during dormancy (roughly Nov 1 to Feb 28). Deciduous fruit trees need a certain total number of chill hours to signal the end of dormancy and set flower buds properly the next spring. The 0-45°F model is the most widely published definition. Other models (Utah, Dynamic) give more accurate results in some climates but are harder to compute from daily data.

Question 2

Where do the variety chill requirements come from?

Accepted Answer

The UC Davis Fruit & Nut Research Center, Dave Wilson Nursery, Stark Bros, University of Florida IFAS, and Rutgers NJAES all publish chill hour requirements for the varieties they sell or research. This tool's variety database is synthesized from those sources. Numbers have ~50 hour uncertainty. A 400-hour variety may set fruit on 350 chill hours in a warm spring or need 450 in a cool one.

Question 3

How does the calculator compute hours from daily temperatures?

Accepted Answer

The Open-Meteo historical archive returns daily minimum and maximum temperatures for your exact location. For each day, the tool models the temperature as a sine wave between tmin and tmax and computes the fraction of the 24-hour period where the temperature was at or below 45°F. This is a standard approximation used by extension service calculators when hourly data isn't available. Accuracy is typically within 10% of true hourly-data chill hours over a full season.

Question 4

What happens if my variety doesn't get enough chill?

Accepted Answer

Three things: weak, uneven bloom; poor pollination because bloom is spread out over weeks instead of concentrated in a single peak; and 'delayed foliation' where leaves emerge slowly or not at all from some buds. The fruit set is poor and the fruit that does set is undersized. Unfortunately, once you plant a high-chill variety in a warm zone, the only fix is to replace the tree. This tool helps you avoid that mistake at the nursery.

Question 5

My winter just started. Why does the tool already show numbers?

Accepted Answer

It tracks from November 1 of the previous year through today. Early in December you might see 50-100 hours; by February most zones have accumulated 500-1000. Check back weekly through mid-February to watch your accumulation climb, and compare against the 10-year average to see whether this is a high or low chill year.

Question 6

Can I get alerts when my variety's threshold is crossed?

Accepted Answer

Not yet. Email alerts are on the roadmap. For now, bookmark the page with your ZIP in the URL and check back weekly. Most fruit growers check once in late December, once in mid-January, and once in mid-February.

Chill Hours Tracker

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