Methodology

Chill hours

We use the standard 32–45°F model: for each hour of the winter we count it as one chill hour if the temperature was between 32°F and 45°F (0–7.2°C), inclusive. This is the model the university-extension sources we cite use, so our figures are directly comparable to theirs.

For each of ~35,000 US ZIP-code and Canadian postal-area centroids we pulled 10 winters (2015–2025) of hourly 2-metre temperature from the Open-Meteo Historical Weather API, which is based on the ECMWF ERA5 reanalysis and elevation-corrected to each point. We count chill hours per season and report the 10-season mean, minimum, and maximum, so you can see both a typical winter and the year-to-year range.

Warm regions (USDA zone 8a and warmer) are reported over the October 1 – March 31accumulation window used by warm-climate extensions; cooler regions use September 1 – March 31. We store every season's value under both windows so alternative chill models can be added without re-downloading data.

Frost dates

From the same hourly data we find, for each year, the last spring date and first fall date at or below 32°F. We report the mean and the 10th–90th percentile range, framed as “typically between X and Y,” because an average frost date alone hides the risk in a cold year.

Hardiness zones

For US locations we use the 2023 USDA Plant Hardiness Zone Map zone-by-ZIP table published by the PRISM Climate Group at Oregon State University. For Canadian locations we show the native Natural Resources Canada plant-hardiness zone, and separately compute a USDA-equivalent zone from the ERA5 mean annual extreme minimum temperature so that variety hardiness ranges (which are on the USDA scale) can be matched consistently.

Variety facts

Chill requirements, hardiness ranges, pollination groups, and harvest windows are transcribed from named university-extension and nursery-specification sources. Every figure on a variety page links to its source in the Sources footer. Where we could not find a citable figure for a field, we omit that field rather than estimate it.

What we don't do

We never recommend a chemical, product, or dose. Spray-related guidance states timing windows only, cited to extension calendars, with a standing note to consult your local Cooperative Extension and follow the label. EveryOrchard is informational and not a substitute for local expertise.