Chill Hours Research Data and Sources

This is the research behind our chill hours guide: every plant we sell, the chill figure we settled on, the range, how confident we are, every source we used with what it actually says, and how we got from the sources to the number. Click any row to open it. You can also download the whole thing as a spreadsheet.

The data sheet

How we did it

What we measured

Our figure is the number of winter hours at or below 45°F (7°C) a plant needs before it'll leaf out and bloom normally. That's the scale most nurseries and extension guides use, and it's what growers compare against their local chill counts. Where a source used a different model (hours between 32 and 45°F, Utah chill units, or Dynamic model chill portions), we recorded it in that unit and noted it. We never converted units with a made-up factor. When a study reported only chill units or portions, we placed the variety against other varieties measured in the same study, and said so in the notes.

Where we looked

For every plant we searched, in roughly this order of trust: peer-reviewed dormancy and chilling studies (ASHS journals, Acta Horticulturae, Scientia Horticulturae, Frontiers and others), university and extension publications, USDA and the original breeding programs, and nurseries that publish per-variety chill figures. We read our own product pages only for origin and parentage, never as a chill source. Nursery figures got extra scrutiny: some catalogs put the same range on every variety they sell, and some numbers get copied from site to site. We flagged those and gave them less weight, and we didn't count two nurseries quoting the same figure as two independent sources.

Lab numbers vs. field numbers

Lab forcing tests, where cut shoots are brought indoors after set amounts of chill, tend to give higher figures than nurseries publish from field experience. Hauagge and Cummins (1991) put McIntosh at 1,086 and Dorsett Golden at 814, yet Dorsett Golden grows normally on about 400 to 450 hours. So for apples and crabapples we used lab results mainly to rank varieties against each other (for example, Siberian-type crabs like Dolgo, Kerr and Chestnut tested below standard apples), and showed the grower-scale figure. Where the two disagree a lot, the range covers both and confidence drops.

Turning sources into one number

• Best figure: the consensus of the strongest sources, rounded to the nearest 50 hours. Not the highest or lowest figure we found.
• Range: the honest spread the evidence supports, including conflicting sources.
• Sports (like Red McIntosh or Red d'Anjou) take their parent's figure, unless the sport has its own published number.
• Seedlings and seed take the parent's figure with a wider range, and never rate above Medium, since every seedling is genetically different.
• No published figure: we estimated from parentage, species, region of origin and related varieties, wrote down the reasoning, and marked it Low. Most hardy prairie, Russian and local Montana selections fall here. Nobody has ever measured their chill.

Shared baselines for estimates

So that estimates line up across the catalog, we used one baseline per type of plant when no variety figure exists, each built from the published figures and studies cited in the rows that use it:

• Standard apples: 1,000 hours (800 to 1,200). Hardy Siberian-type applecrabs: about 700.
• European pears: 800. European plums: 800. Japanese-American hybrid plums: 700. Native American plums: 800.
• Cold-hardy apricots of Manchurian or Siberian background: about 600, rising with later bloom. Northern peaches: 850 to 950.
• Tart, bush and dwarf sour cherries: 800. Gooseberries and red currants: 1,000. European-type black currants: 1,400 unless measured.

Confidence levels

What this can't tell you

Chill needs shift a little with climate, rootstock and how the requirement is measured, so treat every figure as a guide, not a guarantee. If you grow any of these where winters are mild and see something different, please tell us. Field reports are exactly what's missing for most hardy varieties, and we'll update the sheet as better data turns up.

Real winters, counted

Hours at or below 45°F from November 1 through the end of February, averaged over 10 winters (2016-17 through 2025-26), from PRISM daily highs and lows with hourly temperatures estimated by a cosine curve. This is the same method as the calculator on the chill hours guide.

Most-used sources

The sources cited most often across the sheet. Every row above lists its own sources in full.

1. Loading...

Compiled by Montana Fruit Trees, October 2026. Climate data: PRISM Climate Group, Oregon State University, via the NOAA Regional Climate Centers' ACIS web services. Quotes in the source list are short excerpts of what each source states, kept for checking. Questions or corrections: contact us.