Chill hours — the Weinberger "chill hours" model: +1 unit for every hour the temperature sits between 0°C and 7.2°C in winter. Colder or warmer hours don't count. This is a documented simplification of the more detailed Utah model (which weights temperature bands, including negative credit for heat) — we use it because it's the classic, citable baseline.
Chill requirement — the Fruits/Nuts catalogs don't carry a chill-hours field, so each species' requirement is our own modeling proxy, derived from its USDA minimum hardiness zone: lower zone number (more cold-hardy) → higher assumed chill requirement. Not a literature value — a documented, reasonable choice so species visibly diverge.
Forcing — once chill is met, growing-degree-days (base 4.5°C) accumulate; bud break (Bloom) needs ~100 GDD of forcing on top of the met chill requirement — a tree that is warm but chill-starved will NOT bloom.
Frost risk — bloom lasts ~12 days. A freeze (≤0°C) during that window damages fruit-set. Low-chill (warm-zone) species reach their requirement earliest and bloom first — exactly the trees exposed if a late freeze lands before the average last-frost date.
Site — single real site, Yakima Valley, WA (46.6°N), real solar day-length (Cooper declination approximation) drives the seasonal temperature curve. Clock starts real calendar date 1999-01-01.