Gardening glossary
Chill hours
Chill hours are why you can't grow most apple varieties in Florida and why a peach tree from Georgia underperforms in Minnesota. Deciduous temperate-fruit trees evolved to require a measured winter chill before they'll wake up and bloom — it's a safety mechanism that prevents them from blooming during a midwinter warm spell.
The standard "chill hours" model counts cumulative hours between 32°F and 45°F (0–7°C) between roughly November 1 and February 28 in the Northern Hemisphere. The more refined **Utah model** weights different temperature bands differently and even subtracts chill when temperatures exceed 60°F. Both produce broadly similar results in the apple-and-stone-fruit belt.
Typical chill-hour requirements by crop:
- **Apples**: 400–1,000 hours depending on cultivar. Honeycrisp needs ~800; Anna or Dorsett Golden need ~200 and grow in the Deep South. - **Pears**: 600–900 hours for European pears; 100–500 for Asian pears and low-chill hybrids. - **Peaches**: 200–1,000 hours. Tropic Beauty (~150 hours) is grown in Florida; Reliance (~1,000 hours) is grown in Minnesota. - **Sweet cherries**: 700–1,100 hours, which is why they're rare outside Zones 5–7. - **Sour cherries**: 1,000–1,200 hours; very cold-tolerant. - **Plums**: 500–900 hours for European; 300–500 for Japanese. - **Blueberries**: 150–1,000 depending on type. Southern highbush (~150–400) vs. northern highbush (~800–1,000).
Typical chill accumulation by region:
- **Zone 5 (Chicago, Boston)**: 1,500–2,000+ hours - **Zone 7 (DC, mid-Atlantic)**: 1,000–1,400 hours - **Zone 8 (Atlanta, central Texas)**: 600–900 hours - **Zone 9 (north Florida, coastal California)**: 200–600 hours - **Zone 10 (south Florida, coastal SoCal)**: 50–200 hours
If a tree doesn't get its required chill, you'll see delayed and uneven leaf-out, sparse and prolonged bloom, low fruit set, and weak shoots. The fix is to pick cultivars matched to your local chill accumulation — not to over-water or fertilize.