thermal → gdh_chill_dormancy
Plant development in temperate climates is governed by temperature over time, not calendar dates. Two key concepts describe this: - Chill accumulation (winter) - Growing Degree accumulation (spring and summer)
Dormancy acts as the biological gate between the two.
Both are cumulative and time-based.
Growing Degree Hours quantify useful warmth for plant growth, accumulated hour by hour above a base temperature.
GDH is especially useful for: - Glasshouse crops - Protected systems - Short-term growth prediction - Comparing day vs night effects
GDH = sum of (temperature − base) for each hour.
GDD is the daily equivalent of GDH: - Uses daily mean temperature - Common in field crops and phenology models
Agrinomy focuses on GDH where higher resolution improves accuracy.
Chill represents exposure to cool temperatures required for perennial plants to properly reset growth cycles.
Without sufficient chill: - Budbreak is delayed or uneven - Flowering is poor - Yield potential is reduced
Different models weight temperatures differently, but all aim to capture: - Effective chilling temperatures - Ineffective or negating warm periods
Chill is not just “cold days” — temperature pattern matters.
Dormancy is not a single state. It has three biologically distinct stages.
What it is - Growth is suppressed by signals from other plant parts - Commonly controlled by apical dominance and hormones
Key point - Buds could grow, but are inhibited
Examples - Lateral buds suppressed by a dominant shoot - Can be released by pruning or damage
What it is - Buds are internally dormant and cannot grow, even under ideal conditions - This is the critical chill-requiring phase
Key point - Chill accumulation is required to exit endodormancy
If insufficient chill - Delayed or erratic budbreak - Poor flowering - Extended vulnerability period
What it is - Dormancy is released, but growth is limited by environment (mainly temperature)
Key point - Growth will resume as soon as sufficient warmth accumulates
This is where GDH/GDD becomes dominant.
| Phase | Limiting factor |
|---|---|
| Paradormancy | Hormonal control |
| Endodormancy | Chill accumulation |
| Ecodormancy | Heat accumulation (GDH/GDD) |
Only once chill requirement is met does GDH become meaningful.
Agrinomy separates: - Chill accumulation → dormancy release - GDH accumulation → growth progression
This allows: - Better stage prediction - Better year-on-year comparison - Better alignment with phenology and BBCH stages
Temperature history matters more than today’s weather.