Chill Unit Targets for Common Crops (Utah & Dynamic Models)

Many perennial and overwintering crops require exposure to cool temperatures to properly release dormancy and resume uniform growth in spring.

These requirements are described using chill models.

This page provides indicative chill targets for common crops and explains how to interpret them using both Utah Chill Units and the Dynamic Model.

Important notes before using chill targets

Chill requirements vary by cultivar
Chill accumulation is influenced by temperature pattern, not just cold days
These targets are approximate ranges, not exact thresholds
Chill interacts with spring warmth (GDH/GDD) — one cannot replace the other

Chill models explained (brief)

Utah Chill Units (CU)

Assigns positive or negative weights to hourly temperatures
Warm temperatures can negate previously accumulated chill
Widely used, especially in temperate climates

Dynamic Model (Chill Portions, CP)

Models chill accumulation as a two-step biochemical process
More robust in warm or fluctuating winter climates
Chill cannot be “undone” once a portion is completed

General rule:
Dynamic Chill Portions are more stable across seasons and sites.

Indicative chill targets (by crop)

Apple

Model	Typical range
Utah Chill Units	800 – 1,500 CU
Dynamic Chill Portions	40 – 80 CP

Notes - Wide variation between early and late cultivars - Insufficient chill leads to uneven budbreak and flowering

Pear

Model	Typical range
Utah Chill Units	900 – 1,400 CU
Dynamic Chill Portions	45 – 75 CP

Notes - Often slightly higher chill requirement than apple - Poor chill affects flowering synchrony

Cherry (sweet & sour)

Model	Typical range
Utah Chill Units	900 – 1,200 CU
Dynamic Chill Portions	45 – 65 CP

Notes - Highly sensitive to chill deficiency - Flowering uniformity strongly dependent on chill fulfilment

Plum

Model	Typical range
Utah Chill Units	700 – 1,200 CU
Dynamic Chill Portions	35 – 65 CP

Peach / Nectarine

Model	Typical range
Utah Chill Units	400 – 1,000 CU
Dynamic Chill Portions	20 – 55 CP

Notes - Low-chill cultivars exist - Warm winters commonly cause problems in high-chill cultivars

Apricot

Model	Typical range
Utah Chill Units	600 – 900 CU
Dynamic Chill Portions	30 – 50 CP

Strawberry (perennial physiology)

Model	Typical range
Utah Chill Units	200 – 400 CU
Dynamic Chill Portions	10 – 25 CP

Notes - Especially relevant for June-bearing types - Chill influences flowering synchrony and yield potential

Grapevine

Model	Typical range
Utah Chill Units	100 – 400 CU
Dynamic Chill Portions	5 – 20 CP

Notes - Chill requirement is relatively low - Spring temperature becomes dominant early

What happens if chill is insufficient

Common symptoms include: - Delayed budbreak - Uneven flowering - Extended flowering window - Reduced fruit set - Increased management complexity

Chill deficiency cannot be corrected by warm spring temperatures alone.

What happens if chill is exceeded

Generally: - No negative effect - Dormancy release simply occurs earlier - Spring temperature (GDH) becomes the main driver

Excess chill does not compensate for lack of spring warmth.

How to use chill targets practically

Track chill accumulation season-long
Compare accumulated chill to cultivar requirement
Use chill fulfilment as the signal to:
start monitoring GDH closely
anticipate budbreak risk
plan frost protection and labour

Which model should I use?

Situation	Recommended model
Cool, stable winters	Utah Chill Units
Mild or fluctuating winters	Dynamic Chill Portions
Comparing seasons/sites	Dynamic Model
Regulatory / legacy data	Utah Model

How Agrinomy uses chill models

Agrinomy supports: - Utah Chill Units - Dynamic Chill Portions

Both are presented so users can: - Compare models - Understand risk - Avoid false confidence from single metrics

Key principles

Chill unlocks growth potential
Heat drives growth after unlock
Chill and GDH are sequential, not interchangeable
Cultivar knowledge matters

Winter temperature history shapes spring performance.

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