As dormancy begins to break and the chill of a wet winter still lingers, growers across pome growing regions of WA are asking: Did we accumulate enough winter chill this season? The answer is cautiously optimistic.

Compared to 2024, the 2025 season got off to a better start, with chill accumulation beginning earlier and progressing steadily. However, we’re still tracking within the lowest 10% of chill years historically, which may impact flowering and foliation in some regions.

Key Highlights:

Manjimup reached its average of 70 chill portions, a positive sign
Bickley (Perth Hills) exceeded its average, reaching 57 chill portions.
Donnybrook, however, is expected to fall short of its average, reaching only 50 chill portions.

Winter chill is essential for deciduous fruit trees to break dormancy properly. Insufficient chill can lead to delayed or uneven flowering, impacting fruit set and yield.

Rainfall was above average in the South West, with some areas recording their highest winter totals ever. Temperatures were warmer than usual, with maximums 1–3°C above average, which may have reduced effective chill accumulation in some locations.

Understanding Chill Models: Three models are commonly used to estimate chill:

· Chilling Hours Model – Simple but less accurate in warmer climates.

· Utah Model – More nuanced, but sensitive to warm spells.

· Dynamic Model – Most reliable, especially in variable climates like WA.

The following outlines the positive and negatives of each model.

1. Chilling Hours Model (Weinberger, 1950)

Method: Counts hours with temperatures between 0°C and 7.2°C.
Pros: Simple and easy to calculate (e.g., in Excel).
Cons:

Uses a hard temperature threshold.
Ignores chill at slightly higher temperatures.
Doesn’t account for negative effects of heat.

Usefulness: Considered outdated and less accurate, especially in warmer regions like South Africa and Australia.

2. Utah Model (Richardson Model) (Richardson et al., 1974)

Method: Assigns weighted chill units based on temperature ranges (1.4°C–12.4°C), including negative chill units for high temperatures.
Pros: More nuanced than the chilling hours model.
Cons:

Requires proper calibration for each location.
Can accumulate negative chill early in winter if misapplied.

Winter Chill Units accumulated using the Richards Model in the South West of WA and in the pome growing regions of South Africa. — Winter Chill **Units** accumulated using the Richards Model in the South West of WA and in the pome growing regions of South Africa.

3. Dynamic Model (Erez et al. 1990)

Method: Models chill accumulation in two biological steps:

Formation of an intermediate product (destroyable by heat). Following a bell-shape response function to temperature with an optimum at 6°C, tapering to zero at -2 and 14°C.
Conversion to a stable dormancy-breaking factor.