Prune Chilling Prediction Model

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Franz Niederholzer, UC Farm Advisor, Sutter/Yuba Counties

Deciduous fruit trees have a mechanism to avoid damage from cold or freezing weather. We generally refer to this as winter dormancy*  – the annual life stage of the tree between leaf drop and bud break. Winter dormancy has two stages that can’t be visually separated in the field.

In the first part of winter dormancy, technically called endodormancy, tree growth is limited by some unknown factor inside the plant itself – actually in each plant bud. A certain amount of cool temperature is required to end this first stage of winter dormancy. This is referred to as the Chilling Requirement. Temperatures between roughly 30^oF – 60^oF contribute towards ending this first stage of winter dormancy, with temperatures between 35^oF – 50^oF contributing the most chilling.

In the second part of winter dormancy, technically called ecodormancy, growth is controlled by an external factor  –  temperature. Each species of deciduous plant requires a certain amount of heat to begin growing after the first stage of winter dormancy has been completed. So, deciduous fruit trees first need some cool weather and then some warm temperatures to start growing. Different tree species need different amounts of chilling and/or warm temps to begin bloom.

In cool winter climates like the Sacramento Valley of California, sufficient chilling to end the first stage of winter dormancy accumulates for many deciduous tree crop species before cool weather ends. The lack of warm weather is what suppresses bud break.

The use of rest breaking agents (RBA) – oil, hydrogen cyanamide (Dormex®), various nitrogen fertilizers plus penetrators, etc. – can cut short the first stage of dormancy and allow for the accumulation of heat units towards the end of the second stage of dormancy and bud break. This can result in earlier bloom. However, a certain amount of chilling must accumulate before the RBA is applied – or it won’t work. Researchers report the starting point for using RBA to advance bloom is roughly 70% of the Chilling Requirement**.

Chilling is estimated several ways. The most biologically accurate model is the Dynamic Model, developed in Israel and tested around the world in the last 20 years. It measures chilling on an hourly basis, and accumulates units called “chill portions”. Chilling portions are abbreviated “CP”. At maximum chilling conditions, the Dynamic Model accumulates 0.83 CP per day. In the Sacramento Valley using the Dynamic Model, total chilling accumulation from late October through March 1 is between 65 – 85 chilling portions.

The chilling requirement for flower buds on fruiting spurs of ‘Improved French’ prune is 35 – 40 CP. The flower  buds on last year’s upright shoot growth and leaf buds may need more than 40CP, but the chilling needed by key buds to target for oil and other RBA is just under 40CP. The oil timing that most consistently advances bloom is 30 – 40 CP. The amount of heat that follows oil treatment determines how much bloom advance you get.

Note:  Horticultural oil is the only RBA agent that has consistently performed well in University of California field trials to manipulate prune bloom. Dormex® is not labeled for prunes. Nitrogen fertilizers plus penetrators can advance bloom but can reduce fruit set compared to untreated trees and can burn buds is applied too late in the winter (February).

*It is important to make the distinction between winter dormancy and other forms of dormancy. In the summer, water stressed trees stop growing and do not resume growing until they are watered. This is a form of dormancy that is different from winter dormancy.
**The Chilling Requirement is a misnomer. Different buds on a tree require slightly different amounts of chilling. Also, the more chilling that accumulates in a winter, the less the heat required to begin bud break. So while chilling is required to end the first stage of dormancy and heat is needed to end the second stage of winter dormancy and begin bud break the process is not precise.

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Table 1.  Chilling requirements of some common tree crops in the Western North America using different chilling models.

¹ Info from A. Erez. Temperate Fruit Crops in Warm Climates. 2000. Kluwer Academic Publishers.
² Info from K. Ryugo. Fruit Culture. 1988. Wiley Co.
³ Niederholzer, unpublished data

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