Winter Chill

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Seasonal Gardening Tasks

• Plant bare-root pluot, peach, nectarine, and other fruit trees now.
• Spray fruit trees with copper sulfate to prevent peach leaf curl, a fungal disease.
• Start an asparagus bed for tender shoots.
• Plant strawberry plants for summer harvests.
• Sow cool-season vegetables like cabbage, broccoli, and lettuce from seed.

Understanding Fruit Tree Dormancy

Deciduous fruit trees enter winter dormancy after dropping their leaves, which protects them from cold damage and provides a required rest period before blooming. Each variety has a specific chilling requirement—measured in hours below a certain temperature—needed to break dormancy and bloom.

Chilling Requirements and Bloom Timing

• Chilling hours are typically measured as hours below 45°F (or between 32°F and 45°F, depending on the model).
• Low-chill varieties bloom earlier, risking frost damage to blossoms or immature fruit during late spring frosts.
• High-chill varieties may fail to fruit in warm climates if they don’t receive enough cold hours.
• Apples have the highest chilling requirements, followed by apricots and peaches.
• Insufficient chill can reduce harvests by up to 50%.

Chilling Models and Local Conditions

• 45 and Under Model: Every hour below 45°F counts as one chill hour.
• 32-45 Model: Only hours between 32°F and 45°F contribute to chill accumulation; temperatures below 32°F do not count.
• Terrain affects chill hours: Open slopes may receive more chill than sheltered areas, while higher elevations (e.g., 1000 feet above the valley floor) may receive less.
• The Willits Valley averages over 1000 chill hours annually (Hopland Field Station records 1024 hours below 45°F and 987 hours between 32°F–45°F).

Selecting Fruit Trees for Your Area

• Choose varieties with at least 700 chill hours for best results in the Willits Valley.
• Peach trees with 800–1000 chill hours typically bloom in early March, reducing early frost risk.