Walnut trees are valued for their timber and nutritious nuts. Temperature is crucial for their growth, health, and productivity. Managing temperature throughout the walnut tree’s life cycle—from dormancy to harvest and post-harvest storage—is important for cultivating healthy trees and ensuring good yield.
Temperature Requirements for Walnut Trees
Walnut trees need a specific period of cold temperatures in winter to break dormancy, accumulating chill hours. These are measured as the cumulative time the tree is exposed to temperatures between 32°F and 45°F (0°C to 7°C). English walnut varieties typically require 400 to 1,500 chill hours, while some American varieties, like black walnuts, may need over 1,400 hours. Insufficient chill hours can cause delayed or erratic bud break, poor flowering, and reduced nut set.
During the active growing season, walnut trees thrive within a specific temperature range. Optimal temperatures for vegetative growth and nut development are 75°F to 85°F (24°C to 29°C), though some black walnut varieties tolerate 41°F to 95°F (5°C to 35°C). Temperatures below 50°F (10°C) slow growth and hinder nut maturation. Extreme heat above 100°F (38°C) causes stress, leaf scorch, and negatively affects nut quality, as kernel temperatures rise quickly.
Walnut trees are vulnerable to frost, especially during key developmental stages. Late spring frosts can damage newly emerged shoots, flowers, and young nuts, leading to blackened leaves, shoots, and flowers, or a black ring in young nuts. Early fall frosts also harm immature wood before dormancy. Temperatures dropping abruptly to 22°F to 28°F (-5.5°C to -2.2°C) can injure non-dormant trees, and even dormant trees can be damaged below 20°F (-6.7°C).
Managing Temperature for Walnut Tree Health
Careful site selection is a primary step in managing temperature impacts on walnut trees. Choosing a location with good air drainage helps prevent cold air pooling in “frost pockets,” reducing frost risk. Adequate sun exposure is also important for optimal growth. Applying mulch around the base of trees helps regulate soil temperature, insulating the root zone from extreme heat and cold.
Proper irrigation strategies influence soil temperature and enhance a tree’s resilience to temperature stress. Maintaining adequate soil moisture, especially before a heat wave or cold snap, buffers soil temperatures and supports tree health. For frost protection, particularly for young trees, methods like covering can be employed. Selecting walnut cultivars suited to the local climate’s chill hour accumulation and frost patterns is another proactive approach.
Temperature’s Impact on Walnut Nut Production
Temperature conditions throughout the growing cycle directly influence the quantity and quality of harvested walnuts. Insufficient chill hours during winter dormancy can lead to reduced flowering and poor nut set in spring. Similarly, severe spring frosts destroy developing flowers and young nuts. Both issues directly reduce the overall yield of walnuts.
Extreme heat during nut development, such as kernel fill, compromises nut quality. Temperatures consistently above 90°F (32°C), and especially above 100°F (38°C), can result in shriveled kernels, lower oil content, and darker kernel color. This quality impact can occur rapidly, with loss within hours once kernel temperatures reach a threshold. Temperature also influences nut development and maturation rate, affecting optimal harvest timing.
Storing Harvested Walnuts
After harvest, maintaining proper temperature preserves walnut quality and extends shelf life. For optimal freshness and to prevent rancidity, walnuts should be stored at cool temperatures. Shelled walnuts can be refrigerated at 35°F to 40°F (2°C to 5°C) for up to six months. For longer-term preservation, walnuts can be frozen at 0°F (-18°C) or below, extending their shelf life for up to 12 months.
Humidity levels also play a role in preventing spoilage. While low temperatures preserve quality even without strict humidity control, avoiding high humidity, especially above 80%, is advisable. Excessively high temperatures accelerate rancidity, leading to unpleasant flavor and aroma, and increase the risk of mold growth and pest issues. For short-term storage, a cool, dry pantry at 59°F to 68°F (15°C to 20°C) is suitable for a few weeks, but refrigeration or freezing is recommended for long-term preservation.