The Washington Navel Orange tree is a popular citrus variety, cherished for its sweet, seedless fruit. However, it is particularly susceptible to cold temperatures. Understanding its cold hardiness is crucial for gardeners to ensure its survival and fruit production.
Understanding Washington Navel Orange Cold Hardiness
Washington Navel Orange trees are classified as sweet oranges, which are generally more sensitive to cold than some other citrus types like mandarins, but hardier than lemons and limes. Temperatures between 28°F and 32°F can lead to light frost damage, particularly to new growth and leaves. Leaves may show dark water-soaked areas, turn tan or brown, or curl. If temperatures drop below 26°F for at least two hours, fruit can sustain damage.
More severe damage, including death of branches or the entire tree, typically occurs when temperatures fall below 20°F. Citrus trees do not become fully dormant like deciduous trees, making them vulnerable to year-round growth. This sensitivity means Washington Navel Oranges are best suited for USDA Plant Hardiness Zones 9-11, where winter temperatures rarely dip below 20°F. Even within these zones, unexpected cold snaps can pose a significant threat to these subtropical plants.
Factors Influencing Cold Tolerance
Several factors influence a Washington Navel Orange tree’s ability to withstand cold. Younger trees are more vulnerable to freeze damage than established ones. Healthy, vigorous trees are better equipped to tolerate cold events than those stressed by pests, diseases, or nutrient deficiencies.
The tree’s microclimate, or planting location, also affects cold tolerance. Structures like houses or fences can provide protection by absorbing and re-radiating heat. Cold air pools in low-lying areas, so trees on higher ground or slopes fare better. Windbreaks also shield trees from harsh, drying winds during a freeze.
Soil conditions also contribute to cold tolerance. Moist soil retains and radiates heat more effectively than dry soil, insulating roots during cold nights. Well-drained soil is preferred, as poor drainage limits root development. Adequately hydrating the soil before a cold event can provide a buffer.
Protecting Your Tree from Cold
When cold temperatures are anticipated, protect Washington Navel Orange trees with several steps. Deeply water the soil a day or two before a freeze; moist soil holds and radiates heat, insulating the root system. Remove mulch from the dripline to allow the soil to absorb more solar heat during the day.
For smaller trees, cover them with frost cloths, blankets, or burlap. Ensure the covering extends to the ground to trap warmth radiating from the soil. Avoid the material touching foliage, as this can transfer cold. Temporary structures, like stakes, can support the covering and prevent crushing branches.
Incandescent Christmas lights (C7 or C9 bulb types) can be strung through the canopy to provide warmth. These bulbs generate heat, unlike modern LED lights, offering a few degrees of protection, especially with a cover. Ensure lights do not directly contact leaves or covering material to prevent damage. For young trees, banking clean soil 1-2 feet around the trunk insulates the graft union and lower trunk from extreme cold.
Recovering From Cold Damage
After a cold event, assessing damage to a Washington Navel Orange tree requires patience. Initial symptoms include blackened or wilted leaves, or soft, discolored branches. Resist immediate pruning, as the true extent of injury may not be clear for weeks or months. New growth in early spring can die back if underlying wood is still damaged.
The best time for significant pruning is late spring or early summer, after frost danger has passed and new, healthy growth appears. This allows clear identification of dead or recovering parts. When pruning, cut back into healthy, green wood, making clean cuts.
Support the tree’s recovery with proper post-damage care. Ensure adequate water; deep watering helps rehydrate and support new growth. Avoid overwatering, especially in poorly draining soils. Fertilize lightly and with balance after new growth establishes, to avoid stimulating overly vigorous, susceptible shoots. Patience is key, as extensively damaged trees may take a year or more to fully recuperate.