The Washingtonia palm, encompassing the California Fan Palm (Washingtonia filifera) and the Mexican Fan Palm (Washingtonia robusta), is a popular, fast-growing choice for landscape design. These towering palms, native to the arid regions of the Southwestern United States and northwestern Mexico, bring a distinct tropical appearance to many environments. Because their native habitat is generally warm and dry, their ability to withstand freezing temperatures is a primary concern for gardeners cultivating them outside of these ranges. Understanding the species-specific cold tolerance and implementing proactive protection strategies are necessary steps for ensuring their long-term survival in cooler climates.
Defining Cold Tolerance by Species
The cold hardiness of Washingtonia palms varies significantly between the two species, with the California Fan Palm exhibiting superior tolerance to low temperatures. Mature specimens of Washingtonia filifera can generally survive brief temperature drops into the range of 15 to 20 degrees Fahrenheit, often with only minor damage to the foliage. The desert fan palm’s greater resilience is partly due to its native environment, which includes desert oases where temperatures can fluctuate. Established, acclimated individuals have been reported to tolerate temperatures down to 12 degrees Fahrenheit, though this level of cold will result in substantial frond loss.
The Mexican Fan Palm, Washingtonia robusta, is noticeably less cold-tolerant, making it more susceptible to injury during moderate freezes. This species often begins to show significant frond damage when temperatures fall below 23 degrees Fahrenheit, with the survival threshold for mature palms typically hovering around 18 to 20 degrees Fahrenheit. The duration of the cold event is important, as a brief dip below the threshold is less damaging than a prolonged freeze lasting several hours or days. Younger palms with less trunk mass have a lower tolerance than older, well-established specimens.
Recognizing Signs of Freeze Damage
Visual assessment of cold injury requires patience, as the full extent of the damage may not become apparent until two to three weeks after the freeze event. The most common sign of non-fatal cold stress is the browning or necrosis of the outer, older fronds, which may appear bleached or water-soaked. This foliar damage is largely cosmetic and indicates that the palm is using these expendable leaves to protect the central growing point. The most serious sign of injury is known as “spear pull,” which occurs when the newest, central frond, called the spear leaf, can be easily removed by hand.
Spear pull confirms that the base of the spear leaf, located near the apical meristem, has been killed by the cold. This dead tissue provides an entry point for secondary fungal and bacterial pathogens, leading to a condition called bud rot. The rotting tissue around the growing point releases a foul odor and, if the apical meristem itself is compromised, the palm will not be able to produce any new leaves. Since palms possess only one growing point, the death of the apical meristem is a terminal injury that results in the loss of the entire plant.
Active Winter Protection Techniques
Proactive measures taken before a freeze ensure a Washingtonia palm survives cold weather outside its hardiness zone. The first step is to keep the palm dry in the days leading up to a predicted cold snap. Excess moisture in the crown can freeze, leading to extensive damage to the sensitive growing tissue. For palms small enough to manage, the fronds should be gathered and tied gently together into an upright bundle using soft rope or twine.
The next step involves insulating the crown, where the apical meristem is located. The gathered fronds and the upper portion of the trunk should be wrapped tightly with a breathable material, such as burlap or commercial frost cloth. For prolonged or severe freezes, external heat sources can be incorporated beneath the wrapping for added protection. Options include specialized heat cables or stringing incandescent C9 Christmas lights, which emit a small but effective amount of heat.
Once the heat source is in place, the entire wrapped column should be covered again with the frost cloth or burlap to trap the warmth. Protecting the root system is also necessary. Apply a thick layer of organic mulch, such as shredded bark or straw, around the base of the trunk. This heavy layer of mulch insulates the root zone, preventing deep soil freezing and maintaining a stable temperature for the palm’s roots.
Strategies for Post-Freeze Recovery
After a damaging cold event, the urge to prune away all the brown, dead-looking fronds should be resisted. These damaged fronds still contain carbohydrates and nutrients that the palm can reabsorb, and they provide a layer of insulation for the growing point against any subsequent cold snaps. Pruning should be delayed until the danger of additional freezing temperatures has passed, often waiting until the start of the next active growing season. Once warm weather returns, the brown leaves can be removed, but only if they are entirely necrotic.
If a palm exhibits spear pull, immediate action is necessary to prevent a secondary infection from becoming fatal. The rotting spear leaf should be removed from the crown, and a broad-spectrum fungicide, such as a copper-based product, must be applied down into the resulting cavity. This treatment helps to sterilize the wound and protect the apical meristem from bud rot. Survival is only confirmed when new, healthy growth emerges from the center of the crown in the following spring or summer.