Washingtonia palms are popular landscape trees, especially in warmer climates. Understanding their cold hardiness is important for successful cultivation, particularly for those in transitional climate zones or experiencing unexpected cold snaps.
Washingtonia Palm Species and Cold Tolerance
The two primary species of Washingtonia palms exhibit varying degrees of cold tolerance. Washingtonia filifera, known as the California Fan Palm, is more cold-hardy. This species can reach heights of 60 feet or more, featuring a stout trunk and a dense crown of palmate leaves. W. filifera tolerates temperatures down to 15°F (-9°C) and is suited for USDA Hardiness Zones 8b and warmer.
Washingtonia robusta, the Mexican Fan Palm, grows taller and more slender, often exceeding 80 feet in height. Its trunk is noticeably thinner than W. filifera, and its fronds are smaller. W. robusta is less cold-hardy, surviving temperatures down to 20°F (-6°C) and thriving in USDA Hardiness Zones 9a and warmer.
Factors Influencing Cold Hardiness
A palm’s ability to withstand cold temperatures depends on more than just the absolute minimum temperature. The duration of cold exposure impacts survival; prolonged periods below freezing can cause more damage than a brief dip. A rapid drop in temperature can be more detrimental than a gradual cooling, as palms have less time to acclimate.
Moisture levels in the soil play a role in a palm’s cold resilience. Wet soil combined with cold temperatures can be more damaging to roots than dry cold. Wind exposure can exacerbate cold damage, increasing the chilling effect and leading to desiccation of fronds.
The overall health and age of the palm influence its hardiness. Healthy, mature palms are more resilient to cold stress compared to young or stressed specimens. A palm’s ability to gradually adjust to colder temperatures, a process known as acclimation, contributes to its survival during winter months.
Protecting Washingtonia Palms from Cold
Selecting a sheltered location for planting offers protection from cold winds and frost pockets. Before an anticipated freeze, ensuring the palm has adequate moisture in its root zone helps. Well-hydrated palms are more resilient to cold stress.
Wrapping the trunk with burlap, frost cloth, or blankets provides insulation against freezing temperatures. Tying up the fronds into a compact bundle protects the sensitive growing point, or crown, of the palm.
For severe freezes, supplemental heat sources are useful. Heat lamps, old-fashioned incandescent Christmas lights (not LED), or small outdoor heaters placed near the palm provide warmth. Container-grown Washingtonia palms can be moved indoors to a protected area during cold snaps.
Assessing and Recovering from Cold Damage
After a cold event, signs of damage in Washingtonia palms include browning or blackening of fronds. A more severe indicator is “spear pull,” where the newest, unopened frond can be easily pulled from the crown, indicating damage to the growing point. It is important to exercise patience when assessing the full extent of damage.
Do not prune immediately after a freeze, as the full impact may not be apparent for several weeks or even months. Wait until spring to fully assess which fronds are truly dead. Only prune fronds that are completely brown and crispy, as partially damaged fronds can still contribute to the palm’s recovery.
Continued watering and appropriate fertilization aid a damaged palm’s recovery. Providing necessary nutrients helps the palm regain strength and encourages new growth. A palm is likely beyond recovery if the spear pull test indicates a rotted or mushy growing point, as this is the primary area for new frond development.