Washingtonia robusta, the Mexican fan palm, is perhaps one of the most commonly planted palm trees around the world, and yet its native home in northern Mexico is limited to a relatively few water canyons scattered here and there in Sonora and Baja California. Rows of it’s dizzyingly tall, slender trunks topped with a relatively small crown of fronds are the icons of southern California’s older neighborhoods – it’s hard to find a photo of Hollywood that doesn’t include at least a few. So how did this relatively scarce palm become so widely grown throughout the world’s warmer climates? The answer is simple – this is a very tolerant, fast growing palm.
Washingtonia robusta is a denizen of water canyons in otherwise desert to semidesert regions. It is one of the tallest palms in the world, commonly topping 15 meters, with exceptional specimens growing an additional 10-15 meters higher. Its unbranched gray trunk is generally quite slender, usually not more than 30 centimeters in diameter, but swells considerably at the base. It is ringed by close set leaf scars and in tall trees tends to curve a bit as it ascends in long, lazy arcs.
The glossy fronds are palmate in form, are rich green in color, and have hanging leaflet tips not unlike Livistonia chinensis, though not as pronounced. The fronds are quite large, a bit longer than wide, with mature fronds growing up to 1.5 meters long. They form a large round crown, and tend to remain attached to the trunk even after they die, creating the tell-tale skirt common to this genus. The petioles grow up to a meter long and are host to rows of large orange hook-shaped sawtooth spines.
In spring and early summer long sprays of tiny white flowers are produced on hanging branched flower stalks that grow beyond the limit of the crown, each extending up to 3 meters in length. By the fall thousands of small dark brown fruits are produced on them, making this palm a potential weed in the right climate.
The Mexican fan palm is found in nature in seaside facing water canyons and oases near the coast of Sonora (generally north of Guaymas) as well as Baja California. Its range is by no means continuous, but rather highly scattered, suggesting a remnant distribution from a time when conditions allowed it to be more widespread. Its northern distribution is close to its sister species, Washingtonia filifera, but apparently they never are found growing naturally together.
In spite of its rather restricted native distribution, this palm is far from rare these days, in fact in many places it is considered a potential pest. In parts of central and southern California it has naturalized itself such that the California Invasive Plant Council considers its impact on native ecosystems a “moderate risk”. Many people who have grown this palm and let it go to seed have found out just how easily it spreads, sometimes forming “lawns” of seedlings. It is not uncommon to see young trees growing literally in the cracks of concrete walls and sidewalks.
In California it especially likes to invade riparian areas where it can form pure stands – not a good thing in a state where natural waterways are rare. In Florida it is ranked a category II invasive, that is, naturalized populations are increasing, but so far have not altered native ecosystems significantly. Still, planting it within 500 feet of native ecosystems is illegal within Miami-Dade County in southern Florida. It is also grown in Hawaii, but to my knowledge is not yet considered a pest species there. Seeds are dispersed by birds eating the fruits.
While the small fruits of W. filifera traditionally have been used by native people as a food source, I’ve found little reliable evidence that W. robusta has been used in a similar fashion. The fleshy part is very thin, nevertheless it is said to have a sweet flavor like a date. One thing’s certain, you’d have to be pretty hungry to want to eat these small, hard fruits.
This tree has been given a number of common names, some banal, others more colorful. Mexican fan palm and Washington palm are the most common, but I prefer skyduster. After seeing a really tall one you can easily imagine it effortlessly sweeping the clouds of dirt. Another name (more commonly used for W. filifera) is petticoat palm due to the large “skirt” of dead, hanging fronds that both species can accumulate over time.
This tendency to form a skirt in W. robusta is more typical in younger specimens, but once they attain some height they begin to shed old fronds within a couple years of dying. W. filifera by comparison tends to keep its skirt unless cleaned of old fronds (and the petiole bases). These skirts can attain formidable volumes and are host to all kinds of wildlife – birds, snakes, rodents, insects – you name it. This “petticoat” is also a fire hazard.
For these reasons it is necessary to remove dead fronds in urban settings or if trees are planted near residences. Municipalities spend a lot of money on cleaning and maintaining street specimens, one reason why this tree is becoming less used in urban settings, especially southern California. The reverse is true here in Japan however – each year I see more and more of them being planted along streets, in parks, and next to apartment buildings.
Frond removal must be done carefully to protect the health of the tree. It is common practice to over prune them, leaving just a few fronds in the crown. While this rarely kills the tree, it can become a problem if done on a regular basis, since the tree may not be able to photosynthesize enough to provide adequate nutrition to the crown and adjoining trunk. Trees that are pruned this way again and again can lose vigor and the upper trunk can become spindly such that the crown dies or breaks off in a high wind. Therefore, it is necessary to leave at least 50% of the green fronds intact. The petiole bases can either be removed or left on, but ultimately they too will fall, creating a mess around the tree and are a potential hazard during high winds.
This species is perhaps most remarkable for its potential height, sometimes exceeding 30 meters (over 100 feet), making it one of the world’s tallest palm species. Since it isn’t a very massive palm, ultra-tall trees have an elegant, graceful appearance, accentuated by their slightly curved, thin trunks. It is said that Florida trees are short since lightening strikes tend to take out trees that get too tall – natural lightning rods. Therefore, places like southern California where lightning isn’t as common, are best for this species to attain its highest potential (literally).
The growth rate of the Mexican fan palm is one of the fastest amongst the commonly grown species of palm. In climates with sufficiently warm summers (say daily highs consistently above 25 C) established trees can grow astonishingly fast. Under the best conditions they may grow up to 1.2 meters (4 feet) per year, but are more likely to put on half that height under normal garden conditions. In cooler climates where this species can still be grown, anticipate much more modest growth.
Germinating W. robusta from seed is straightforward, especially if fresh seed is used. No special treatment is needed – simply plant the them in any reasonable compost that’s evenly moist, keep them in warm conditions, and within a couple weeks germination will begin. From seed you can grow a large tree in just a decade’s time, so this is one palm you may consider growing from seed rather than buying as a larger specimen, particularly if you want to have a grove of them. They do transplant well as adults, but transporting and planting costs can make that expensive.
The Mexican fan palm is fairly cold hardy, especially in drier climates. Common estimates rate this palm to at least -6.5 C (20 F) with frond damage occurring several degrees higher, depending on relative humidity and soil moisture (the drier, the more cold resistant). In general W. robusta is more suited to moister climates while W. filifera is a true desert species that is more resistant to cold snaps provided conditions are dry (continuously wet cold conditions can be the death of it however). It is thought that the artificial hybrid of the two, known as Washingtonia “filibusta”, should grow fairly well in cooler climates with wet winters, combining the moisture resistance of W. robusta and the cold hardiness of W. filifera. Areas such as coastal Oregon and Washington states might be good places to try this hybrid, but don’t expect too much cold resistance.
In terms of the USDA cold hardiness zone system, I would rate W. robusta a solid zone 9 plant, more marginal in zone 8b, and risky in zone 8a, at least in the eastern USA. You can extend this range if you protect in winter, but given the size of these trees and their growth rate, protective measures will become a difficult task in short order. In the Pacific northwest, the wetter regions of zone 9 are probably out for this species. In more humid climates expect damage to the fronds when temperatures go below -4 C (~25 F). It is said that in low humidity the fronds are hardy down to -5 C (23 F). Regardless, as long as the crown remains alive, trees will quickly recover over summer especially if temperatures are high. This palm is also very heat tolerant, taking well over 40 C in stride.
In general W. robusta is very easy in the right climate, in fact it grows too well in many cases. It can grow in almost any soil type as long as it isn’t actually soggy – clay, loam, or sand – and is tolerant of a wide range in pH as well, optimally not too acidic, but able to grow in extremely alkaline soils. It has a high tolerance for salt, and so is a good choice for planting near coastlines. It also is very wind resistant, even tall specimens. Fertilizer can be given if trees are planted in poor, sandy soils, but be careful not to overdo it or you will have a giant on your hands quickly. Older trees are usually resistant to disease, however young trunkless plants are susceptible to rots, in particular fusarium wilt, caused by the fungus Fusarium oxysporum (which can effect adults as well).
With many positive attributes, what negatives are there to mention about this palm? As said earlier, it is a potential invasive pest. This isn’t its worst feature however. Oddly enough, the speed at which this palm reaches lofty heights is its ultimate doom as a common lawn specimen. While some people like the skirt or petticoat of dead fronds, most folks don’t. When a tree is fairly short it is easy enough to remove old fronds with a simple extendable ladder. However, once a tree gets taller than say around 7 meters (~25 feet), a professional tree pruner is needed to remove old fronds – something that needs to be done on at least a yearly basis. This of course is expensive, and if you have more than one tree it can become prohibitively so. That is why most residential trees retain their skirt until it falls off naturally – a process that may take many years.
Another negative are the super-sharp, hooked spines on the petioles. During pruning great care must be taken so as not to be hurt when cutting fronds off, or from falling fronds. Protective gloves are a must. Moreover, in high winds it is common for old fronds to dislodge and fall from great heights – not something you want to fall onto you. Also, just after flowering it is recommended the flower stalks are removed to prevent the myriad seeds from dispersing into the landscape. This too is costly to do, but luckily has to be done only once a year. Neglecting to remove flower stalks most certainly will result in volunteers showing up near the parent tree. If the conditions are right large “lawns” of seedlings can result, and will require removal before they begin to get big (something they do very quickly).
A dream palm or dreaded weed? Which one you’ll have to decide for yourself. Regardless, the Mexican fan palm is guaranteed a place in private and public spaces for years to come.