The striking silhouette of a tall, narrow palm tree, often seen lining city streets or standing against a desert sky, is a globally recognized image. These palms are distinguished by their remarkably straight, slender trunks that shoot upward, supporting a crown of large, fan-shaped leaves at the very top. This unique form is achieved through a specific biological structure that allows for impressive height without the massive girth seen in traditional trees.
Identifying the Iconic Fan Palms
The tall, skinny palms commonly admired belong primarily to the genus Washingtonia, which is often referred to simply as Fan Palms. This genus includes two main species that are widely cultivated and known for their height: the Mexican Fan Palm (Washingtonia robusta) and the California Fan Palm (Washingtonia filifera). Both species are recognizable by their palmate, or fan-shaped, fronds that radiate outward from the top of the trunk.
The Mexican Fan Palm, as its scientific name robusta might suggest, is surprisingly the more slender of the two species, often growing taller and more rapidly than its relative. Its trunk is typically narrow and straight, culminating in a relatively small crown of dark green leaves. The California Fan Palm, on the other hand, is the only palm native to the western United States and is generally shorter and stockier.
These palms are prized for their ability to provide an immediate tropical aesthetic, with the Mexican Fan Palm being particularly favored in landscaping due to its fast growth rate. Under optimal conditions, a Mexican Fan Palm can grow up to three feet in height annually.
Key Differences Between the Main Varieties
A primary distinction between the two species lies in the trunk’s dimensions and taper. The California Fan Palm (W. filifera) develops a massive, non-tapering trunk that can reach a diameter of 100 to 150 centimeters. Conversely, the Mexican Fan Palm (W. robusta) has a noticeably more slender trunk, typically 50 to 80 centimeters in diameter, which often tapers sharply from the base toward the crown. This tapering effect contributes significantly to its “skinny” look as it ages.
The maximum height also differs, with W. robusta generally achieving greater stature, commonly reaching 70 to 100 feet, while W. filifera typically stays within the range of 59 to 82 feet. Another visual marker is found in the dead fronds that hang down the trunk, often referred to as a “skirt.” While both species can retain this skirt, the California Fan Palm’s trunk often remains rougher due to persistent leaf bases, whereas the Mexican Fan Palm can become “self-cleaning” as it matures past about 30 feet, shedding its old leaves for a smoother surface.
Closer inspection of the foliage reveals further differences in the fan-shaped fronds themselves. The Mexican Fan Palm has darker green, smoother leaves and petiole bases that are distinctly reddish-brown. The California Fan Palm’s leaves tend to be a lighter green or grayish color, and its petioles, the stalks connecting the leaf blade to the trunk, have a less pronounced color at the base. Furthermore, the W. filifera fronds often have distinctive threads of white fiber along their margins, giving them a frayed appearance.
How They Achieve Their Impressive Height
The narrow, towering structure of these palms is a consequence of their unique biology as monocots, a group of flowering plants different from traditional trees, which are dicots. Unlike a true tree, a palm trunk does not have a cambium layer, which is the ring of tissue that produces new wood and causes a tree trunk to grow outward in girth each year. Therefore, palms do not possess annual growth rings.
The trunk diameter is essentially established early in the palm’s life, and once that initial width is set, the plant only grows vertically. The height is achieved by the continuous growth of vascular bundles, which are specialized cells for transporting water and nutrients, that run vertically through the stem. This process allows the palm to push its single growing point, known as the apical meristem, straight upward.
This structural design, which concentrates growth on vertical elongation rather than radial expansion, allows the palms to quickly reach heights that help them compete for sunlight. The combination of rapid vertical growth and a fixed diameter is what creates the iconic tall, skinny appearance of the Washingtonia palms.