UNITED STATES—A tree is a woody perennial plant with a single tall trunk and branches. Banana trees and tree ferns lack both branches and wood. Arboriform yuccas develop branches but are not woody. Palm trees are no better. Some develop a few trunks but without branches. Doum palms that develop branches are extremely rare in California. No palm is actually woody.
Palms are trees only because of their size and form. In other words, most are big and tall. The most compact of palms are no smaller than Japanese maples, which are also trees. Realistically though, palms are merely large to very large perennials. They are monocots like grass, bamboo or cordyline. Some horticulturists classify them as herbaceous trees.
Only California fan palm is native to California, and only to remote desert oases. All other palms are exotic. Spanish Missionaries imported date palms to produce dates within arid regions. Only a few other palms were similarly utilitarian. The majority are desirable only for their distinctive form, texture and evergreen foliage. They are genuinely ornamentals.
Only the first exotic palms in California were fruit trees.
All palms are evergreen. Fan palms produce rounded palmate leaves on sturdy petioles. Feather palms produce elongated pinnately compound leaves on sturdy rachises. Many fan palms also produce wicked teeth on their petioles. Many feather palms also produce dangerously sharp spines on the bases of their rachises. Even lush palms can be mean.
Not many palms get big enough to provide much shade. Many types are shady in groups though. Without branches, palms are not conducive to containment or redirection. Some eventually grow tall enough to shade neighboring gardens instead. Unfortunately, palms that encroach too closely to high voltage cables require removal. They do not go around.
Palm trunks do not widen as their canopies grow higher above. Palms with plump trunks grow at ground level for several years before they can launch. Their single terminal buds must first grow as wide as their mature trunks will ever get. Their foliar canopies likewise grow no broader than they were when they launched. They only grow higher. Most large palms develop distended basal adventitious roots that can get quite wide, though.
Highlight: Mexican Fan Palm
California fan palm is the only palm that is actually native to remote regions of California. However, Mexican fan palm, Washingtonia robusta, is much more common. Technically, its native range is not far away. It had been the most popular of palms prior to the 1990s. It is more adaptable to more populous climates. Also, it naturalizes within some regions.
Mexican fan palms grow vigorously while young. Growth decelerates with maturity. Very old trees can grow very slowly without completely stagnating. Wild specimens may have potential to survive for a few centuries with such slow growth. Cultivation and irrigation of home gardens may limit this potential. Few local specimens are more than a century old.
Mexican fan palms are taller and slimmer than California fan palms. Their trunks lean as prevailing wind blows them. They easily grow fifty feet tall, and eventually get a hundred feet tall. Such tall and leaning trunks can suspend their canopies over adjacent gardens. Their pleated and palmate leaves are about three feet wide, on petioles with nasty teeth.