Friday, August 7, 2015

What is a Palm and How Do We Care For Them?

Unpruned but healthy Mexican Fan Palm Washingtonia robusta at Fullerton Arboretum, CA

Areas around the world with a Mediterranean climate such as The Middle East, Florida, The Caribbean Islands and even places in the American Southwest have both native and introduced species of palms that grow well.  Las Vegas and Palm Springs are especially well-known for having palm lined streets and palms in many landscapes.  These popular plants are often referred to as 'trees' but they don't really grow nor behave much like most of the trees we are familiar with.  What are they and how do we take care of them?  We're going to briefly discuss those two questions in this blog post. 

This information is based on personal experience as well as discussions with teachers at the College of Southern Nevada's Ornamental Horticulture Department and University Research Extension Service circulars from both The University of Nevada Reno and The University of Florida. The extension services of these and a few other universities have even more detailed information about palms than we are going to go into here.  We encourage you to look into it if you are interested in learning more.  We can also discuss these topics in depth in the comments section if you would like.

First, what is a palm really?  There are two families of plants that are commonly called 'palm trees': Palmae and Cycadaceae.  Most are in the Palmae family, but at least one commonly planted 'palm' in the Southwest is in the Cycadaceae: the Sago 'palm' Cycas revoluta.  

Female (in background) and Male (in foreground) Sago Palms (Cycas revoluta)

Both palms and cycads are dioecious meaning that there are separate plants in each species: one being female and the other male.  You have to have a male plant nearby for the wind to carry its pollen to the female plants if you want fertilized fruit.  Some species of palms have edible berries or dates as long as they come from fertilized flowers.  This is beneficial to landscape designers since if you don't want to deal with the fruits falling on the ground, you can plant a male of the species.  On the other hand, if the people who will be near the plant the most have hayfever, you can plant the female plant instead to reduce the pollen count around the property.  Most of these plants do not flower until they are several years old, the exact age varying among species and depending on the health of the plant.  You can also prune off the flower stalks before they start dropping pollen or fruit without hurting the plant.  Removing flowers is harmless.  Removing leaves is harmful.  

Neither palms nor cycads are true trees.  They have significant differences in their anatomy and their behavior from what most real trees are like.  Palms are more closely related to grasses than to pines or deciduous trees and have anatomy somewhat similar to grasses although they are not anywhere near as tolerant of being pruned or cut as most grasses are.  Cycads are more closely related to pine trees in that they have a similar sort of flower structure as other gymnosperms (broadleaf plants that do not produce showy flowers).  Most trees will recover rather quickly from being pruned, even severely cut however palms and cycad species either recover extremely slowly or not at all depending on where the cuts happen. 

The most critical difference between palms and cycads versus most other plants we are familiar with is how they grow.  Both have their primary growing point at the very top of the plant.  If the buds at the top of the main stem are removed or killed (by frost, sun scald, pesticides, insect damage etc.) the plant will never recover from it and will be stunted at best, or begin to decline and die rather quickly at worst.  They cannot sprout at the base of their trunks as many trees can in response to damage.  They also lack true bark.  What some call the bark on these plants are just overlapping leaf bases and a porous material similar to what you see in the middle of a grass stem.  If this is damaged, it never scabs over and leaves wounds that make the plant more susceptible to insects and disease. 

Most species of palms also start very short and stay that way for many years as their trunk diameter gradually increases.  Once the trunk has reached the maximum diameter for the species (also depending on the plant's health), the trunk will begin to grow taller.  When the palm has reached its maximum height for its species (groups of palm species are capable of achieving different heights with species such as California Date Palm and Canary Island Date Palm being among the tallest at around 60 feet tall and pigmy palms such as
Phoenix roebelenii and Sago palms that only get about eight feet tall at the most), it will begin to age and decline, living less than 100 years in most cases.  The shortest species, tend to be multi-trunked and spread out more, so they need more square footage of clearance where they are planted. Trying to prune a multi-trunked species so that it looks single trunked is a huge mistake and will make the palm fall apart as it grows bigger.  Almost all palm species should have at the very least 10 to 20 feet of space or more between the mature sized trunk and traffic areas, power lines etc. to avoid having to prune them because they are 'in the way'.  Never plant a palm or cycad right up against any solid object such as a wall, fence, sidewalk etc. or you are asking for trouble later on. 

There are two main appearances of palms: 'Feather' and 'Fan'.  The feather types have leaves that look like a bird's feather since they are so deeply lobed that they appear to have hundreds of long, narrow 'leaves' growing out from the central axis of the leaf in a sort of herringbone pattern.  Fan palms look more like the palm of a persons hand or ceiling fan with wide leaves that appear to be almost completely entire with lobes of varying length (depending on the species) extending beyond the solid part almost like little fingers from a persons hand.  The solid part usually looks corrugated like a piece of tin or carboard.  All types of palms produce strong, stringy material from the leaf margins with the strings being longer or more abundant on some species than others.  California Fan Palm Washingtonia filifera, for example, has so many of these threads that it is how it got its name.

Fan Palm leaf style vs Feather style

California Fan Palm Washingtonia filifera with its threads

In general, the feather type palms tend to do better in full sun than the fan palms do here in the Southwest, so it does make a difference which type you plant in certain locations.  It especially matters what exact species you plant as some species get much bigger, height or width, than others. 

recommend Pygmy Phoenix roebelinii and Sago 'palms' for most residential purposes in Las Vegas as they don't get very tall and are easier to care for than the palms that get huge.  If you have the room and want a really hardy palm California Fan Palm Washingtonia filifera which is actually native to the Southwest and grows wild in oasis in the Sonoran and Mojave deserts.  The younger of a plant you get, the better it will survive the transplanting process and the healthier it may be later on.  It is sometimes easier to tell what sex the plant is once it is several years old. 

Palms should be planted in the late spring, after the last hard freeze, but way before temperatures start getting up into the 100's.  It is a myth that palms establish best when it is really hot.  They should be watered almost daily as they get established, but only need water occasionally once the root system has anchored in. When they are watered, it is important that they are irrigated long enough for water to moisten the soil down to the bottom of the rootball.  Light watering will leave them drought stressed.  However, that does not mean that they will tolerate saturated soil for long. They need to be planted in soil that drains quickly such as a fine sand or loamy sand.  They do best with irrigation water that soaks into the soil around the root zone of the trunk (from the trunk out several feet, depending on the species and age) rather than having water sprayed at them. In fact, our high pH water can actually cause salt damage to the trunks and leaves of palms when it is sprayed.  

Palms also need a lot of Potassium (K) and usually need it supplied by a low Nitrogen (N), low Phosphorous (P) but high Potassium fertilizer.  Over-supplying Nitrogen and Phosphorous runs the risk of polluting the ground water since the soil needs to be fast draining and over-fertilizing will not benefit the plant.  Potassium deficiency is the leading cause of most of the poor performance we see in palms.  Having leaves that droop, turn brown and 'need' pruned off (they never need pruned off as long as humans don't decide that they have to be) every year is a sure sign that the palm is lacking K and maybe other nutrients. It is also probably not getting enough water either. 

Most palm species have leaves that live three to five years at least, and as long as 10 years when the plant is really healthy.  Also, the 'dead' leaves are beneficial to the plant as they provide physical support to the younger leaves above them; protect the trunk from temperature extremes; and gradually release their nutrients back to the rest of the plant as long as they are still attached.  In fact, yearly pruning can lead to chronic K deficiencies that actually wind up causing even more rapid leaf drop and shortening the palm's lifespan. 

Some palm species hang on to their old leaves longer than others but all of them will shed the leaf when it is totally drained of mobile nutrients and has dried out completely.  Some shed dry leaves a few at a time over a very long time period, like evergreen trees do, while others will shed several all at once.  How clean you feel the area around your palm must be is something to consider when picking a palm species to plant.  If you are a neat freak or have others around you who dictate that nothing can stay on the ground, it's best to either not plant a palm at all, or to pick species that produce very little fruit, the smallest fruit, drop leaves less often and, if you can get them, the male of the species.  That species usually winds up being the Sago 'palm' as it has tiny berries, delicate leaves and drops them very seldom. 

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