Surly the largest commonly grown ornamental palm in the world is the Canary Island date palm, Phoenix canariensis. A close relative of the true date palm, P. dactylifera, this species is much more widely grown due to its ability to live in a broad range of climates and soils. Beyond that, this palm is truly stunning as a mature tree.
Phoenix canariensis is a massive palm with a single trunk growing typically up to 20 meters, but vigorous specimens have reached twice that height. The thick trunk is up to nearly a meter across with a wide, bulbous flaring base. It is prominently marked with petiole scars that have a stacked appearance, and is usually light brown in color.
The fronds are pinnate, growing from 4 to 6 meters in length on mature trees. The individual leaflets (called pinnae) are simple and narrow, between 20-40 centimeters long each. The central shaft of the frond, the rachis, widens at its base and is armed with long, sharp spines. The overall appearance of the fronds is like a long feather. A mature specimen can hold a hundred or more fronds at one time, forming a dense, nearly rounded crown up to 10 or more meters across. Like other palms the petiole bases can persist for some time, an attribute particularly noticeable on pruned specimens.
Flowering starts in early spring. The bright orange branched flower scapes are born abundantly, but remain tucked within the the crown even in at full maturity. These palms are dioecious, meaning male and female flowers are found on separate trees. Fruits are ripe by fall or early winter, and though fairly large, are not particularly fleshy, and so do not make a good date for eating. A single large seed is contained in each fruit.
P. canariensis is an endemic of the Canary Islands, a volcanic island chain off the southwest coast of Morocco in the Atlantic Ocean, and though owned by Spain, are largely self governed. This palm is found throughout the main islands, however its distribution is scattered and localized due to centuries of human activity. The largest remaining populations are found on La Gomera. The introduction of exotic Phoenix species (especially P. dactylifera) has put the remaining populations at risk since these palms easily interbreed, creating hybrid trees that can be difficult to distinguish from wild ones.
They can be found in a wide range of habitats, typically in dry subxeric Mediterranean areas and more rarely in wet cloud forests, known locally as laurisilva, consisting of broadleaf subtropical evergreen trees. In some areas P. canariensis is the dominant species, and these communities are known as palmerales. Nowadays these once natural associations are highly influenced by humans who have introduced exotic species or expanded them by planting more palms. This species grows from sea level to around 600 meters elevation in its native habitats. For more about this palm in its native habitat please read this fascinating article from the journal of the International Palm Society: Phoenix canariensis in the wild.
In of spite being from the relatively low elevations of subtropical islands, the Canary Island date palm is remarkably cold hardy. Most sources state large trees can take -10 C (14 F) for brief periods and rebound, though fronds are damaged at much higher temperatures, taking no more than a couple degrees of frost before die-back begins. Nevertheless, consistent reports of them surviving in USDA cold hardiness zone 8 persist, at least in areas where summers are hot. Having said that, as long as the average temperate remains a few degrees above freezing in winter, this species seems to hold on. Reports of specimens growing in cold weather latitudes also abound, for instance the warmest parts of the British Isles (including London), Tasmania, and even the coastal areas of British Columbia, Canada and along the coasts of Washington and Oregon in the US if given protection in winter. On the other hand, it is able to live in truly tropical places as well, making it one of the most temperature resistant palms in cultivation.
While adapted to rich volcanic loams, it is tolerant of a wide range of soils as long as the drainage is good. It is at home in sands and even clay based soils, tolerating moderately acidic to alkaline pH conditions. Once established, trees can withstand both drought and flooding as long as these conditions do not persist too long.
Ideally it is grown in full sun in a deep, well drained loam in a climate with long, hot summers, and frost-free winters. In the southeastern US, for example, it would be best suited to USDA cold hardiness zones 9 and 10, would be marginal in zone 8b, and at risk in zone 8a. Not surprisingly, peninsular Florida is home to many large specimens. In southern Japan it is grown without much effort as well, due to the relatively mild winters, hot summers, and rich volcanic soils.
Propagation is from seed. Fresh seed will germinate within 2 to 3 months at temperatures above 30 C. If grown optimally seedlings can form pinnate leaves within a year or two. Trunk development starts around the fifth year. Young plants grow trunk slower than mature trees, with 30 centimeters of annual trunk growth possible for large, established specimens.
The Canary Island date palm is often transplanted as a mature tree. Root balls are cut up to a meter away from the trunk and two-thirds of the older fronds are removed to prevent water loss. The remaining fronds are bundled and bound with rope. To protect the trunk, fronds, and apical bud of the crown, wooden splints should be used. Damage to the trunk and especially the apical bud, can lead to the death of the tree. Additionally, newly transplanted trees need support scaffolding until firmly established. It goes without saying, a mature tree can only be handled by professionals with proper equipment – both a crane and large flat bed truck being necessities.
One of the more interesting aspects of this palm is the amount of epiphytic flora one single tree can support. In humid climates it is very common to see the lower crown and adjacent upper trunk festooned with ferns. Nephrolepis sp. (Boston ferns) in particular seem to enjoy this palm as a host, at least in Florida and Japan. In its native islands it is host to a number of endemic plants that are not typically epiphytic – Sonchus species (sow thistles), Aeoniums (succulents), and various ferns. Apparently the petiole bases and fibrous trunk are perfect perches for plant roots, and retain enough water and nutrients to allow their growth.
As with any plant, it has its problems, not the least of which is its truly massive adult size. I can’t tell you how many times I’ve seen young ones planted right up against house foundations or next to walkways. In both cases they outgrow their designated area in just a few seasons, pushing fronds right up the eves of a building (or though windows!) or completely blocking passage along walkways. Simply put, this is not a tree for small places. Remember that a mature crown can be 10 or more meters across. Also, it takes many years to produce a trunk high enough to walk under. As for small urban lots, even if you have the space, do you really want one tree to dominate your entire yard? In the end, this is a species for open landscapes with room to grow – as a large lawn specimen, in wide parks, and so on.
Phoenix canariensis is also susceptible to nutrient deficiencies, particularly on poor, sandy soils. Potassium deficiency symptoms show as a yellowing of the oldest leaves accompanied by necrotic (dead tissue) spotting. As the condition progresses leaf tips begin to die as well. Since these often break off, the older fronds look quite ragged, while younger fronds can be completely unaffected. This deficiency not surprisingly also shortens the life of older fronds, thus weakening the tree. Magnesium deficiency is seen as a yellowing of the pinnae at the margins while the centers stay green, again, usually effecting older fronds. It is not uncommon for a tree to have both deficiencies at once, with the older leaves dying off prematurely from lack of potassium, the mid-aged fronds showing the yellowing of magnesium deficiency, and the newest fronds looking fine.
The solution is simple – fertilize your trees regularly! In warm summer climates with high rainfall you may need to feed with a high potassium fertilizer up to three times a year (particularly on sandy soils). Be sure to get a fertilizer that has magnesium as well, or you can use epsom salts (magnesium sulfate) alongside your desired fertilizer. Slow release pelletized fertilizers should work quite well. In general, deep rich loam based soils should have enough natural nutrients to keep your palm happy. The removal of old fronds as a “remedy” for these deficiencies in fact weakens the tree even more since you are cutting off stores of nutrients that yet remain in the leaves.
More seriously, P. canariensis is susceptible to various diseases and pests. Many of these are unfortunately fatal. Weevils, particularly in the genus Rhynchophorus, prey on this tree, often with disastrous results. In the southeastern US palmetto weevils (R. cruentatus) burrow into the heart of the palm, which leads to its death. Weevils are attracted to damaged trees, so after pruning or transplanting a tree, it is recommended to treat cut or damaged tissues with insecticide as a prophylactic. Cut petiole bases can also be tarred over. Once a tree is infested however, the only solution is to remove it and destroy it before adult weevils leave the tree and potentially infest nearby palms. The Mediterranean area, Middle East, and north Africa have been progressively infested by the Asian red palm weevil, Rhynchophorus ferrugineus, affecting not only this species, but the commercially important date palm, Phoenix dactylifera. Recently it has been detected in the Americas as well, notably in Laguna Beach, California in 2010.
Younger trees tend to be most prone to weevil infestation. Symptoms include the yellowing and wilting of the fronds starting at the center of the crown and progressing downward to the older fronds. Unfortunately, by this time the tree has become completely infested, with death being the most likely result. Systemic insecticides can be tried, but if the damage is too great it isn’t likely to save a tree. A wide variety of novel products have been devised to trap and kill them, and natural fungal controls (Beauveria bassiana for example) are being studied, but truthfully, this bug as got a leg up on the situation.
Another problem for this tree is fusarium wilt caused by the fungus Fusarium oxysporum f. sp. canariensis. This fungus isolate is specific to P. canariensis, and unfortunately infected trees are doomed since there is no known chemical or biological control. Symptoms start with a dying of leaflets along one side of the rachis accompanied by a reddish-brown strip. Older fronds die first, creating a skirt of dead fronds. The fungus works its way up the crown to younger fronds until the tree dies.
Luckily, it seems the main reason for transmission of this fungus is through unsterilized pruning equipment. The upshot is that tools used for pruning must be adequately disinfected every time between pruning trees, no exceptions. Fortunately a number of cheap materials can be used to do this, notably household bleach (25% solution for 5 minutes) and rubbing alcohol (50% solution for 5 minutes). Clean solution should be re-made every couple hours if continuous pruning is necessary. Note that if chainsaws are used, then both the complete chain and saw bar need to be disassembled and disinfected between trees. While this may seem extreme, the results of infecting multiple trees can be far worse.
Diseased trees must be removed and either burned or put in a landfill. Every effort should be made to clean up the work site as much as possible of any infected tissue. One should not plant a new P. canariensis in that location since the fungus can continue living in the infected rootmass for as long as it persists. So the best cure for this disease is prevention. Interestingly, petiole/rachis blight, caused by a variety of fungi, has similar symptoms to fusarium wilt, but is non-lethal. The only way to know if it is fusarium is to have infected tissue tested. If the results are negative, the trees don’t need to be removed, and may recover by increasing air circulation, making sure the trees aren’t nutrient stressed, and also by applying broad-spectrum or systemic foliar fungicides. In the end though, managing petiole/rachis blight is an inexact science, but at least the condition is non-lethal.
A word about pruning palms in general – don’t overdo it. Too often palms are given such a severe pruning that it is a drag on the tree’s resources. Over pruning can lead to many problems including loss of vigor, reduction in the width of the trunk, and most importantly it can lead to making the tree more susceptible to attack by parasites and fungi. Remember that when you cut living tissue you are opening the palm up to outside diseases. A good rule of thumb is to never remove more than two thirds of the crown at any one time and ideally only dead fronds should be removed. In the case of the Canary Island date palm, severe pruning of the crown to create what’s known as the “pineapple” shape is something to be discouraged, especially if the tree has been recently transplanted. A healthy tree is in the end much more resistant to outside pathogens, so that means growers should aim to maximize overall vigor through adequate watering, fertilizing, and using a sensible pruning regimen.
In summary, this is a fascinating and surprisingly tolerant palm species. If you’d like to give it a go, you’ll need to consider the tree’s special requirements, and evaluate if it is sensible to take a stab at growing one yourself.