A Tale of Two Washingtonias

Does provenance play a part in the hardiness and growth habits of different plants of the same species? An interesting trial designed to find out is explained in this article.
Greville Arnold-Jenkins, 6 Vermont Road, Upper Norwood, London, SE19 3SR
Chamaerops No. 10, published online 23-09-2002

Enthusiasts growing temperate palms are invariably on the lookout for cold and frost-hardy subjects to survive the rigours of a northern winter with little or no protection. Trachycarpus and Chamaerops can always be relied upon to do the job. Palms that are less hardy, such as Washingtonia, Syagrus (Arecastrum), and Phoenix will usually require protection at some stage in average winters. There is, therefore, the desire to know if the subject has any exceptional ability to withstand frost. Natural variability should mean that some palms of the same species will be hardier than others.

I have had an opportunity to test this out in recent years with Washingtonia filifera, and the results, so far, have been extremely interesting. Two plants from two very different sources have been grown on in identical circumstances to monitor such things as speed of growth, duration of cold/frost hardiness and general response to cultivation.

First, I shall explain how the plants were obtained as this will shed light on the importance of PROVENANCE.

In March 1990, my family and I stayed with friends in Laguna Beach, Southern California, the home, of course, of these vigorous palms. My plant came from the garden of my host. It was a tiny seedling, no more than 6 months old, growing in a border right against the house. The day we left for home I carefully dug it out, avoiding any root damage, washed the soil from the roots, wrapped the whole plant in damp tissue and put it in a plastic container.

I was keen to grow on this seedling because I was fascinated by the parent tree. The palm which produced this seedling was growing some 12 feet away in the front garden of the next-door neighbour. Interestingly, it was the only specimen in the immediate neighbourhood so there was no doubt about the seedling's parentage. Its trunk was unusually thick - nearly 4 feet in diameter, and stood some 25 feet high.

The site and climate experienced by this palm merit attention. Laguna Beach is situated between Los Angeles and San Diego on the Californian coastline at 33N, 15W - its coastal setting being part of its natural habitat. Washingtonias abound in this area but become conspicuously absent with altitude. Three miles in from the coast, the land rises to 1300/1600 feet above sea level. This was the elevation at which the palm was growing. Early morning jogs through the neighbourhood revealed only a handful of these palms at this high level. Could they be experiencing a slightly more adverse climate in the winter months? The answer to that was "Yes!" Minimum temperatures of -8¾C (18¾F) are not uncommon in winter, though usually the air is very dry and still when this occurs, and strong daytime sunshine pushes the temperature up to 8¾C during these cold snaps. Even so, the climate is rather cooler than one might expect.

My conclusions were that this palm should be tougher than usual because of the "adverse" nature of the local climate; its stout appearance certainly indicated that. I was therefore eager to see how its progeny would grow here in my London garden.

The second plant that has been grown for comparison came from the Canary Islands. In March 1991, during a holiday in Gran Canaria, I was unexpectedly showered with some tiny black "pellets" whilst walking through a public park. They turned out to be seeds from a very tall specimen of Washingtonia filifera. Squabbling birds had knocked them out of the inflorescences. I picked up a seed and decided to take it home to see how it would compare with the young seedling obtained from California the year before.

The climate of Gran Canaria, unlike warm-temperate California, is genuinely sub-tropical. Lying at 27.5N 15W, its sea-level locations have never experienced a frost. The lowest temperature likely to have been experienced at this site is probably just 10¾C (50¾F). Daytime temperatures between 20-30¾C (68-86¾F) and near unbroken sunshine are experienced all year. The palm, standing some 40 feet high, was a lot leaner and lankier than its Californian cousin. Close scrutiny of seed and plant determined that this was not its near relative, W. robusta. Ifs seed was to provide an interesting comparison to determine how important provenance is when growing cold-hardy palms.

Before describing the cultivation of the two plants, some important differences between the two should be mentioned. Firstly, the Californian palm would have started growth some eighteen months earlier than the "Canarian" equivalent, albeit in slightly hostile conditions. Disturbing the seedling some six months after germination may have briefly checked its growth. The seed from the Gran Canaria specimen germinated in about two weeks and experienced immediate cultivation conditions in a standard 3" pot.

Wherever possible, the cultivation given to both plants has been identical. Although cultivation of each commenced a year apart, their first springs and summers were hot and sunny. Indeed, the first half of 1992 saw similar conditions. Thus, both palm seedlings experienced similar weather to their respective parents during the growing seasons. The plants were kept in an unheated greenhouse until the end of April and then put outside against a sunny south facing patio wall.

Both plants have remained outside in their respective pots into the early winter to experience temperatures close to freezing before being transferred to the greenhouse. In the winter, ventilation is given to the greenhouse whenever the temperature was above 5¾C (41¾F) and sufficient heating to keep it just frost-free when colder weather occurred.

At the time of writing (mid-March, 1993) the difference between the two plants is now remarkable, even allowing for the 18-month separation in germination times.

The palm from the Canaries has not yet produced its characteristic fan-shaped leaves - there are only 4 simple leaves on the plant. The circumference of the base is just 5 cm. The palm from California has, by comparison, performed spectacularly. Since potting up in April 1990, it has produced thirteen new leaves in its three growing seasons (some of those have of course died when successors followed). Its base has a circumference of 20 cm and is already showing growth as spring approaches.

The growth rate of the Californian specimen is clearly faster but what has been observed too is its duration of growth. This palm begins to move in March while still in the greenhouse and continues visible growth right through until October. The palm from the Canaries only grows between June and September and then at a much slower rate.

This past winter, the Californian palm has been purposely exposed to slight frosts. Minus 2¾C (29¾F) has been permitted without protection before colder nights of -4¾C to -6¾C (21¾F) towards the end of December, 1992, forced it inside. No visible damage has occurred. The palm from the Canaries has not been allowed to experience frost and will not be planted out until it too has produced 13 leaves and a further comparison about its hardiness can be made. I shall be up-dating their progress as time passes.

I hope my observations will prompt other palm growers to furnish us with similar information on other species.

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