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
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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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