COLD HARDINESS 201
How is the cold hardiness of a palm species ascertained? It's
simple, people grow the palm in climates that are slightly colder or
harsher than the palm's natural habitat. For example, Flame Thrower
Palms (Chambeyronia macrocarpa)
come from New Caledonia, which is a sub-tropical island in the South
Pacific. Collectors began growing them in Mediterranean climates and
desert climates to see if they were versatile enough to survive.
After much trial and error, these growers discovered that they can grow in
desert climates if properly watered and protected from sun and wind (there
is a nice specimen growing in
Rancho Cucamonga, CA). They can also be grown with minimal
maintenance (irrigation and fertilizer) in Mediterranean climates such
as coastal Southern California, France, and Italy.
The cold hardiness of a certain palm species is much more involved than
just a number. For instance, if you are told that a Bismark Palm (Bismarkia
nobilis) is "cold hardy to 18 degrees" it does not mean that the palm will
instantly fall over dead the moment the air temperature falls to 17
degrees. More accurately, the Bismark will begin to lose its ability
to photosynthesize sunlight once the temperature falls below this point.
The key word to remember is duration
and there are two factors involved:
Duration of Cold
If the temperature hits 18 degrees for just 30 minutes, the Bismark
will sustain minimal leaf burn (probably just at the tips) and likely will
not die. However, if the temperature drops to 18 degrees and remains
there overnight, the palm will probably sustain complete leaf burn
(causing all leaves to die) followed by freezing in the growth point, or
heart. Once the heart is damaged beyond recovery, the palm will
likely die a slow death over the next few days. Sometimes, the
freeze damage to the heart is minimal enough to cause slow decline and
death over the course of weeks or months. Once a palm is defoliated
(loses all of its leaves) it loses its ability to photosynthesize light
and push new growth.
Even if the heart is
not killed by the cold, the loss of leaves
basically cuts off the palm's
food source
Bismark Palms in habitat- central plateau,
Madagascar. (G. Levine)
and the tree will starve to death. Some
species are
resilient enough to force new growth even after all leaves have been
killed. The chances of this happening become better if the weather
quickly warms up, the growth point is treated with a fungicide to prevent
further disease, and the palm is already mature (able to utilize reserves
of sugar and water from its stem).
Duration of Warmth
More important than duration of cold is the duration of warmth. The
duration of warmth has to do with the climate in a certain area.
(Don't confuse weather with climate. Weather
refers to the current conditions in a particular area.
Climate refers to the meteorological
conditions, including temperature, precipitation, and wind, that
characteristically prevail in a particular region). We will use the
Coconut Palm (Cocos nucifera) as
an example in this case. The Coconut Palm is generally known as a
Zone 11 plant (cold hardy to 40 degrees F). If the temperature falls
below 40 degrees, the palm begins to halt photosynthesis (as explained
above). Coconut Palms grow well in Southwest Florida, which is
considered a sub-tropical climate. The average annual
temperature in SW Florida is 72.5 degrees F.
Now lets take a look at
coastal Southern California, which is considered a Mediterranean climate, where
a few collectors are successfully growing Coconut Palms outdoors.
Learn about
 climates.
The average annual temperature in coastal Southern California is 63 degrees F. Thus, on any given day, the temperature in SW
Florida is an average of 9.5 degrees warmer than it is in coastal Southern
California. This translates to 15% more warmth on average throughout
the year.
Now lets make a connection between the 15% higher average
warmth and the cold hardiness of the Coconut Palm. Two equally-sized
Coconut Palms, one in SW Florida and one in coastal Southern California,
are both subjected to 35 degrees from a northerly storm for a period
of three hours. Both trees are growing in beach sand and both have
received the same amount of water to the roots. The three hours of
cold has caused both trees lose 25% of their fronds. The difference
comes when the weather returns back to average in both locations.
Since the Coconut Palm in Florida receives 15% more warmth annually, it
has an advantage over the Coconut Palm in California by producing 15%
more new growth, 15% larger leaves, and a stem that is 15% thicker in
diameter (roughly).
Is it beginning to
become clearer why duration of warmth is important? Even if both
locations drop down to 35 degrees for the same amount of time and the
same number of days in a given year (both are Zone 10b), the Coconut has
a better chance at survival following cold snaps because it is warmer,
on average, throughout the year. The Florida Coconut will also
grow faster, larger, and with less care than the California Coconut.
The other major factor that affects cold hardiness is relative humidity.
This famous
Coconut Palm is growing in
Newport Beach, CA and is considered the most
northernly-growing Coconut
in the Western
Hemisphere!
Cold Tolerance 301
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