Predicting Cold Hardiness in Palms
Climate adaptedness, I feel is a better term than Cold hardiness
The nature of one’s growing season has a profound effect on “cold hardiness”. Cold hardiness zone maps will provide indicators as to the minimum temperature a species may have been subjected to. However, that data is insufficient. Therefore, we must look at the physical map for that specie’s natural distribution range where this will indicate many other vital statistics; Latitude, Altitude, and environment type. The environment type indicates the amount of vegetative cover, topography, total sunshine in hours, total rainfall in mm, number of rainy days, etc..
Thermally, the summers of climates that have some cold weather limitations in winter fall into three groups:
Group 1: warm - daytime temperatures in summer are consistently warm and remain elevated during the night. Over 2500 GDD per annum* (semi-tropical and low desert climates).
Group 2: mixed - warm daytime temperatures in summer may be mixed with cooler days or cool mornings. There is a considerable drop in temperature during the night. 1000-2500 GDD per annum* (warm Mediterranean and semi-arid inland climates).
Group 3: cool - warm daytime temperatures are the exception rather than the rule. Nights are consistently cool. Fewer than 1000 GDD per annum* (mild maritime and subtropical montane climates).
We should then breakdown this still further, by including specific local environment conditions at that local; I.e., Latitude, Altitude, and environment type.
All of these group examples above are “Hardiness Zone 9b” climates, all very different. Whether or not any given palm will adapt to the given winters depends, in large part, on how thermophilic the palm is, not just on how well it tolerates occasional frost, which is how most people read hardiness zones to be. Rhopalostylis sapida, for example, is not thermophilic at all. It grows slowly in temperate conditions and giving it additional heat does not accelerate its growth. Butia capitata is moderately thermophilic. It grows slowly in temperate conditions but prefers subtropical conditions and giving it additional heat does accelerate its growth. One could classify “cold-hardy” palms as belonging to climate group 1, 2, or 3 depending on what kind of summers they prefer. The problem with the recognised list of “temperate palms” is that it mixes palms from all three groups with little regard for limitations imposed by the nature of the growing season, and environment type associated with the specific specie.
Another limiting factor is seasonal precipitation. In ideal conditions, most palms would prefer equi-distributional rainfall. With cold winters, however, dry winters are best and a strong rainfall peak in spring or summer produces the best growth. Having a marked rainfall peak in winter adds another limitation to what kind of palms will grow in a “temperate” climate.
*Growing Degree Days per year calculated on a base of 12°C. As the temperature most temperate palm species commence growth.
GDD are calculated by taking the average of the daily maximum and minimum temperatures compared to a base temperature, Tbase, (usually 10°C). As an equation:
GDDs are measured from the winter low. Any temperature below Tbase is set to Tbase before calculating the average. Likewise, the maximum temperature is capped at 30°C because palms generally do not grow any faster above that temperature.
For example, a day with a high of 23°C and a low of 12°C would contribute 5.5 GDDs.
Example climate models compared with Cornwall in the United Kingdom.
Here we examine examples of the climate models for species:
- Chamaerops humilis
- Butia capitata var. odorata
- Washingtonia filifera
- Parajubaea torallyi var. torallyi
- Rhopalostylis sapida
- Trachycarpus fortunei
|Av. Night Temp.||4°C||4°C||5°C||6°C||8°C||11°C||13°C||13°C||12°C||9°C||7°C||5°C|
|Av. Day Temp.||8°C||8°C||10°C||13°C||15°C||18°C||19°C||19°C||18°C||15°C||12°C||9°C|
|Record min. temp. -8°C. But this was a one-off. Otherwise -4°C. 887 GDD per annum|
Total sunshine hours = 1620, Total rain = 950mm. Total rainy days = 178.
So from this data we can see that despite the large amount of rain, the UK still gets a reasonable amount of seasonal sunshine. For those who don’t already know this, this makes the UK a great place for gardening. This amount of sunshine is attributed to the long summertime day length, a consequence of being so far north of the equator. The downside is the minimal sunshine during the winter months, combined with the fact that most of the rain falls in the winter. There is little summertime extreme heat, and little temperature swing between summer and winter. The winters are extremely mild. Finding palm species, that thrive in these conditions is challenging.
|Av. Night Temp.||2°C||2°C||5°C||7°C||10°C||15°C||17°C||17°C||14°C||10°C||5°C||2°C|
|Av. Day Temp.||9°C||11°C||10°C||18°C||21°C||27°C||31°C||30°C||25°C||19°C||13°C||9°C|
|Record min. temp. -10°C. for this provenance. 1551.2 GDD per annum|
Total sunshine hours = 2790, Total rain = 445mm. Total rainy days = 87.
Madrid is better compared with London rather than Cornwall, because London shares the same average Min. Temp. (2°C), and the same record Min. Temp. (-10°C), both being a zone 9a. Here in Cornwall, we actually have a better hardiness zone than both those zone 9b. We all know that Spain is a better place to grow palm trees, so why is that? - 58% more sunshine than the UK, this almost entirely due to the 91 fewer rainy days. The consequence of this being far less available water, 505 mm less than the UK. Making this climate hot, dry, and arid.
Chamaerops humilis will grow almost anywhere in the British Isles, and we don’t ever get temperatures low enough to harm it. However, it grows very slowly here.
|Av. Night Temp.||6°C||6°C||8°C||9°C||12°C||15°C||17°C||16°C||15°C||15°C||9°C||6°C|
|Av. Day Temp.||14°C||14°C||17°C||20°C||23°C||26°C||28°C||28°C||26°C||26°C||18°C||15°C|
|Record min. temp. -4°C. for this provenance. 1961.8 GDD per annum|
Total sunshine hours = 2850, Total rain = 951mm. Total rainy days = 71.
So this is basically a Mediterranean climate, but with twice the rainfall over fewer rainy days. The rainy days are also spread quite evenly throughout the year, providing excellent growing conditions, with a long growing season. The exact same amount of rain as the UK, but fewer rainy days, which in turn produces more sunshine hours. It is clear why Butia capitata grows so well in the UK. With more sun it would be completely at home here.
|Av. Night Temp.||4°C||6°C||8°C||12°C||16°C||21°C||25°C||24°C||21°C||13°C||7°C||4°C|
|Av. Day Temp.||18°C||21°C||24°C||28°C||33°C||38°C||40°C||38°C||36°C||30°C||24°C||19°C|
|Record min. temp. -13°C. for this provenance. 3315.4 GDD per annum|
Total sunshine hours = 3990, Total rain = 190mm. Total rainy days = 40.
At first glance at the record min. temp. -13°C., and climate zone 9a most people will assume that Washingtonia filifera will grow easily in the UK. This palm is adapted to a very harsh and arid environment. It likes a lot of sun, high temperatures, dry air, and any cold snaps to be very short. It does like its roots in plenty of water, but it totally dislikes freezing moist air, and for this reason this palm can be killed at a mere -4°C in the UK. It can be grown in the UK, but requires a little attention.
|Av. Night Temp.||1°C||2°C||3°C||4°C||6°C||6°C||6°C||6°C||6°C||4°C||3°C||3°C|
|Av. Day Temp.||17°C||17°C||18°C||19°C||19°C||18°C||17°C||17°C||18°C||18°C||18°C||18°C|
|Record min. temp. -4°C. for this provenance. 1064.5 GDD per annum|
Total sunshine hours = 2430, Total rain = 575mm. Total rainy days = 124.
This is a climate comparable to the UK except for the fact that the temperature swings between Min., & Max. temperatures occurs between day and night within a single day, not summer and winter seasons, as in the UK. We can assume that Parajubaea torallyi dislikes seasons, or it would have migrated south down the Andes mountain range after the last Iceage. And we can assume that this palm dislikes high temperatures, or it wouldn’t be growing at such a high altitude in the tropics. Parajubaea torallyi grows at the highest altitude of any palm (3,400 m).
Parajubaea torallyi seems to tolerate the UK temperatures, because we have minimal temperature swing between summer and winter. However, it grows very slowly.
|Av. Night Temp.||3°C||3°C||6°C||8°C||9°C||11°C||12°C||12°C||11°C||8°C||6°C||3°C|
|Av. Day Temp.||12°C||12°C||13°C||15°C||16°C||18°C||19°C||19°C||18°C||18°C||14°C||12°C|
|Record min. temp. -5°C. for this provenance. 642.5 GDD per annum|
Total sunshine hours = 1890, Total rain = 2910mm. Total rainy days = 186.
Here we have a climate almost exactly the same as the UK, except for the massive 1960 mm of additional rain falling in almost the same umber of wet days as the UK. This is the southernmost range of Rhopalostylis sapida on mainland New Zealand, and is the ideal provenance to grow in the UK. This palm also grows in areas of New Zealand with much less rain than this. Nevertheless, we can assume that the rain in the UK would not deter this palm from thriving here. The palm seems to prefer higher temperatures if it can get them, and temperatures below -5°C. can easily kill it. Rhopalostylis sapida grows very slowly both in the British Isles and New Zealand.
|Av. Night Temp.||1°C||1°C||4°C||10°C||15°C||19°C||23°C||23°C||19°C||14°C||7°C||2°C|
|Av. Day Temp.||8°C||8°C||13°C||19°C||25°C||28°C||32°C||32°C||28°C||23°C||17°C||12°C|
|Record min. temp. -12°C. for this provenance. 2113.9 GDD per annum|
Total sunshine hours = 1860, Total rain = 1135mm. Total rainy days = 98.
Here we have the climate, which is the best match to the UK. This particular provenance of Chusan Island is wetter and sunnier than the UK, but if you compare the average climate over the whole, wide range of Trachycarpus fortunei in China then you will find that it matches the UK very well. Trachycarpus fortunei enjoys the longer day length in the UK summer, and it can be said that the palm actually grows better in the UK than it does in China.
Finding an exact match to the UK climate is impossible, due to the fact that the British Isles lies so far north of the equator, where both winter and winter nights are long. No other place on the planet, which shares similar winter temperatures is situated so far from the equator. Studying hardiness zones alone does not indicate species suitable for growing in the UK. All palms, which can be grown here would prefer more winter sunshine than they can find in the UK. On the plus side; the British Isles’ mild winters permits us to grow more species, albeit uncomfortably than any other place situated this far from the equator, 50°N - 60°N. Studying climate modelling statistics of palm’s provenance’s in this way has so far identified over 130 (and counting) species as likely candidates for trial in the British Isles. But only by actually trialing them in the ground here can we identify the specie’s tolerances.