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Palms in Colorado Springs (USDA Zone 5b)
Palms in Colorado Springs (USDA Zone 5b) revised 11/2007 Cenotes de las Palmas
Rhapidophyllum hystrix - Florida Needle Palm

This is the most cold tolerant known palm. At 2 Lazy2P Gardens, R. hystrix survive -10F without damage. Temperature to -15F produces moderate damage from which these palms usually recover. The oldest, a one gallon specimen planted in 1992, has an 18 inch tall trunk, 12 inches in diameter (2006)

Soil is Pierre shale clay mixed with Pikes Peak granite scree, pine needles and compost, is neutral in pH, and has good drainage. A deep mulch of pine needles insulates the palms.

Between April and October, needle palms grow 2-4 new fronds. Ample water is  given during that time. In fall, water is withheld, as in drought, to harden new growth. During winter, slow growth continues, adding one leaf and one new spear.

In the 2005-6 winter, low temperature to -15F caused some hystrix to defoliate and lose a spear from the main trunk, but lateral shoots survived.
Photo above -Trachycarpus takil (nainital) after five USDA Zone 5b winters, and several subzero 0F periods, coldest  to -15F. These T. takil outperformed R. hystrix in the 2004-5 and 2005-6 winters

USDA Zone 5b PAGE TWO
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Photo #1 - Rhapidophyllum hystrix at the initial planting in 1992.

Photo #2 - The October 1997 blizzard the original Rhapidophyllum hystrix survived, low temperature -10F, snowfall 26", windspeed to 97mph, snowdrifts to 8 feet deep.
Left above, T takil and fortunei 'greensboro' during March 2006 blizzard. Below, the palms in June 2006, beginning active growth of 3-5 new fronds.
Right, T nainital takil after five years of growth. During winter subzero 0F cold, palms are covered with canvas tent during prolonged subzero 0F cold, and Christmas lights used below -6F . Below, the unprotected perennial "Greensboro" that refoliates every summer
Excellent sources for reliable cold tolerant palms:


Hypothesis- "If some palm species are successfully grown in USDA 5 and 6 climates, in locations such as Massachussetts, Michigan, Sweden, Norway and Bulgaria, then it is reasonable for those species to be tested in Colorado." Palms grew outdoors in Colorado until the Laramide Orogeny formed the Rockies. 35,000,000 years ago the climate became too severe, and W. filifera marched down the Colorado river to California.

The idea of growing a palm outdoors in Colorado Springs seems ridiculous. Through trial, error and reasonable scientific method, pioneers in other locations found species that could be successful. If palmito yucca are able to tolerate USDA Zone 5 conditions, then there must be true palms with similar tolerance.

Why Some Palms Succeed in Colorado USDA Zones 5 - 7

Several factors combined to allow successful test and growth of some palms in Colorado Springs.

First is temperature. December and January means are 40-45F, and 12-18F. Extreme lows of subzero 0F cold, while expected, are usually brief, lasting only a few hours each morning they occur. While record low temperatures prove the climate is USDA Zone 5, those extremes are rare and usually brief. Many winters are Zone 6 and Zone 7, with brief extreme lows a few degrees above or below 0F. Daytime temperature usually rebounds dramatically, and within a few days, highs well above freezing return. High temperatures of 50F and even 60F are expected on some winter days, every winter.  Daytime high temperature allows palm growth to continue through winter, and the average lows only appear to slow, not stop growth.

Second is aridity. Low humidity and sporadic snowfall keep volumetric soil moisture low, and induction of a drought mode appears to slightly alter palm fluid chemistry. The ratio of carbohydrate and fat to water is greater, and cell wall formation thickens, lowering temperature and duration requirements for lethal freeze to occur. That combination appears to allow for adaptation so palms in ground survive lower temperatures than elsewhere in wetter climates with similar or even higher temperatures, as well as temperature controlled in laboratory freeze tests.

The only real threat to palm survival is when actual plant tissues reach a temperature range of 0F to ~ -5F, or slightly colder. Lethal freeze occurs in fronds first. Measurements below show how quickly freeze was observed to occur at critical low temperature. When palms lose mass due to lethal freeze, they enter a decline mode that significantly shortens life expectancy, and  produces death from growth exhaustion in attempt to regain mass. Any means of protection to limit loss of mass shortens time and reduces energy palms need to achieve equilibrium and resume growth.
   
Other Palms
Other palms demonstrate an ability to survive subzero cold.

Trachycarpus fortunei - Fortune's Windmill Palm

T. fortunei is listed hardy to +10F in general publications. Research indicates cold tolerance in special conditions is lower. Lethal freezing of leaf tissue in drought mode occurs at ~0F, but  it requires several hours of subzero cold for lethal temperatures to reach and damage the deeply insulated apical meristem. In the arid, Colorado Front Range Zone5-6, because of daytime temperature that usually rebounds well above freezing (32F) fortunei can survive unprotected in air temperature to -6F if the temperature duration  is limited to a few hours, soil is dry, plant is stressed in drought mode and plant is acclimated with gradually colder, drier temperatures. Donov, reported 50 year old T. fortunei in Bulgaria that may survive air temperature to -20F or colder. Progeny of these palms were tested during the 2001-2002 winter, but they showed insignificantly greater tolerance to subzero temperature.

At 2 Lazy2P, during initial tests, T. fortunei lost spears during a mild winter (low of ~0) or suffered total defoliation during a hard winter (low of -10F) or a sudden drop to subzero cold without acclimatization. They regrew new leaves if the apical meristem was not killed. Specimens planted in full sun, with as much heat accumulation and recovery as possible suffered less damage than those protected from winter sun. For spear and bud damaged palms, a treatment of bordeaux in early May prevented rot, and by mid June, new emerging spears replaced the damage. New growth never made up the loss of mass, and lead to discovery of equilibrium and threshold. After a few seasons at best, the palms died, not from cold, but from exhaustion.

One T. fortunei form, 'Greensboro' was more cold tolerant than the species. The palm consistently regrew foliage, acting as a deciduous perennial. With a canvas tent cover used only during the few actual subzero 0F nights, defoliation was prevented, and since 2002, Fortunei 'greensboro' are in vigorous growth, increasing mass and resistance to cold by thickening of cell walls in new growth. They showed no leaf damage during the December 2005 cold, to -15F, tent temperature to -9F, and certain leaf temperature below 0F.

Trachycarpus takil (nainital) - Himalayan Windmill Palm

Palms sold as T. takil may not be true takil, but another Trachycarpus tentatively known as Trachycarpus nainital. They appear more cold resistant than most T. fortunei. There exists hybridization between takil (nainital) and fortunei, so pure strains are difficult to prove. During winter when average temperatures are below freezing and average low temperature ranges from 10F to 20F, pleats of the fronds fold up to prevent snow from accumulating on and breaking fronds, as well as trapping a layer of "warmer air" around the fans. Takil (nainital) fronds most cold resistant are deep, dark green when protected from winter sun, and turn almost black during subzero 0F cold. The trunk base usually has a "kink" common among mountain plants to anchor roots on slope that move from periodic frost heaving and gravity. Like 'Greensboro', takil regrew fronds after winter defoliation, and in 2002, protection by canvas tent method, only during actual subzero 0F periods, prevented defoliation, with takil (nainital) in vigorous growth since. Cell walls thickened to 3x thicker than before seen prior to the 2005-2006 winter, and the palms withstood air temperature to -9F inside the tent, without any lethal freeze during a -15F event in December 2005.

Trachycarpus wagnerianus - Wagner's Windmill Palm

Trachycarpus wagnerianus appear to be similar in performance to fortunei and takil. The smaller, stiffer leaves are more resistant to snow. Endurance of cold during the 2005-6 winter was impressive, though the small specimens lost spears. The palms appear to be in growth mode.

Trachycarpus nanus - Dwarf Windmill Palm

T. nanus may be the most cold tolerant Trachycarpus, simply because of its very low, almost trunkless apical meristem. Nanus was planted in 2002, and after 50% defoliation the first winter, has regained equilibrium and by 2005 was in vigorous growth mode.

Trachycarpus winter growth

One note about all Trachycarpus palms, is they continue growth through winter. Our December and January means of ~40F high, ~15F low do not appear to bother the palms, which typically grow one new frond and spear between Novermber and February.

Nannorhops ritchiana - Mazari Palm

The Mazari Palm, Nannorhops ritchiana has a reputation for significant cold tolerance. Early reports indicated Nannorhops could withstand extreme cold and snow. Nannorhops' legendary hardiness was not observed at 2 Lazy2P, but the trials were very small seedlings.
A larger Nannorhops was tested in the 2003-2004 winter, and suffered ~80% defoliation. This species will not be tried again.

Sabal minor (and varieties) - Minor Palmetto

The Little Palmetto is almost as hardy to subzero cold as the Florida needle palm, but takes more GDD heat to recover damage in the following growing season.    2 Lazy2P has several S. minor and S. minor 'McCurtain' that tolerate frozen soil, and do not exhibit winter bronzing typical of hystrix juveniles and takil. The 2 Lazy2P S. minor have endured low temperature to -16F and periods of extended subfreezing weather with minor visable harm. The lack of an emergent trunk, a parallel with Yucca angustifolia and baccata protects the plant from cold and wind, and is an adaptation from a colder prehistoric period in eastern North America. At   2 Lazy2P, S. minor planted in full sun suffer dessication in frozen soil, and up to 50% of the fronds will be cold burned. In partial shade, S. minor is less damaged, and where snow will bury the palm, it suffers little to no winter damage. One explanation is that unlike R. hystrix, S. minor has no protective acid that sunscreens fronds exposed to winter sun.

Sabal 'Birmingham'

A 3 gallon specimen was planted in summer 2003. It is alive as of 2006, but in decline from defoliation, even with canvas tent protection from subzero 0F cold. It appears to require more GDD warmth for recovery than this climate allows, even though it technically withstands the winter cold.


Chamaerops humilis - European Fan Palm

Chamaerops humilis is another cold tolerant palm, and due to a Western Garden Book reputation as probably the hardiest palm, was the first species tested. It is sensitive to moisture in the soil, especially during dry cold periods. The first palm was planted outdoors at 2 Lazy2 P in 1981. That palm was thatched with straw, a technique used with W. filifera in Japan. C. humilis barely survived one winter, low to -7F.  One weak leaf emerged the following summer, which we understand now as classic decline and loss of equilibrium, and the palm perished from a record cold -21F the following February. Two small C. humilis planted at Fountain Mesa succumbed to a combination of freeze and excess soil moisture. One feebly survived. however it defoliated after near 0F temperature. Humilis is not as resistant to cold as the species noted above and is not the hardiest palm.

Seronoa repens 'silver' - Saw Palmetto

The 23-26 November 2001 event of 64F to -3F caused 100% defoliation. Leaves are killed when ari temperature falls below 8F. C/H2O ratio was 0.111%, indicating a low ceiling cold tolerance of +2F air temperature for no more than an  hour or two.

Washingtonia filifera - California Fan Palm

The 23-26 November 2001 event of 64F to -3F caused 100% defoliation. Filifera leaves are killed when temperature falls below +12F. C/H2O ratio was 0.115%, indicating a low ceiling cold tolerance of +4F air temperature, if the cold does not last more than an hour or two. A filifera frond provided by Don Moorhead had a carb/water ratio of 0.112%, indicating a cold tolerance of +3F in a soil with relatively high volumetric moisture. That palm in a soil with low volumetric moisture could be the most cold tolerant filifera yet found.

Butia capitata - Pindo or Jelly Palm

Butia C/H2O ratio was 0.108% before it was 100% defoliated during the 23-26 November 2001 cold event. Leaves are killed in temperatures below +10F and the palm may not survive if air temperature drops below 0F for more than an hour to two.

Cordyline australis (Dracaena australis) Maori Dragon Tree

Specimens of this palm-like plant were observed to "almost" surive the 5b Colorado winter in tubs, for which they are sold as accent plants. In 1997 and 1998, specimens were planted in the ground at 2 Lazy2P, as garden accents. The roots appear to be hardy, and after a few years,  leaves adapt to freeze, cold and dry sun as well. One Dragontree has grown a stem and looks like a slender, soft yucca.

Palms and Carbohydrate Production & Storage

Two identified factors influence palm tolerance of subfreezing and subzero cold temperatures. First is production and storage of complex carbohydrates during a growing season. For cold tolerant palms of this study, peak efficiency occurs in the range from 60F to 75F. Temperatures above 87F + 3F initiate break down of cellular protein, reducing storage efficiency. Temperatures below 50F retard metabolism and reduces quantity of sugar production and storage. The more complex carbohydrates a palm stores during a growing season, the greater its resistance to subfreezing and subzero cold during winter, providing winter soil is dry. Wet or moist winter soil lowers the carbohydrate/water ratio so vascular damage from ice crystals occurs at higher temperatures. Dry winter soil, the second cold tolerance factor,  increases the carbohydrate/water ratio, so damaging vascular ice crystals do not form until a critical low temperature, well below 0F, is achieved. Thus, dry winter soil and a warm summer with moist soil combine to increase palm cold tolerance.

MODIFICATIONS

After several years of trial, it was determined that a screen from southern sun by conifers, allowing filtered sunlight, with occasional afternoon periods of full sun, provided the best microclimate for palm survival and growth.
Growing Degree Days

A formula for determination of the energy available to a plant during the frost-free growing season appears to have a correlation to recovery potential for cold tolerant palms stressed by either extreme low minimum temperature, or prolonged freeze.

The GDD formula is the daily sum of the high and low temperatures, minus 100, for each day during the frost-free growing season, from 1 April to 1 November. Or it can be calculated by subtracting 50 from the daily high and low temperature (commutative property makes the first method less cumbersome).

The following correlations between lethal freeze temperatures, volumetric soil moisture, and minimum GDD for recovery are determined by observation of plant damage at or below threshold temperature, followed by strength of the recovery during the frost-free growing season. The GDD estimate for recovery is determined by mean minimum GDD in locations where that species is known to survive.

Minimum temperature cold tolerance of selected palm species based on low volumetric soil moisture during temperature event. 0.070%

0.143% -14F (Rhapidophyllum hystrix) (lethal freeze of leaf begins at -10F at rate of 0.35-0.70cm/hr) (~2500 GDD required for recovery)
0.139% -10F (Sabal minor) (lethal freeze of leaf begins at -8F at rate of 0.35-0.70cm/hr) (~2500 GDD required for recovery)
0.133% -9F (Trachycarpus fortunei  'greensboro') (lethal freeze of leaf begins at -5Fat rate of 0.35-0.70cm/hr) (~3100 GDD required for recovery)
0.127% -9F (Trachycarpus takil) (lethal freeze of leaf begins at -6F at rate of 0.35-0.70cm/hr) (~3300 GDD required for recovery)
0.125% -6F (Trachycarpus fortunei) (lethal freeze of leaf begins at -3F at rate of 0.35-0.70cm/hr) (~3200 GDD required for recovery)
0.115% -2F (Washingtonia filifera) (lethal freeze of leaf begins at +5F at rate of 0.35-0.70cm/hr) (~5000 GDD required for recovery)
0.111% 0F (Seronoa repens) (lethal freeze of leaf begins at +8F at rate of 0.35-0.70cm/hr) (~4200 GDD required for recovery)
0.107% +4F (Butia capitata) (lethal freeze of tissue leaf at +10F at rate of 0.35-0.70cm/hr) (~4200 GDD required for recovery)
0.082% +25F (Howea fosteriana) (lethal freeze of leaf begins at +30F at rate of 0.35-0.70cm/hr)

Minimum temperature cold tolerance of selected palm species based on moderate volumetric soil moisture during temperature event. 0.180%

0.126% -10F (Rhapidophyllun hystrix) (lethal freeze of leaf begins at -8F at rate of 0.35-0.70cm/hr) (~2500 GDD required for recovery)
0.119% -8F (Sabal minor) (lethal freeze of leaf begins at -5F at rate of 0.35-0.70cm/hr) (~2500 GDD required for recovery)
0.115% -4F (Trachycarpus fortunei 'greensboro') (lethal freeze of leaf begins at -1F at rate of 0.35-0.70cm/hr) (~3100 GDD required for recovery)
0.108%  0F, (Trachycarpus fortunei and Trachycarpus takil) (lethal freeze of leaf begins at +3F at rate of 0.35-0.70cm/hr) (~3100-3300 GDD required for recovery)

Minimum temperature cold tolerance of selected palm species based on high volumetric soil moisture during temperature event 0.250%

0.122% -10F (Rhapidophyllum hystrix) (lethal freeze of leaf begins at -6F at rate of 0.35-0.70cm/hr) (~2500 GDD required for recovery)
0.117% -5F (Sabal minor) (lethal freeze of leaf begins at -2F at rate of 0.35-0.70cm/hr) (~2500 GDD required for recovery)
0.112% -3F (Trachycarpus fortunei 'greensboro') (lethal freeze of leaf begins at 0F at rate of 0.35-0.70cm/hr) (~3100 GDD required for recovery)
0.105%  0F (Trachycarpus fortunei and takil) (lethal freeze of leaf begins at +5F at rate of 0.35-0.70cm/hr) (~3400 GDD required for recovery)



Thanks to the research efforts of Gerry McKiness, David Francko, Kiril Donov, Allen Hirsh,
Tom McClendon, Joe Bunnewith, Mark Glicksman, Bob Snyder, Pete Sayre, Hayes Jackson,
Tim Behan, Lee Lindauer, Don Moorhead, Gary Hollar, Jeff Picazzio and other cold tolerance investigators, significant knots in the understanding of cold tolerance in palms have been untied.
Email questions or comments to 2 Lazy2P Gardens at Fitzroya@aol.com
For specific climate data for this garden,
Captions to the photo gallery above

Trachycarpus wagnerianus after the 2006-7 winter.

Trachycarpus takil under snow and subzero 0F cold in December 2005, upper right, same palms in tent during -15F event, lower left, Trachycarpus nanus and fortunei greensboro after the -15F event of December 2005, lower right, exposure to a -6F event in January 2006.

This group of palms withstood clear subzero 0F cold, multiple times, and showed several adaptations. First, cell walls had thickened to ~3x thicker than had been seen before. Palm color deepened to a blackish green in subzero 0F cold, but lethal freeze did not occur in the frond. Each of these palms also showed a marked SFH (Slope Frost Heave) trait, of a vigorously sideways creeping stem, believed to be a adaptation for fast anchorage to a slope were frost heaving could cause fatal root shearing.

Each of these palms also continued growth during the 2005-2006 winter, adding one new frond and one new spear between November 2005 and February 2006. January mean temperatures were 42F and 14F.


Periodic temperature in root system of  Rhapidophyllum hystrix

In June 2001, three temperature probes were buried in the root system of a large R. hystrix  (10" of "trunk"), planted in June 1996,  that shows little or no winter damage, grown in partial sun (4-5 hours daily). Probe A was 10cm deep, probe B was 50cm deep and probe C was 100cm deep.

Month   Probe A    Probe  B     Probe C
June      62F             59F             58F
July       70F             60F             58F
Aug      69F             60F             58F
Sept      67F             60F             58F
Oct       58F             56F              58F
Nov      40F             53F             58F
Dec       33F             49F             58F
Jan        32F             44F             58F
Feb       25F             36F             56F
Mar       29F            35F              54F
Apr       42F            51F              56F
May      56F            54F             57F

Volumetric soil moisture 0.042% - 0.115%,
C/H2O ratio 0.127% - 0.143%

Effect of prolonged freeze or short extreme cold on Trachycarpus fortunei and takil.

It is observed that for small specimens, loss of spear occurs before noticible damage to leaves. However with adequate GDD, the bud will replace the lost spear without stress that results in a multi-seasonal decline of the palm. For larger palms, damage to leaves occurs before damage to the spear, and loss of a spear, because the bud is higher in the trunk and more susceptible to cold damage, is a more serious matter.

Fan Palm Equilibrium Threshold
DECLINE

Palms have no true dormancy period. In the mature phase, every leaf lost must be replaced, to maintain a delicate equilibrium of mass. If they do not grow, they decline.

 If a palm is stressed by damage to fronds, loss of spear and/or defoliation by exposure to extreme threshold freeze (short-term subzero cold) or periodic threshold freeze (long-term subfreezing cold), then there is a minimum growing degree day accumulation required for the palm to recover 100% damage and resume growth. If that minimum is not achieved in one growing season, unable to fully replace damaged tissue, a palm expends stored energy to replace greater lost mass, not to grow, and begins a decline period ranging from one to multiple seasons, at the end of which the exhausted palm finally dies.

That explains why there is a notable absence of long term surviving palms in USDA Zones 5-7, even when it is demonstrated that palms can survive common expected winter cold in those regions. It also supports why palms lose fronds and achieve their distinct shape during the mature phase of their lives. Every leaf shed must be replaced by another leaf, to maintain balance.

Limiting damage from lethal freeze (or drought) reduces the number of GDD (see below) necessary to replace mass and resume growth. Perennial protection will extend the life of the palm, even if it is decline mode.

Volumetric Soil Moisture

Volumetric soil moisture is a factor in the severity of winter damage. A dry soil allows some fan palms to shed water through normal transipration so dissolved carbohydrates and fats in vascular and cellular fluid respond like antifreeze and lower the threshold temperature at which lethal freezing of tissue occurs.

However, once a lethal freeze period occurs, either by longevity at freeze or radical low temperature, long term survival of the palm appears dependant upon sufficient growing season length and warmth to replace loss and resume actual growth, and the formula for growing degree day accumulation appears to be a more useful tool for determination of energy requirement than simple growing season length.

GDD Growing Degree Days

The GDD threshold for a palm stressed to loss of spear, defoliation, or both is variable to species, like the duration or severity of freeze exposure. The following are GDD estimations required for full recovery and the resumption of growth. Insufficient GDD may result in the decline period that leads to death of the palm.

The GDD formula used is the 24 hour period high-low temperature sum, less 100, for each day of the frost-free growing season, from 1 April to 1 November. (50 base for each temperature)

Rhapidophyllum hystrix ~2800 GDD
Trachycarpus fortunei 'greensboro' ~3200 GDD
Trachycarpus takil ~3200 GDD
Trachycarpus fortunei 'bulgaria' ~3200 GDD
Trachycarpus fortunei ~3200 GDD
Sabal minor ~3600 GDD
Sabal birminghamn ~4500 GDD
Chamaerops humilis ~4500 GDD
Seronoa repens ~4600 GDD
Butia capitata ~4700 GDD
Washingtonia filifera ~5600 GDD

Adjusted to accomodate decline damage recorded during 2003-2004 winter, with six multiple-hour subzero F periods, two to -10F or colder

The following website has a wonderful photo essay noting the decline mode and growth mode of palms in Georgia, species that we cannot test long term.  These observations support our findings.

 

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