Cold Hardiness and Palms- what does it mean and what can be done about it?
Palms are mostly a tropical species. However, thankfully, many palms can grow in temperate and subtropical climates, providing a wonderful ‘touch of the tropics' to less than tropical climates throughout the world. This article is not about what these palms are (that article was already written a few months ago), but more about cold hardiness itself and what related complexities that pertain to marginal palm growing than one might not guess at just thumbing through a list of ‘cold hardy' palms.
All along coast of California one sees palms in the landscapes nearly every where one goes. And yet California is not a tropical climate, but a combination of subtropical and temperate zones. There are literally hundreds of species of palms that grow in California, though admittedly many are considered quite marginal and only survive happily in the very best microclimates in the warmest growing zones. But some palms can be grown inland where the weather is harsher. And some palms are hardy enough to survive farther up the northwest coast. Many species are cold hardy enough to grow all over the Southeastern US and some even can survive where it snows regularly. Some species do well in Britain, many in the Mediterranean and dozens upon dozens do well in island environments all over the world that one would certainly not consider tropical. For those ‘lucky' enough to live in Hawaii, northern Australia or in any other zone within the tropics, this article is not for you.
Trachycarpus in Canada (photo Growin)
What does Cold hardy mean?
The term cold hardy is not an unfamiliar one to most people growing plants. For palms, any palm that can grow outside a tropical environment is considered to be at least somewhat cold hardy. Obviously any palm that can grow in a temperate climate is very cold hardy (there are very few of these unfortunately). But there is more to this designation than just a label and these more subtle distinctions are relevant to those who live in marginal climates and want to grow palms.
Often when one is considering a certain species of plant for a certain climate, one normally does some research in order to learn if that plant can survive the low extremes of their environment. In other words, if you were to decide you wanted to plant a Bismarckia palm in your garden or park, and you lived in Washington state, you would (I hope) find that the temperature extremes of your climate would not be conducive to growing this species in your zone. However you still would have other choices of palms to try as long as you didn't live inland where the lows were extreme. But if, for example, you to live in southern California which is considered by some a subtropical climate, a Bismarckia might or might not be a good choice depending on your microclimate and a variety of other factors. You should probably know a bit more than just your zone- you need to learn what other sorts of things affect your palm's cold hardiness such as how environment, age, planting time etc. affect your palm's potential survival before you just run out to Home Depot and buy one (which, thankfully, you really could do, since Home Depot lets you return anything as long as you have a receipt).
Bismarckia from Home Depot ready to plant; but in wrong climate it can just up an die on you
Lets say you want to do a landscaping project with palms. Before investing a lot on $$ in certain palm species, you should research the lows in your location back over the last 50 years. Of course, with current weather situations being a bit in flux thanks to global warming, there are no guarantees. Rare cold snaps can wipe out entire collections of palms. However, statistically speaking, you are likely not to experience severe fluctuations more than 1-2 x in a palm's life, and most catastrophic changes may not happen but once every 50-100 years.
Once the absolute low has been learned, you should be able to go to a source (eg : Cold Hardy Palms by Betrock, or Palms for Southern California by G Stein) and learn what palms can survive above your low. You should understand, however, that there is no such thing as an absolute value when it comes to the lowest temperature a palm can survive. There are far too many other variables to consider, many which are too nebulous to accurately calculate at all. This is why some palm growers consider growing palms in marginal climates an art. But if the temperature listed for a palm's low is at least close, then you will at least have a chance it will survive in the right location in the yard. The following are some of the other factors to consider.
The factors that affect a palm's ability to survive or succumb to certain temperature extremes include genetics (and individual variation), anatomy/size and age of the palm, how long palm has been in the ground, temperature variations before and after a potential cold snap, type of freeze or frosts likely to be encountered, wind exposure, sun exposure, rainfall, humidity, soil porosity, presence or absence of mulch, and other environmentally protecting factors such as presence or absence of canopy (taller trees, patio covering, shade cloth etc.), proximity to buildings, wall etc., slope (grown on hillside, mountain top, valley, etc). Each of these will be discussed below but in a far from technical format. For technical details and explanation of the effects of environment on weather, one should look elsewhere. I am only a palm grower, not a meteorologist, physicist or master gardener.
Anatomy and physiology:
Age and size of a palm will affect its ability to withstand frost for obvious reasons. With few exceptions, the older and larger a palm is, the more cold hardy it becomes. Unfortunately a lot of data on palm hardiness, particularly for rarer species, has been based on young plants, mostly because there has been no experience with mature ones yet (and some species just don't seem to live long enough to make it to the adult size in marginal climates). So some of the data one may encounter may be pessimistically skewed. In general, I suggest getting the largest palm you can, realizing that sometimes these larger palms got that way by being coddled in a greenhouse for a long time and will take some time to ‘get used' the real climate. For a few species, buying big is not such a good idea as these palms are just too weak to survive in the outside world (King palms and Majesty palms would be two good examples- plant them smaller... they will get big soon enough).
King palms at different ages- the smaller the plant, the more likely to be killed off by a freak cold snap- palms in last photo are probably safe now (first photo by Jdiaz)
One conceivable exception to this rule is bud placement. The bud is the growing meristem of the palm, and some palms have this safely and securely tucked away below ground level where freezes are less of a concern. Attaleas are such palms and their cold hardiness may actually decrease once they mature beyond the point their bud is protected by the ground. Fortunately for those growing this genus in marginal climates, it takes dozens of years for the bud to make it to the surface in such climates, so Attaleas may still make good landscaping choices for 15 or 20 years. And it is still unknown how sensitive this meristem is to frost as these palms have rarely been tested in such climates.
Attalea seedling in my yard surviving, but bud is well below ground; This plant in Hawaii has an exposed bud now, and if in my yard, might keel over from cold
Positioning of the leaves also protects/predisposes parts of the palm to damage. Palms with large, flat palmate leaves horizontal to the ground are much more likely to get damaged during a frost over palms with mostly upright or drooping leaves. Frost settles on a flattened surface much more easily than on a sloping one (imagine frost like pebbles- fall/slide off a sloping surface or sitting on a flat one). Palms that have large, unsplit leaves are more likely to suffer damage than those with finely split ones- again the frost falls through the slats/spaces between the leaves. How does information help? Perhaps, knowing a freeze is on its way, one could wrap up the leaves so they were all pointing upright... wrapping my also itself protect the leaves some.
a cold snap in my yard damaged this Sabal yapa and this Trachycaprus martianus, but the large, flatter leaves got the most damaged, with the newer upright leaves surviving for the most part
Trimming a palm before a frost can predispose it to more serious damage- leaves tend to protect each other, and a lot less leaves mean less protection. Even if a frost has come along and killed some leaves- until further frost danger is no longer an issue, it is wise not to remove these dead leaves as they can be protective of the entire plant.
Then, finally, of course, genetics plays the biggest role, with some palms innately able to handle temperatures down to -10F while others succumb to temps in the 50s. Why some palms are able to survive climates far harsher than their origins would suggest is not totally understood. But many anatomical and physiologic adaptations that allow a palm to survive certain climate extremes other that cold sometimes also influence their ability to deal with cold, too. Palms that live in arid but tropical climates sometimes have a natural ability to handle cold temperatures thanks to their evolutionary adaptations to deal with drought, winds, flooding, etc. How to predict which palms will survive in a cold climate is not that easy however- trial and error is mostly how it's done.
Three colors of Bismarckia nobilis: blue, silver and green... the green form struggles and often dies in California while the other 2 are much more cold hardy
Either way, once a palm's general cold tolerance has been learned, there is little one can do about this genetic factor, though many keep trying to breed for the hardiest individuals. Archontophoenix Illawara is supposedly a hardier version of Archontophoenix cunninghamiana (King Palm) and some growers will selectively grow this variety of King Palm. Actual data confirming this is lacking though many growers will swear by this form of King Palm. And then there are those overly optimistic growers that are growing Cyrtostachys by the thousands in a cool climate hoping that one or two will survive, and then hope to propagate those few individuals to produce a cold hardy Sealing Wax Palm. Good luck. Still sometimes there is some significant variation within a species- for example Bismarckia comes in green, blue, silver, grey etc. The green form is far less cold hardy, by nearly 8 degrees, than the other forms. The difference is so great one must wonder if they are truly the same species.
Types of frost:
Depending on what area of the world one lives in, one is likely to see either radiational frosts or advective frosts more often. Here in California radiational frosts are more common. This is generally when there is no wind or cloud cover and the warm air escapes into the atmosphere, being replaced by colder air. Daytime temperatures with these frosts are generally above freezing. The damage from one of these frosts depends on duration, absolute low reached and somewhat on the humidity. Very dry air is less likely to allow ice formation on leaves and less damage may occur as long as the frost is relatively short. If it's humid, frost can sometimes form when temps are above freezing (35F) and some palms will get damaged leaves even though it may not actually drop to freezing. Radiational frosts vary a lot with microclimate and surrounding heat sources. For example, lakes, large buildings, highways etc. absorb and distribute a lot of heat, and palms planted near these structures will be less affected by a radiational frost.
My Archontophoenix myolensis colony before severe radiational frost. A year later one of three struggling to survive (closest to the brick wall- a heat source)
Advective frosts are more commonly seen in east coast climates. These frosts are associated with large weather systems (cold fronts) and often are associated with rain, snow, high winds etc. Some people call this sort of frost ‘wind frost'. Microclimate is less of a factor with these sorts of frosts, but still, living near large bodies of water can help (a common situation in Florida). Daytime temperatures for most advective frosts can be below or far below freezing. For these frosts, latitude is far more important that microclimate: the farther south (or north if living below the equator) away from the origin of the storm system the better, as less and less cold will be trapped by the normally warm air and there will be less damage. These frosts tend to be more dramatic and catastrophic and therefore damaging a lot more palms.
Other Weather and Environmental Influences
Wind is usually a protection against frost. It takes a much lower temperature to allow frost to form or damage the leaves- in southern California frost is very rare on windy nights. So a palm that normally would get damage at 29F may survive a few more degrees of cold if the air is moving. This is why citrus growers have giant fans throughout their fields. However, wind itself can do damage to palms, and if it's both cold and windy, some species may suffer desiccation damage to the leaves. The best situation for a cold sensitive palm on a very cold night is for there to be wind, but the palm to be planted in a wind-protected location. This may allow a cold sensitive palm to survive a much lower temperature than it might normally be able to.
On the other hand wind has its down sides, particularly if it is associated with an advective frost (which is typical for such a frost). These winds occur below freezing and wind chill can further lower the temperature, doing severe damage to senstive foliage. Additionally wind itself is dessicating and can literally suck the life out of healthy, water-filled leaves of sensitive species leaving them either dead or far more susceptible to damage by cold. Wind associated with hot weather (like California Santa Anas), or onshore salty winds are also severely damaging, but that has nothing to do with cold.
Wind did not help this poor Syagrus coronata- in fact all parts exposed to wind above the wall died
Sun exposure also has an effect, even though most frosts in marginal climates happen overnight. A palm in full sun will often warm up faster the following day and the overall frost damage may not be as severe. Unfortunately full sun exposure usually goes along with lack of surrounding protection (canopy, large building, shade cloth, etc.) which may negate the benefits of being in full sun in the day. These unprotected palms are much more likely to be damaged from a radiational frost.
Rain and low temperatures are a rare combination in southern California, but Florida marginal climates often see this combination during one of their advective frosts, and the combination can be both damaging and protective. Rain and low temps allow for ice formation, which will damage a lot of sensitive foliage. However, ice formation can also be protective, forming at about 32F-34F (depending on wind chill) and protect the leaves from temps that may continue to drop into the low 20s. This is why many palm growers in these marginal climates will water their palms all night during one of these high-humidity advective frosts.
Soil can also affect a palm's hardiness, particularly if its well mulched and porous, or not mulched at all. Mulching soil is a very good practice for increasing the nutritional value of the soil, keep it well draining (few palm roots like to sit in choking mud) and protect the roots from solar radiation and from drying out. Healthy, vigorous palms seem to have an increased resistance to being killed by frosts, than to sickly, weakened ones. However, mulch does have a down side. Radiational frosts are affected by the environment, and the earth has a set temperature in the high 50s, even if It's freezing cold above it. This heat radiates up during times of cold and can help to protect low-growing palms and seedlings during radiational frosts. So in marginal climate zones it might be a good idea to scrape away the mulch below a palm, particularly before predicted frosts, and allow the earth's heat to escape and protect the plant. Plumeria and citrus growers use this logic to good effect and many recommend NO mulch around plants in winter.
Microclimate is extremely important when it comes to palms being able to survive temperature fluctuations, not just for being able to survive excess cold, but excess heat, wind, sun etc. In a climate where radiational freezes are the dominant cold exposure, microclimate can make the difference in being able to grow 50 species of palms versus 400 species of palms. Many growers in certain towns may have microclimates that allow them to grow species no one else is able to grow depsite them all living in the same climate zones. What factors affect microclimate?
Canopy is probably the single most important factor in a palm species' ability to survive certain climate zones. A recorded temp of 25F may reflect what the actual unprotected air temperature is, but temperatures under canopies may be 5-10 degrees higher. Some of the oldest and wisest palm growers suggest that growing a canopy for 5-10 years FIRST should be a priority before planting any marginal palms (in reality, most growers simply are not that patient, and consequently lose a lot of plants they may not have otherwise). Even a thin layer of shade cloth can trap a significant amount of radiating heat from the earth and keep frost from forming on the fronds. There is always an active discussion on the palm sites about what are the best canopy trees. This is definitely an area one should research if one is really interested in growing marginal palms.
Two Trachycarpus latisectus after freeze- one out in the open fried badly, while other against a well and under a Brugmansia did fine
Surrounding Heat sources:
Similarly palms grown near or against buildings, walls, or even other foliage, as well as ponds or lakes, will often stay a lot warmer thanks to radiated heat from those heat-absorbing bodies during a frost. And for freezing winds, structures and other foliage can protect against wind chill. Palms that are very marginal and love sun should be planted ideally against a large warm structure facing southwest to keep the warmest as long as possible until dark comes. I have grown several species of palms that I am sure would have otherwise succumbed in such locations.
Famous Coconut growing in Newport Beach, California against a west-facing wall in perfect sandy soil and wind protected- an extremely rare survivor in California (photo by PanamaJack)
Slope and Elevation:
Hills and valleys are very different than flat surfaces when it comes to very cold weather. High elevations can have a protective advantage as long as they are not so high that they push the zone down a notch. I am not talking about mountains versus deserts, but just hills versus valleys. Cold air flows off hillsides, so palms planted near the tops of hills will often have much less cold damage during a frost than those planted near the bottom, or in a valley, where cold air tends to collect and sit for long periods. Hills also get much more sunlight and more ground warmth from being heated all day long, further protecting plants on its surface. Hillsides may experience more wind, too, which can be protective, or damaging depending on the circumstances. I have seen several magnificent palm collections in marginal climates that show little or no damage year after year thanks to being grown on steep slopes or hills, while surrounding gardens suffer from damaging frosts yearly.
normally a very tropical palm (Copernicia macroglossa) seen on the left in Thailand, that most Californias fail to eke even through a single winter, is surviving on this hilltop property in Vista in photo on right, in a perfect microclimate for it.
The reason it is not recommended to plant palms right before winter is their roots don't tend to move much when it's cold, so stay in a pot-shaped ball the entire winter, making them much more likely to dessicate during times of drought. For example, here in California Santa Ana winds will dessicate the leaves of unacclimated or newly planted palms in the winter since the roots are not able to absorb water from surrounding soils yet. And these compact rootballs are more likely to suffer from lack of oxygen during times of heavy rainfall. Water seeps down to the rootball and fills in the hole, often taking too long to leach into the surrounding soils, particularly here in California where high clay content is common in the soils. Both situations make for a weakened palm, much more likely to suffer or die during a cold snap.
Fertilization is not quite as detrimental in terms of frost damage as it might be in trees and shrubs. Fertilizing shrubs/trees late in the season often promotes new tender tissue growth which is far less resistant to freezes than hardened, woody plant material. But palms don't grow like this and some light fertilization near the end of the season can keep them healthier over the winter. Obviously fertilization mid winter is not going do much as very little growth is going on then with marginal species (very cold tolerant ones may still benefit). Some feel one should increase the Potassium in the fertilization formula in the late summer to fall as potassium is associated with a plants ability to handle some freezing temperatures. Whether this actually is true in palm culture is not known, but fertilization with increased potassium does seem to improve Plumeria's ability to withstand frost, so that may be something to consider as winter approaches.
This is not by any means the last topic to consider when trying to decide your palm's chances for survival in your yard and how you can affect them. There are many other forms of hardiness to consider including heat, wind, drought, poor soil, and salt tolerance, as well as some things called ‘cool hardiness' and ‘humidity hardiness'. Those will be discussed in another article. But this should give the would be palm grower some things to consider when making palm growing and placement decisions.