What is cold hardy?

This winter has been a doozie thus far, with record low temperatures being set throughout the USA. With the cold experienced there will be lots of dead plants in gardens at the end of the season but also some surprising survivors. However once the numbers are tallied the disappointments will likely outnumber joyful discoveries leading to frustration. The frustration and sorrow will lead some to ask, what is a cold hardy plant? The answer of course is dependent on many factors. In a broad sense, most would say that a plant is hardy for a given area if it survives the defined temperature range for the given area. This seems straightforward enough but it may be a bit too simplistic and not completely accurate.

The US government and the Arnold Arboretum separately began developing maps that were intended to delineate average temperature ranges throughout the USA. These maps would help farmers make decisions on what they could possibly grow in a more standardized fashion.

Seemingly prior to the Great Depression, the determination of cold hardiness or what was hardy in a given area was based on local availability, family and cultural tradition. All information tended to be local. At that time the US government and the Arnold Arboretum separately began developing maps that were intended to delineate average temperature ranges throughout the USA. These maps would help farmers make decisions on what they could possibly grow in a more standardized fashion. Eventually the USDA hardiness zone map became more popular and is what most folks use to communicate plant hardiness from place to place. Based on observation and data collected over time, plants gain zone ratings based on their ability to survive the average cold of a given zone. So if a plant can generally survive a given temperature, say down to 10 Degrees F, but dies when exposed to colder temperatures, then it is noted as hardy to zone 8 and is unlikely to survive a colder zone. Sounds simple and straightforward right? Well, not so fast. There are many factors, other than simple temperature lows, that influence whether a plant can survive a winter in your area:

  • - Wet or dry winters;
  • - Cloudy or sunny conditions during a cold spell;
  • - Presence of snow or ice;
  • - if the plant is subject to strong wind or is sheltered;
  • - if it the plant is in a pot or in the ground; 
  • - how long the plant has been in the ground;
  • - and a multitude of conditions and factors that we mortals can barely understand and those that we cannot yet conceive.
 

There is no set of hard and fast rules or a governing body that determines a hardiness designation for a given plant. A hardiness designation for the most part is an accumulation of known data and can vary as time proceeds, so should only be used as a general guideline, not a static immovable number.

Over the years of being in this business I have received feedback from customers or have followed plant forums and periodically hear the lamentations of many that say that they had a supposedly hardy plant for their zone die in conditions that the they would describe as average or sometimes less than average cold. This is a very common thread among Agave growers. One species that I often see such frustration over is a plant from the Agave parryi complex. Many of the varieties of this species are rated cold hardy to zone 6 and 5b, yet folks in zone 7 and 8 will often say that their plants were damaged or killed from what seems to be no more than average cold. Puzzlement, frustration and disappointment are often displayed as they feel cheated or fooled. That somehow they were sold a bill of goods and were lied too. It is easy to feel this way when you follow a strict definition of what should be "cold hardy" based on air temperature. But the problem is much more complex and nuanced.

Several years back North Texas had a cold spell come through and many "hardy" Agaves, including those from the parryi complex, were damaged. That year there was an initial cold spell in late October with frosty conditions followed by a warm mild period of several weeks in which the highs were in the mid 70’s and 80’s. Then, a Blue Norther rolled in during January and temperatures went from the 80’s down into the 20’s in a matter of hours. This cold snap also had an extra sting: ice. I received a flurry of emails after the event asking what the heck happened. "Agave parryi is hardy to zone 5 right? We barely had the mid 20’s. Why were my plants damaged? Did I have the right plant or what?” I was perplexed at first but after evaluating the conditions and discussing the phenomenon with others the conclusion was that the mild weather prior to the hard cold snap encouraged the plants to come out of their initial dormant state and a slight growth spurt was triggered. This actively growing tissue was no longer “hardened off” or in a dormant stable state and was thus susceptible to the sudden drop in temperatures and was damaged by the seemingly mild cold. The accompanying ice also inflicted extra damage because ice can inhibit the free exchange of oxygen, carbon dioxide and others gasses, causing a sort of suffocation effect. Ice conducts cold temperatures very efficiently and is not insulating like snow can be, therefore exacerbating the cold’s chill. Ice also acts as a magnifying lens and can temporarily magnify sunlight. On a sunny day after an ice storm, before the ice melts away, sunlight can heat up frozen tissue beneath the ice too rapidly causing cell walls to rupture. If the thaw occurs more gradually, plant tissues have a chance to get back to equilibrium without rupturing and damaging cells. So if you have an ice storm hope the warm up following the event is cloudy.

Unstable temperature patterns are a persistent vexation to Southern gardeners and one of the many challenges we face. Our winters are or can be mild for periods of time and then harsh in a flash. This instability of climate is hard on plants and causes great frustration for gardens. This fluctuation in conditions is especially hard on leaf and stem succulents. The funny or ironic thing is that plants hardy to stable cold, zone 7b can get severely damaged in area of zone 8, just because of this instability in cold. I often wrestle with this phenomenon when I discuss plant hardiness results with Tony Avent of Plant Delights Nursery, in Raleigh, NC. There are some Agaves (some clones of Agave xylonacantha for example) which were seemingly hardy for Tony in North Carolina yet die or are severely damaged in milder cold conditions here in zone 8b. I am still not completely sure of what is happening but it may be that the winters in North Carolina come on more gradually and stay relatively consistent through the winter, allowing the plants there to acclimate more efficiently and remain dormant effectively. The same plant further south is subject to the more unstable temperatures patterns and finds it hard to remain in a consistent state of dormancy.

This is not only a problem for leaf succulents but can be problematic for other plants as well. Tony also reports that Lapierousia laxa and Oxalis boweii are great, hardy plants for him. Yet here in Southeast Texas the same bulbs are periodically blasted back and severely damaged by hard frosts. The difference from what I can surmise is that these bulbs go into complete dormancy in NC, only initiating growth after conditions become suitable in the spring. Here in Texas the weather is generally mild enough that these plants begin growing in the fall and want to grow all winter. The problem is that periodically we get blasts of very cold air that are too cold for the actively growing plants to effectively tolerate. It is a frustration indeed to have a happy, lushly growing plant, most of the winter, get smacked to ugliness in one or two severely cold nights. It is enough to make you want to cry.

This particular scenario is common in the southeast, yet in other parts of the world other variations in conditions have their manipulating and destabilizing effects. The Northern USA gets terribly cold, at least by my standards, but areas that receive lots of snow are often able to grow a wider set of bulbs and temperate plants compared to those that receive minimal snow. Because snow tends to act as an insulating blanket, areas with lots of snow are often able to grow many plants from slightly warmer zones. The snow insulates the ground, reducing the effects of the air temperature above the snow, keeping the soil from freezing as severely as areas that are snow free.

Beyond Pure Temperatures

Other than climate and temperature extremes, how well a plant is established in the landscape can have a major effect on its hardiness. I touched on this above of how actively growing plants are more tender than fully dormant plants but how well established a plant is before the onset of cold plays a part as well. I always stress, when discussing planting times for leaf succulents and marginal subtropical plants, to plant them as early in the season as possible to give them time to acclimate before the onset of winter. Established plant tends to tolerate cold conditions better that a plant planted late in the season. The early planted plant has time to use up excess nutrients and to adjust its metabolism for the seasons ahead. Leaf succulents, palms and other subtropical plants are generally more prone to being tricked into out of season growth spurts when they should be shifting into dormant mode. To a plant that has been cooped up in a pot all summer the somewhat ideal conditions of fall (in the south our falls often mimicking spring like conditions) are just too inciting or tempting and they begin growing at a time they should not.

Leaf succulents, palms and other subtropical plants are generally more prone to being tricked into out of season growth spurts when they should be shifting into dormant mode.

My favorite example of this is with Dyckia x ‘Soda Straws’ when several years back I planted one in the garden for trial. It survived its first winter unharmed. The next fall I thought that if one was good then a grouping would be even better, so I planted several more plants of the same clone only feet away. That winter offered up similar temperatures as the year before but the late planted plants suffered the cold severely while the original plant had no damage whatsoever. Since that year the original plant has survived even harsher cold, so I know the clone has hardiness. The difference in how the plants reacted to the cold depended on their state of metabolism prior to the onset of cold. I have seen this occur over and over again through the years with different species, so I know it is a valid theory.

This same “lack of establishment” or incomplete dormancy can be induced by conditions other than planting time. Time of fertilization can have a similar effect, even if plants that have been in the ground for multiple years. We always recommend fertilizing leaf and stem succulents early in the growing season avoiding late season fertilization, so the plants have time to get any growth spurts out of the way and harden off prior to the onset of winter.

Timing of irrigation or rains can also affect how plants react to winter cold. Especially over the last several years Texas has experienced drought year after drought year. During these droughty years many plants go into states of semi-dormancy, induced by the harsh conditions of heat and lack of moisture. They just sit hoping for rain and better times. Unfortunately as it often plays out, beneficial rains and better conditions often come late in the growing season, triggering growth that is vulnerable to frost damage when they need to be going into dormancy. It seems so cruel to go for month after month of brutal hot dry summer only to get abundant rain at a time of year when plants want and need to go to sleep. Instead of a case of “too little, too late” it becomes “too much, too late” and is often more dangerous than beneficial. In these years you have to take steps to protect some plants, at least during the first few cold spells, to get them better acclimated for the rest of the winter.

Size Matters

Some plants grown in marginal climates are only hardy with size and maturity. Palms from the genera Brahea and Sabal lend themselves to this. The seedlings of Brahea bella or B. armata often can barely take frost, yet when protected as young plants for several winter seasons and given time to gain some size and become fully established, can take remarkably cold temperatures. However this is only going to work with some plants, a huge coconut palm is not going to be hardy no matter how big and established it is. A recent example of size and maturity mattering for me has played out here over the past several years with the cactus Neobuxbaumia polylophus. Three years ago I set out a group of young seedlings. By the time the winter arrived the plants were about 6” tall. I neglected to protect them and they froze in the low 20’s, a temperature that mature plants of this species can easily tolerate! The same year an established 4’ tall specimen, already in the landscape, was unharmed by the cold. The following spring I planted more seedlings, determined to protect them, which I did and got them through the following winter. This year the larger plants, (now 2’ tall) were not protected and they survived the upper teens undamaged. I had a different set of one year old plants (that were only 8” tall), which I neglected to protect and they froze dead. So size does matter. Whether it is the mass of the plant that has the positive effect or there is a beneficial chemical compound present that is only efficiently produced with age and size I do not know but something is at work allowing survival.

Microclimates

Selecting a good location, or microclimate, to place a planting in is a technique that is often employed by gardeners wanting to push the limit of a given climate. The temperature reading at one location in a yard does not necessarily mean the temperature is ubiquitous throughout the area. Temperatures in a yard or region can vary significantly depending on the lay of the land; the proximity to buildings, bodies of water to other plants and other factors. Probably the most well known example of microclimate planting is the placement of plants on the south side of a walls or structures in order to protect them from the brunt of north winds. Another example includes planting on slopes or near the top of hills. This can be beneficial over planting at the base of hills or in creek bottoms because cold air settles and pools in low lying areas causing a temperature differential between upland and low lying areas. There may be only a few degrees difference between the slope of a hill and a low lying area below but it can make an enormous difference for the survival of some plants. Downtowns and busy central city zones offer warmer temperatures compared to the surrounding countryside because of all the vehicle traffic and heat escaping from structures. This heat island effect allows folks in cities to grow plants that can often not be gown in the areas just a few miles beyond. Sometimes just the canopy of a grouping of trees can reduces the effects of radiational cooling preventing frost from developing beneath the tree’s canopy thus minimizing damage from light frosts. These are just a few examples microclimate variation but there are many others. The point is that the location of a planting or the micro-climate influences a plant’s cold hardiness or winter survival.

Duration of cold

The damaging effects from a cold snap that lasts just a few hours can be significantly less than the damage that occurs from prolonged cold exposure. As an example, many of the Dyckia species and hybrids that we grow can take overnight low temperatures into the middle and low teens undamaged. However fewer can tolerate the temperatures in the teens for days at a time. This illustrates that a simple temperature reading is just part of a much more complex story. It often takes years and years of trial and experimentation to determine if a plant is truly “hardy”. Surviving on year or one cold event does not a hardy plant make; the opposite of course is also true, a plant dying one year at a given temperature does not mean it lacks hardiness completely.

Climate as a whole

England and parts of the Pacific Northwest seldom get hot or at least not to the degree that we do here in Texas. Their winters are not that severe but they often have difficulties keeping subtropical plants alive during the winter. The winter low temperatures in Texas and in the Pacific Northwest are on average fairly similar but subtropical plants that are hardy here are often not hardy there. The difference is that some subtropical plants need periods of heat in the summer to help create the necessary sugars and other compounds in their cells preventing them from freezing. Many years back we used to ship plants to England and Europe. A succulent collector in England attempted to grow Agave americana ssp. protoamericana but without success. We inquired about the conditions and low temperatures that the plant was subject to, trying to figure out what was going on. On the surface the temperatures and conditions experienced did not seem out of line with what the plant was known to tolerate elsewhere. During this same time period we were also exchanging information with Tony Avent in North Carolina about plant hardiness and knew that this species was very cold tolerant, surviving far colder temperatures in North Carolina than what the English fellow was reporting. For a while we were perplexed but started comparing the two climates. The main difference we felt was that England’s summers are pretty mild and pleasant for the most part and real heat is pretty uncommon. Yet North Carolina can have streaches of pretty steamy heat (though mild in comparison the Texas). Though there are most likely other influencing differences contributing to the hardiness differential but we felt that the lack summer heat in his part of England was the major factor.

Another example, but from the opposite direction, plays out with our experience with Agave montana. True Agave montana grows in the states of Tamaulipas and Nuevo Leon, Mexico at an elevation range of between 9000’ and 12,000’. The conditions at this elevation tend to be very mild and even (temperate in nature) or at least no sudden or drastic changes. It seldom becomes exceeding hot and doubtfully never exceedingly cold. Even if warm or hot periods do occur the nights always cool off (falling into the 50’s or lower often through the summer). Rainfall mainly occurs in the summer with dry winters. Though rainfall is not exceedingly abundant the area is often bathed in clouds and fog thus helping to “temper” the climate even further. Though the area experiences a level of humidity it is a cool humidity, not steamy. In contrast, the climate in Central Texas at elevations of 600' to 800' is defined by triple digit high temperatures for months at a time. During the worst of summer the nighttime lows seldom drop below the mid 70s and sometimes barely manage to break below the mid 80s. Outside of a consistently hot summer the weather during the rest of the year in Texas tends to fluctuate dramatically. Fall and winter temperatures can fluctuate rapidly between warm -mild and cold-crazy with no discernible predictable pattern. Precipitation year round is inconsistent and erratic and can be non-existent for long periods and then overabundant to the point of flooding. The steamy heat and humidity, well they are legendary!

Agave montana, being from the very mild climate described above, has proven very difficult to cultivate in Central Texas. I know, we have tried and failed with it many times and I see similar feedback on a regular basis about this plant. It seems to languish when it is hot and steamy, struggling to grow often dying before it makes a complete summer. If it does survive a summer it tends to be sensitive to relatively mild cold, even though it is subject to freezing temperatures in its native habitat. One theory for this lack of cold tolerance is that in hot night climates, like Central Texas has, the hot, steamy conditions disallow proper metabolic rest. Any Southerner that has tried to get a good night’s sleep, during a sultry summer night without air conditioning, can testify to how hard it is. This lack of sleep, so to speak, leaves a plant stressed and exhausted. In this exhausted state the plant’s metabolic functions become distorted and the plant is notable to create all the compounds needed to adjust to seasonal changes. They then suffer from cold or freeze at temperatures, which in a healthy state they are able to tolerate. This is just one theory and there are likely many others. Whatever the actual scientific reasons are, growing plants from gentle, high elevation climates, successfully; long-term in Central Texas is a real challenge. Aloe polyphylla, Nolina sp La Siberica (a.k.a. Nolina hibernica), Agave havardiana and other high elevation plants offer suffer similarly.

The above are just a few examples of how conditions influence the hardiness of plants. Simple temperature readings tell only part of the story and a zone designation on a map should only be a starting point and not be taken as a hard and fast rule. Having gardened now for a number years I have found plants listed as hardy to zone 10 easily tolerate zone 8 and I have also grown plants listed as hardy to zone 6 not survive a zone 8b winter.

Conclusion

As gardeners and plant collectors we are often pushing the limits and boundaries of what is or has been grown in our area. We become fascinated with plants we saw in past travels. We fall for a particular group of plants that then become an obsession and are curious to learn more. Some of us are just contrarian by nature and want to grow things because others haven’t or say can’t be grown in a particular area. We often just want to expand our world or create a haven apart from the harsh realities of our world. This just seems to be human nature. We just can’t be satisfied with what is present, we want more. There is not necessarily any harm in trying to push the boundaries but we have to remember that when we grow certain plants or push certain natural limitations there will often be pushback from nature. Maybe there is a reason there are no leaf hardy agaves currently native to eastern and southeastern USA. There is a native agave relative (now sometimes listed as an Agave, depending on the authority you follow), Manfreda virginica, however it is leaf deciduous and maybe there is a reason for it. The climate over the millennia is just not conducive to the long term establishment and survival of plants with permanent, succulent leaves and never will be. We can play with them during warm intervals in the climate but periodically the climate reasserts dominance showing who is ultimately in charge. We should not be surprised because climates have and always will be in a constant flux.

Because the weather causes periodic heartbreak does not mean we should forgo the joys of growing any particular plant or group of plants. We just have to be flexible, educate ourselves and maybe work a little harder. In truth sometimes it means we have to abandon the concept of growing particular plants outdoors yet there is no reason we can’t then enjoy the process of growing them in pots. Greenhouses were not only designed to grow out of season tomatoes, cucumbers and seasonal poinsettias, they are also useful and enjoyable for leaf succulents, orchids, tropicals and a myriad of other things which our climate outside does not allow.