Winter 2016/17 will go down as a harsh winter. The combination of prolonged subfreezing weather with multiple bouts of snow and freezing rain exacted a price on our gardens. I write the tags for Xera and aside from witty repartee I also am tasked with assigning cold hardiness for each plant. Let me tell you that it isn’t as easy as picking a number out of a hat. Years of observation from both myself and Greg have gone into how I give each plant a rating. Just so you know I make our ratings relevant to our specific climate. I’ll get into that as well as the many other factors that fall into line. And I will tell you that I am far from correct. The capriciousness of weather and climate and microclimates is vast. And I swear that once I have assigned a cold hardiness number I will invariably be proven wrong. Visiting a garden and voila! what I thought was completely tender is there thriving in uninhibited replete glory. I like when that happens. This years weather was NOT funny for avid gardeners in our region. It was a prime example of the most extreme conditions that tests plants. Every freeze is different and I learn something new each time. I work to pass that on to gardeners because above all else I want gardeners to succeed with our plants.
Averages miss the details
Its important to possess a working knowledge of weather to pair with growing plants. This is obvious. We are all familiar with USDA climate zones and most gardeners will tell you what zone in which they reside. Well, it isn’t that simple. Temperatures are compiled at official reporting stations over a 30 year period and averaged to achieve a temperature and then assigned a rating. The last period of record used to assign our USDA hardiness rating was 1980 to 2010. During those years our average annual low was 19.3ºF- USDA zone 8b (15º to 20ºF) at the Portland Airport. This isn’t up for discussion. In fact, all over western Oregon and Washington we are in zone 8. You may say well in my garden its zone 7 or even zone 6. Nope, the law of averages and reality is against you. That doesn’t mean we don’t experience the kind of cold that can relate to those numbers its just that an annual average is not very helpful for long lived plants. Remember that averages are made up of extremes which can’t be ignored in a garden and such things as microclimates, soil conditions, and the age and health of a plant all have an effect on hardiness. Most of all simple temperatures miss the all important nuance of the actual weather and how it acts on our gardens.
Hardiness for the long haul
There is another way of looking at the climate with real impacts on our plants- that is expected extremes over time. For instance, we average 6 years in ten with zone 9 winters. (ultimate lows above 20ºF) and four winters that are zone 8 (10º to 20ºF). That is for the Portland Airport and its a better way of interpreting hardiness. As it turns out we have dropped below 20ºF every four years for the last 18- the same interval between each arctic air intrusion. We have become used to mild intervals and are shocked when a colder than that winter comes a long. (What we locally call Phormium killers). For a better predictor of plant performance I consider that equation and the even more specific number of the coldest temperature we have experienced in the last 10 years. In Portland that is 11ºF- Zone 8a. Locations in the city are subject to the urban heat island. Concrete, glass, and asphalt retain heat and release it giving the city a warm cushion. In outlying areas it is considerably colder and those areas have a different cold profile and average. For instance, Hillsboro sees an average of three zone 7 winters in a 10 year period and their lowest 1o year temperature is 3ºF (zone 7a). Though the averages puts them squarely in zone 8 significantly colder conditions occur regularly. This must be factored into plant performance. Even more specific are the highs that occur during arctic episodes. Many plants can endure very cold overnight lows but highs above freezing gives them a period of recovery. During our arctic episodes we see subfreezing high temperatures. This continuous stretch of subfreezing weather causes more damage. This year was a prime example of that. So, I factor that into a plants hardiness rating as well. For instance, many Australian plants can endure very cold overnight lows but they almost always rise above freezing there during the day. This is why you see such plants as Acacia pravissima rated zone 7 but it fails to live through our quad annual freezes- it dies during the day with no daily period of recovery- hours on end below freezing.
The &$#%$%% East Wind
Another way plants can be damaged is by the subfreezing desiccating effects of wind. Our arctic blasts are invariably accompanied by howling east winds from the Gorge. Couple that with insanely low humidity (dew points) and you get a recipe for freeze dried plants. Wind, which can actually keep things milder by stirring the atmosphere is horrible for plants when temperatures are below freezing. Wind evacuates transpiring moisture from leaves, and lowers the point of freezing, frozen soil prevents the capillary action of moisture replacement- semi tender plants, evergreens and even perennials suffer. Be wary of temperatures below 28ºF and winds above 25mph. (If you don’t own an anemometer use the tried and true Beaufort Method of observed effects: 25+mph winds move the largest branches of trees, cause overhead electrical wires to whistle, and fully extend flags- often with some raucous snapping). The longer subfreezing wind persists the more damage will occur. The closer to the Gorge or the higher in local elevation gives you more wind. In my years of observation conditions such as this can erase fully 10ºF of hardiness from a plant- a whole zone. That means the temperature may dip no lower than 26ºF (zone 9b) but the effects of wind produce the amount of damage you would expect at 16ºF (Zone 8b) in a calm environment. And this effect is perhaps the most confusing to gardeners and the reason why people come up with crazy zone ratings for their own patch of land, they have observed damage equivalent to that rating. Another example, 18ºF and roaring wind can fry even zone 7 plants. If a plant is particularly susceptible to subfreezing wind I factor that in too.
Plant protection. Is it worth it?
In a complicated way it can be worth it. In the Portland area when you protect a plant you are shielding it from the wind. Wrapping plants doesn’t keep them warmer. It just makes them freeze more slowly. Plants are not warm blooded- they make due with adaptation and we can slow the process of freezing. If you really want to protect a plant place a wind proof screen around it and add an emergency light that projects real heat. I get a kick out of strings of new cool LED lights wrapped around palm trunks…the effect is not protection its more Las Vegas if anything. Even the huge old dangerous glass lights from childhood only heat a tiny amount of space around each bulb- and can cause damage to plant tissue. You are better off providing an actual heat source to raise the ambient temperature. Wrapping containers is another way to make the gardener feel better but only prolongs the inevitable. Clear bubble wrap can act like a mini-greenhouse and provide some daytime warmth but you are better off moving the whole plant to a protected location. Even an unheated garage can make a huge difference. At Xera we cover all plants INSIDE the hoop houses with clear plastic. This prevents the loss of heat from the ground, provides an added layer of protection and if our arctic air comes with sunshine gives a little bit of daytime heating. The plants may still freeze in their pots but that is slowed as well as thawing is slowed providing a smoother transition and the ability to recover during the day. Remember that plants in pots are MUCH more at risk because cold air surrounding the container will quickly and thoroughly freeze the soil and the roots. For potted plants that you do not intend to move make sure they are hardy to at least one zone lower than we expect. Zone 7a or lower. Even then the roots on plants vary greatly by species in their tolerance for freezing. And the longer the freeze the more thoroughly frozen the roots will be.
A perfect world
For a plant to achieve maximum hardiness to cold several things must be right. First, the plant should be grown in the most optimum conditions that suit it. That means a borderline hardy plant (Zone 8b) should be sited with protection. That could be the top of a south facing slope, under the partial overhead protection of trees or on a heat accumulating south or west facing wall. Age plays into hardiness in a big way. A good example is Olives (Olea europea ). When young they are significantly less hardy to cold than when established with at least a year or two of life in the ground. The difference is remarkable. Small specimens can barely recover from lows of 20ºF while big trees handle temperatures down to 5ºF with little damage. Most plants exhibit this characteristic. So the bigger the better. Another complication in our climate is saturated soils. Good drainage is key to so many plants – I feel like a broken record writing this on tags but for many plants its key to surviving arctic spells when soil moisture turns to suffocating crystals and plants lush with moisture are suddenly confronted with arctic air. From perennials such as Echinacea, Salvias and Agastache, to most drought adapted shrubs and trees it imparts hardiness. And I’m not talking about removing all the soil and lacing it with pumice- slopes, even gentle slopes count for a lot. Annual top dresses of organic material increases the oxygen in the soil. So many plants need to breath and just a moment of work can yield great success.
Summertime and heat and ripening
One last component to rating the hardiness of a plant is the amount of heat that it requires to achieve cold hardiness. Plants from very hot climates are adapted to a certain amount of ripening to achieve their ultimate cold tolerance. High summer temperatures converts green wood to brown, it accumulates calories in foliage and it can spur plants into bloom to complete their annual life cycle. They are adapted to this. When plants receive inadequate hardening they are way more susceptible to cold damage in the ensuing winter. Another facet is growing a plant in soil that is too rich. Many plants from mediterranean climates are adapted to dry somewhat impoverished conditions. Rosemary, Cistus, Arctostaphylos are adapted to a period of summer dry conditions. This pushes them into a second type of dormancy and prepares them for winter. Some plants just don’t like it fat and happy. Its important then to know where a plant is native, lower latitudes, deserts, subtropical environs this can tell you a lot about a plant. An example is Loropetalum (Fringeflower) it is native to subtropical SW China which can get surprisingly cold in winter. In the Portland area we receive just enough heat for this plant to survive our winters. Its not recommended for Puget Sound because their cooler summers don’t allow for ripening and it can freeze away at surprisingly warm temperature. A shocking number of plants register this same necessity. Daphne, Nandina, Camellias are just other examples where this is a factor. And I figure this into our local climate when assigning hardiness.
When things go horribly wrong
For almost all plants autumn brings a period of shorter days and colder and colder conditions nudging plants safely into dormancy. Ideally, in our climate, you want successive mild frosts to harden plants for colder conditions that meet the middle of winter. Well, in the best of all possible world that happens. Instead our mild maritime climate saturates plants with moisture and temperatures hover in the 30º to 50ºF range. You actually want mild freezes to convert the chemicals in your plants to freeze protecting sugars. And all but the most tropical plants possess this capability. Even, Phormiums and Bananas can do it. But when an early (Nov 15-Dec. 15) arctic blast descends or a late freeze when plants have lost their dormancy real disaster occurs. Even perfectly hardy plants that are still in lush growth mode can fall victim. And I’m not talking about borderline plants I mean ALL plants. Luckily, that is a rare occurrence and many, many years can pass between these devastating conditions. In Portland it has occurred historically every 15-20 years or so. Its infrequent but good to know when you are assessing plants for our climate.
Lets put it all together
So, there are many factors in plant hardiness. My best advice is to follow the instructions on the label. And if there is inadequate information look up the plant. Pay attention to its native origins and when in doubt ask us for the most specific instructions possible. If a plant requires poor soil and little summer water- its for a reason. At Xera we don’t just grow plants because they are exotic – banish the hardiness, we have a real commitment to plants that succeed here. Its important not to dismiss an entire genus if you have lost it to a freeze- there is great, great variability within each category and we are always looking for the most dependable. We test plants rigorously before we release them to the public, in our own gardens as well as above ground containers and in the harshest possible environs at our wholesale nursery. I hope this sheds some light on how and why cold damage occurs. This past winter brought an exceptional test in that realm. Be patient, many plants can show recovery even deep into summer. We look for three things regarding plant damage. First immediate damage, second about 2-3 weeks following arctic conditions real damage is apparent and finally the first day above 70ºF in spring when a plant really breaks dormancy. That is when the most conspicuous problems become apparent. We’re looking forward to a great year at Xera. Climate adapted plants for gardeners in the Pacfic Northwest.