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Now that we’ve set the stage for general landscaping principles and practices, it’s time to get to the heart of the matter. You live in the north, and that’s why we created this guide. You see, while it’s true that all landscapers across the country have the same goals and apply the same basic design principles to achieve them, we in the north have additional challenges that require special attention. Yes, landscapers in the south have an easier time achieving their objectives than their counterparts in the north. Consider this somewhat like step one of a recovery program; only once we have acknowledged the challenge can we then begin to deal with it. Our dear friends in the south sometimes belittle our landscaping efforts here in the north, essentially summarizing that since we don’t have a wide palette of plant materials and since we spend “half” of our year in winter, then naturally our landscapes are more “limited”. Translation - they think that northern landscapes are inherently boring by their very nature. I think this might be unfair criticism, however, because I can prove that a northern landscape can be just as appealing and interesting as any in the south. In fact, we are actually blessed with four distinct seasons, something they’ll never have in California or Florida, and that’s like having four fresh changes of landscape every year! All we have to do is work this to our advantage. So let’s start by commiserating together in a group hug, to get it out of the way. Yes, our collective northern climate is extreme, with long, harsh winters and baking hot summers. Yes, it is true that there are only a certain number of plants that are adapted to the vagaries of the northern climate. Yes, it is true that we have fewer choices for specific landscape applications, that it is more difficult to find a specific color, blooming season, or blend of all characteristics in a single plant.
Now for the great big revelation. There are ways for the northern landscaper to successfully work within these limitations, and the smart northern landscaper will study these ways and utilize them in their designs. In fact, it is possible for us to develop the optimized northern landscape as a style unto itself, a style that can be the envy of our southern friends, because it is a landscape they can never dream of ever having. Think about it; they will never be able to grow a spruce tree, they will never see the reflection of Christmas lights on the fresh snow, or work with a blazing fall palette of colors. How’s that for a reversal? The truth is, northern landscaping in itself ends up being a style of expression, shaped by our climatic realities into something distinctive and unique. By using the four distinct seasons to our advantage, by studying and carefully selecting our plant materials for ornamental characteristics or function, by creatively controlling our environment, we can create our own style unique to our part of the world. Remember, the definition of beauty is broad, and is much more than just loud, flowering shrubs all year long; all landscaping principles are equally as important in the total presentation. Northern landscaping is not a pipe dream, it just takes an understanding of the circumstances and a special design effort. The concept of hardiness zones was created as a means to categorize plants according to their general climatic tolerances and adaptabilities. It is a way of knowing which plants can be expected to perform or even survive in a particular climatic extreme. Hardiness zones are by no means the only important determinant of a plant’s likes and dislikes. In fact, they only characterize one aspect of a plant’s adaptability; cold tolerance. The zones do not take into account precipitation, soil preferences, or light requirements. They are definitely not the only factor to consider when selecting a plant for your landscape, but they are a very important one and should be understood. Here’s the background behind hardiness zones. There is a minimum temperature threshold for every plant below which the living cells of the plant start to die, which leads either to serious damage or ultimately the death of the plant. There is much variation between plant genera (families) and even some variation within particular species, but every plant has some lower limit. That’s why the showy hibiscus of Florida will not survive anywhere in the north (except indoors); its tissues die below about 30°F. To help characterize plants according to their specific range of cold adaptability, scientists created the hardiness zone system. They started by examining the extreme lowest annual temperatures for various regions around the country, essentially the coldest temperatures recorded each year, typically occurring in January and typically overnight when temperatures are at their lowest. They then took an average over a number of years, determining the “average minimum temperature” for each region. This can be thought of as the very lowest temperature any particular region of the country would be likely to experience in an “average” year. By plotting thousands of these regions across North America, they were able to create a map with distinct trends and characteristics. To make this map easier to read, they divided the regions into groups, separated by ranges of 10°F, and they called these zones, as follows;
Then, the scientists examined the performance of plants in each of these extreme climates to see at which low temperatures they started to experience severe damage or death. A plant that exhibited severe damage or died at -34°F would be rated hardy to zone 4, since it could survive the entire range of -30 to -20. It would grow in some parts of zone 3 (most likely southern parts), but not reliably. There are certain differences to note between the United States and Canada, as they have adopted somewhat different zone definitions. In the United States, the Department of Agriculture (USDA) has created a zone map along the lines presented in this discussion, based solely on minimum temperature. In Canada, Agriculture Canada has also created a zone map, but they expanded it to include other growth-related conditions such as growing season, precipitation and snow cover. This is somewhat more comprehensive, although it may complicate the study of cold tolerance alone. For the purposes of this guide, I will refer to the USDA definitions, if only because they cleanly represent cold tolerances free from other considerations. For full-size and up-to-date detailed maps, you can visit the following sites; USDA Map Because the hardiness zone system is so easy to use, nurseries across North America have adopted this as almost a universal standard to describe what can be expected to grow where. As a gardener or landscaper, however, one must be very aware that hardiness zones only describe cold tolerance, and that is only one of many growing requirements for any specific plant. Other significant factors include the length of the growing season (which is somewhat related to the hardiness zones), annual rainfall, exposure, soil pH and type, etc. Duration of cold is another factor; a long cold snap in the plant’s “safe” zone may actually do more harm than extreme low temperatures. Another factor to consider is snow cover; a good, deep snow cover acts as insulation, and can increase the “apparent” hardiness zone by one or even two ratings. It is true that winter is the key limiting factor in regards to plant success for a number of reasons. However, there are summer limitations as well that may contribute to an otherwise “hardy” plant not performing or even dying. Exposure is a big stress factor for plants; openness and persistent drying winds in summer can damage the plant well before the onset of winter. Excessive summer heat, drought, or flooding can also contribute to the general decline of a plant. The open prairies offer an even more difficult challenge to plants, where good snow cover and protection from wind are certainly not guarantees. As a rule of thumb, remember this; whether caused by a dry year, wind damage, water damage, soil incompatibility, etc., a plant that goes into the winter in a stressed condition will have a much more difficult time surviving. In a stressed condition, you can pretty well knock off a zone from the hardiness scale, and if you’re trying to grow an out-of-zone plant, well then this is a sure-fire way to kill it. Note that the concept of hardiness zones applies mostly to trees and shrubs, since they are usually rather completely exposed to the harshness of the winter environment in its totality. Because of snow cover for the root zones, herbaceous perennials are much less susceptible to winter stresses, and as a result, hardiness zones are only a guideline at best. Obviously, since annuals aren’t expected to survive the winter, hardiness zones don’t apply to them at all. Know your zone, know the ratings of your plants, and plan accordingly.
Winter doesn’t only take its toll on humans. It is also very tough on plants; after all, how would you feel if you had to spend 6 months outside in winter, buffeted by the howling winds, gritting your teeth through chilling winter lows, and not even being able to warm up inside a nice, warm house! This may be a little dramatic, but it gives you a certain measure of respect for plants that can survive our bitter northern winters. It is therefore worthwhile to understand a little about the mechanisms of winter and how it stresses plants. At first, to us humans, winter seems to be only about cold. That is in fact one of the major attributes of winter from a plant’s perspective. However, if cold was the only concern, then a surprising number of additional plants would be quite happy in our northern climate. The truth is, there are other factors in winter that add together to really make winter tough on plants. Wind is one obvious culprit, but did you know that sun can also do some serious damage to plants in winter? Let’s look at these factors one at a time. Cold is pretty obvious; below certain temperatures, cells will die. The reason behind this is quite fascinating. Cells are actually able to block the formation of ice inside them down to surprisingly low temperatures, given that water freezes at zero degrees centigrade. Ice is bad for cells because it forms crystals, which kill the cell by rupturing it much like a spear. It is actually because cells have mechanisms to allow them to avoid the formation of ice crystals that they can survive down to such low temperatures. Below these temperatures, however, their protective mechanisms fail and ice starts to form, and the cells start to die. If only a few die, then the plant will show winter injury the next spring; if too many die, then the entire plant can be killed.
Wind is more devious. Since the ground is frozen in winter, obviously, plants can’t transpire water, and so any water that is lost in the winter through evaporation cannot be replenished. Even though leaves are the main evaporators of water, stems and bark also lose some water throughout the year, even in winter. Wind speeds up the evaporation rate, and if the plant loses too much water in winter before the spring thaw, it eventually dies of drought! Protecting plants from drying winter winds is therefore important to their happiness, and applies to almost all plants with the exception of the very hardiest. Luckily, most urban centers have enough homes and buildings to offer significant wind protection, and in the country, a good shelterbelt can provide the same benefits. Sun damage in winter? Sure! In fact, the sun (actually sunlight) can actually damage different parts of the plant differently. First of all, the sun’s rays are warm. If they shine on bark on a winter day, they can warm it up above the freezing point, especially in late winter when the sun is at a much higher angle. However, when the sun recedes behind a building or sets, and the bark temperatures drop back down to winter temperatures, the cells in the bark can form ice crystals simply because they can’t react fast enough to the sudden drop in temperature. The result is bark splitting or sunscald, and can be fatal to some trees. Sunlight in winter is also particularly bad for evergreens. Because they retain their leaves over winter, they are all the more susceptible to drying problems as with wind. Sunlight warms up the leaves and causes moisture to evaporate faster. Combined with a bit of drying wind, the result is called winter burn (also called wind burn or winter browning), and shows up as browned leaf edges, or entire brown leaves, depending on the amount of damage. Broadleaf evergreens (such as rhododendron or boxwood) are much more susceptible than needled evergreens (such as pine and spruce) because of the increased surface area, and that is why very few broadleaf evergreens can successfully endure a northern winter (less the further north you go). One way to avoid the combine damages of wind and sun is to wrap plants in burlap or some other opaque protective material. These not only block the damaging rays of the sun during winter, but also act as a windbreak and even help maintain a favorable climate around the plant. Be sure not to remove the protective cover too early in the spring, until the ground has thawed and the plant is once again able to take up water from the ground.
The concept of frost damage is closely related to winter and plant hardiness. All regions of the north have a limited growing season, and native plants have developed means to ensure that their tissues stop growing well in advance of frost. Growing tissues are generally unable to tolerate low temperatures and even frost, and can only endure the low temperatures of winter once they have ceased growth and properly hardened. Plants that require a longer growing season than that of your region may not be able to respond to the cues of the oncoming fall season, and may not be able to harden their tissues before the first frosts. The result is frost damage, or killing of newly growing tissues, and is a common injury with out-of-zone plants. In such cases, simply prune back to the nearest growing tissues in spring. On the reverse side, frost damage can also occur in spring. Some sensitive plants are too eager to start growing in spring, before the threat of last winter frost is over. This is particularly troublesome with early spring flowering plants in which flower buds open before the leaves. For plants like forsythia or flowering cherries, flower buds can be killed by an early spring thaw followed by freezing temperatures. Since the principal value of the plant lies in its ability to flower, the ornamental value of the plant is then lost, although the plant itself may still survive. This is also a problem with many fruit trees in the north, particularly peaches, cherries and apricots. Again, the consequence is no flowers, and therefore no fruit. Now that we understand the nature of winter and its ravages, you will be pleased to know that there is much we can do to gain the upper hand and defeat winter. It only takes understanding, planning and some attention. The easiest way to landscape in the north is to use plants that are suitable for your environment, plain and simple. These include both native plants and imported plants from similar environments around the world. These are typically the materials sold at your local nurseries. This is the surest way to guarantee a low-maintenance, trouble-free landscape. Be cautious of plants sold through mail order or on-line stores, as they are not always aware of your particular hardiness zone or growing conditions. Some people complain that there is a limited selection of truly hardy plants for the north, and they are somewhat correct. There is a limited selection, but by knowing the intimate behaviors of the available plants and how to use them well in a northern landscape, you will find it’s not as limited as it first appears. Don’t listen to the southern gurus who belittle the lofty poplars or willows as landscape elements. In the south, these may not make sense to use, but in the north, they can be your friends. All you have to do is understand their needs and limitations, and use them accordingly. It is not only possible but strongly encouraged to add “out-of-zone” plants into your design; the safest are those rated to one zone higher than yours. First and foremost, remember that plants do not necessarily “know” their zone ratings, and can often surprise us with where they will grow. Just be sure to give them exactly what they require for optimal growing conditions; the right soil, moisture, aeration, protection and light. If you avoid summer stresses and your plants go into the winter healthy, they should perform well. Some plants may actually acclimate with age, exhibiting less and less damage as they get older and more mature. You may expect to encounter one or two particularly harsh winters where no matter what you do to protect your plants, some winter damage will occur; in these cases, simply prune back the dead wood the following spring, and let the plant heal itself.
This is a good time to bring up the number one word heard time and again by northern gardeners, and the ultimate secret to growing out-of-zone plants; microclimate. What is a microclimate? Well, a climate is a set of environmental (i.e. weather) conditions in a particular region of the world, so a microclimate is exactly that on a much smaller scale. It is essentially the climate at the scale of a yard, a garden, or even one or two plants. You see, while there are certainly very distinct weather patterns in any given part of the world, there is a surprising variation in the actual weather for a specific location in your yard. Did you know that the average temperature inside a shelterbelt can be 1-2° warmer than outside? Did you know that the a garden against a wall in a tightly packed urban residential area will typically experience its first frost almost 1 week later that outside of the city? The concept of microclimates is related to the ubiquitous word “shelter”, which appears everywhere in the gardening world. We are always advised to plant our out-of-zone plants in a “sheltered location”. So what exactly is this “sheltered location”? This is a location with a favorable microclimate. Such a location is sheltered from strong winds, in summer and in winter; gets a good snow cover to protect the roots from freezing (root damage can occur at much higher temperatures than above-ground parts); and, is out of the sun in winter and early spring. In other words, it is a location that naturally minimizes each and every potential stress to a plant, and thus minimizes the risk of any of the plant injuries discussed earlier. So how do you go about creating a favorable microclimate? Look for the following when siting a plant or a garden; - a location not exposed to wind, for example against a house, protected by evergreens on the windward side, or below a solid fence; - a location neither in a valley or depression, where frost tends to collect at night, nor on top of a hill, which is too exposed to wind; - on the south side of a house or building only if NOT evergreen, as the reflections of the sun off a wall can damage evergreens in early spring; - a location that gets and maintains good snow cover throughout the winter; this is critically important for non-hardy perennials and small shrubs, which count on the insulating effect of snow to get them through the winter, but also for trees, to protect their roots. Provenance is another concept important to northern gardeners and landscapers. This basically means that even within species, there are variances in hardiness depending on where the seed was obtained. For example, Red Maple can grow all the way from Florida to Quebec. However, seed from Florida produces plants that are much less hardy than those grown from seed in Quebec. For this reason, it is important to know where your plants are from when you buy them at the nursery. Note that not all plants are this way; some exhibit much more stability independent of their origins. It is also important to realize that different cultivars of the same species often have different preferences, including hardiness; some are less hardy than the species, and some are actually MORE hardy than the species. Be sure to understand these differences and use only cultivars that are best adapted to your climatic region.
As if weather was not enough, there are more winter challenges that you must be aware of. Animals and rodents can actually do more harm than the weather itself in a bad winter, as they are left scrounging for food which is not readily available. Deer browse on branches, while rabbits eat both branches and bark. Rabbits and mice can actually girdle a young tree by chewing the bark in a complete circle around the trunk, which invariably kills the plant. If you’re lucky, these neighborhood nuisances will just stunt the growth or shape of your plants. There are various deterrents available. Plastic or fabric tree wraps are by far the most functional, working by shielding the bark from the animals or rodents. They also serve a second valuable winter function by reflecting late winter sunlight, thus offering protection against sunscald and frost cracking. There are also spray-on or paintable animal repellents which are often quite effective. These are applied to the bark and branches of the tree, and discourage animal munching with their bad taste. For larger trees, you can wrap some chicken wire or fine mesh around the base of the tree. Finally, if you need to protect your whole garden, nothing beats a solid impenetrable fence. |
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