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It is important to understand something about Eucalyptus cold-hardiness if you wish to grow one in a cool or cold climate. Hardiness is not as simple as looking at a plant hardiness zone map and finding the corresponding zone to which the plant is rated. If only it were so easy! But unfortunately, most climate zone maps are inaccurate, and references to the hardiness of the plants are frequently inaccurate as well. In addition, it is also possible to flex the zones a little bit by taking advantage of the microclimates in your own garden. The general health of the tree will also affect its hardiness. And besides all of this, we have a couple of other complications which are unique to Eucalyptus.
What exactly do I mean by cold-hardiness anyhow? There are a couple possible ways to define hardiness:
1. Some people think that if a plant is really cold hardy in a given area, there should be no way that it could ever be killed or even damaged from cold weather no matter how poorly it is mistreated. So, if you plant a little tiny vulnerable euc outside in November, and the following winter the worst freeze in 50 years occurs, then the plant had better survive without any scorched leaves or it is not hardy. This is absurd because even native plants are often killed in such freezes. Yet a lot of people seem to have this mentality, as if the plant should require no extra effort on their part than just to be stuck in the ground.
2. Some people think that if a plant is hardy, it ought to be able to reach maturity in a given climate. Some frost damage is acceptible from time to time as long as it is not much worse than the damage that occurs to native plants. Perhaps a cold winter will cut the plant back now and then and all the leaves will freeze off, or a late spring frost may damage the new growth, but in general the plant keeps on increasing in size until it reaches maturity. Perhaps even a winter might come along once every 50-100 years that is severe enough to completely kill the plant. But so long as the plant survives long enough to perpetuate itself for a number of years, it can be considered hardy.
3. Some people think that as long as you can get the plant to survive a winter outside, whatever it takes, then the plant is hardy. Even if it freezes to the ground every winter, and you have to plant it up close to the house for even the roots to survive, you can still call it hardy because it is still clinging precariously to life. Since Eucalyptus are generally grown as larger trees rather than dieback perennials, I tend not to take this approach when defining hardiness on my web page. But I do commend people for pushing the limits, as there are always pleasant surprises, and it is good to be able to enjoy eucs in colder areas.
So I will go by the second definition when I refer to "hardiness."
Eucs do not adapt well to severe cold in general compared to a lot of other kinds of trees. The timberline is the highest elevation at which trees will grow in the mountains where they are native. In Australia the timberline is slightly lower than the timberlines of other continents at similar latitudes - although eucs have adapted to growing in the high mountains you don't find them quite as high up as you would, say, firs and pine trees in northern California, or Nothofagus in the Andes of Chile. Their predominance at lower elevations in Australia suggests that cold-hardiness in eucs is the exception rather than the rule, and the genus as a whole really tend to be warmer climate plants than many other plants from similar latitudes.
It is a common misconception that if some eucs grow in cold, snowy mountains in Tasmania and southeast Australia, then they must be able to grow in colder areas of the Northern Hemisphere as well - places like the upper Midwest, or eastern Europe. But this is not so. There are important differences between these two kinds of climates.
Southeastern Australia and Tasmania have maritime climates. Maritime climates are characterized by generally cool, mild weather lacking extremes of temperature. Even though the mountains are of course colder than the lowlands in Australia, the moderate nature of the maritime climate extends to the mountains. So even though the mountains may be very snowy and have long winters, there may be no temperatures even close to the extremes which occur in our colder Northern Hemisphere climates. For example, Mt. Wellington, Tasmania, which is home to some well-known hardy euc species such as E. coccifera and E. urnigera, records a great deal of snow every winter, but has never recorded any temperatures below 12°F. (The coldest place in Australia is actually not in Tasmania, but in the mountains of New South Wales, where temperatures down to -8°F have been recorded. However, Mt. Wellington would be a more typical example of a place where your average hardy euc might have originated.)
By contrast, places like the upper midwest have continental climates. Although they may recieve less snow than the mountains of Australia, they do have much more severe cold. This is a very significant factor: since eucs do not adapt that well to cold generally (as discussed above), then they are not going to adapt to even worse cold than they do where they are native. A euc collected from Mt. Wellington that has survived 12°F hundreds of times may not survive 5°F even once, regardless of other factors such as warm summers, better soil, etc. And we know that it may get much colder than that in continental climates of the Northern Hemisphere.
Freezes are generally classified as advective or radiational. Radiational freezes occur on still winter nights when the weather is generally fair and there is no movement of the air. Advective freezes occur when cold air pours out of the Arctic, often accompanied by snow, wind, and days of below freezing temperatures. They are often referred to as "Arctic blasts." Advective freezes are unknown in Australia because (unlike much of the Northern Hemisphere) there is nowhere for a large pool of cold air to come from. The nearest cold land mass, Antarctica, is seperated from Australia by hundreds of miles of ocean which moderates the temperature. Therefore, eucs are not well adapted to advective freezes. The wind that occurs with advective freezes often dessicates their leaves, especially when they are unable to gain access to soil moisture because the soil freezes.
There are some other differences between these two kinds of climates. When the roots are buried in snow for the winter, such as in the Australian mountains, they are protected from freeze damage by the insulative properties of the snow. This is not always reliable in continental climates where snow may come and go throughout the winter. Continental climates also have wide swings in temperature. The air may warm up in late fall or early spring, which triggers the euc into growth, leaving it vulnerable to damage if a severe freeze occurs soon after. In Australia, the seasonal changes in temperature are so gradual that the eucs have been conditioned by weeks of light and moderate frosts before any severe cold comes.
So, for these reasons, the hardier eucs are generally better off in maritime cliamtes such as the Pacific Northwest, Britain and New Zealand; than they are in continental climates such as the eastern USA or eastern Europe. This discussion continues in greater specificity under The nature of the freeze.
In spite of these problems, eucs will still grow in cold climates, with certain limitations. For example, one man has even grown eucs in Cincinatti, Ohio for many years, where the climate is very much unlike Australia. Although it is too cold there for any eucs to reach a large size (as of yet), they are still worthwhile ornamental plants in Cincinatti.
The reason it is still possible to grow eucs in climates unlike where they are native is that they are somewhat adaptable. However, the more dissimilar your climate is to Southeast Australia, the more problems you are likely to encounter. In USDA zones 5b, 6, and 7a of the eastern USA and other continental climates, eucs will probably only grow as dieback perrenials, and require a lot of mulch for even the roots to survive, generally speaking. (Exceptions to this will probably be found, especially in areas where temperatures are a bit less extreme such as the Eastern Seaboard.) In USDA zones 5a and colder it is generally too cold for eucs, although I would not discourage experimentation in this area.
Some maritime climates such as the Pacific Northwest, Scandinavia, and Britain are able to grow a wide range of eucs, even though advective freezes do occasionally occur. To begin with, the severity of the freeze is reduced considerably because the coldest air is blocked by mountains (in the Pacific Northwest) or moderated by water (in Britain). So it is really not nearly severe as an advective freeze in a continental climate. Britain probably has a slight edge over the Pacific Northwest for growing eucs because the water surrounding it actually moderates the cold air, whereas in the Pacific Northwest the mountains merely block the worst of it.
Subtropical climates (such as Florida) and desert climates (such as southern Arizona) may be too hot for most of the hardy euc species - depending on how adaptable your species happens to be. In such areas it is generally better to select subtropical or desert species from Northern or Western Australia that are not exceptionally cold-hardy and are outside the scope of this page.
There are four main factors that must be weighted that determine how cold-hardy a Eucalyptus is for a given location:
1. Species selection.
By this I mean simply that not every species of Eucalyptus is equally
hardy,
but some tend to be hardier than
others. None of the less hardy ones is listed on this page.
In addition to
the seed provenance factor, as discussed
below, the hardiness of some species does not relate directly to
the amount of cold they are subjected
to in the place they are growing. Many species simply happen to
have more vestigial, or "built-in,"
hardiness than they need to be able to survive the cold of their native
haunts. E. kitsoniana,
E.
morrisbyi, and E. mannifera are examples of this--though they
grow at relatively low elevations where severe frosts seldom occur, and
in the company of other species that are not at all hardy, they have enough
extra hardiness to survive in a colder zone than where they grow naturally.
2. Seed provenance. Here is where things get more complex. This is probably the most important and commonly overlooked factor, and cannot be overemphasized. Within a given species, let us say for example E. gunnii, the hardiness of an individual tree of that species may vary from others of that same species by as much as 10 - 20 degrees or so. The reason for this is a difference in the genetic traits of each individual tree, which is probably most simply dealt with by tracing it to seed provenance: seed of E. gunnii collected at low elevations where winters are not severe will generally not produce plants as hardy as those grown from E. gunnii seed collected on a cold snowy mountaintop.
However, elevation and latitude are not the only factors in climate--the Australian high country is riddled with all kinds of microclimates, inverted timberlines and cold-air drainage basins, just like any mountainous area. Because of these microclimates, seed collected at 5,000' elevation might not necessarily yield hardier plants than those grown from seed collected at 4,000' elevation, for example. Perhaps the 4,000' site is on a valley floor where cold air pools on a frosty night. But as a general rule, higher elevation seed yields cold-hardier trees.
To make matters more confusing, eucs are so genetically variable that two E. gunnii growing side by side in the wild, in the same microclimate, may happen to differ vastly in hardiness from each other for no reason other than their genetically variabile nature. In some instances, old commercial plantings in cultivation have produced the hardiest trees for a species. For example, provenances that were introduced to New Zealand's Southland years ago have provided us with the hardiest plants of some species, since cold temperatures there have "weeded" the tender ones out. Only those species with an extremely restricted natural range such as E. parvula and E. mitchelliana are not likely to exhibit this kind of variability in hardiness (there are exceptions even to this rule).
PLEASE NOTE: the Eucalyptus hardiness ratings used on this page (and, unfortunately, in many catalogs which might be selling plants or seed from cold-tender provenances) are based on the hardiest individuals possible for each species. For example, not every E. gunnii is going to have a chance in USDA zone 7b, only those from the most cold-hardy seed provenance. It is therefore important to be familiar with your seed source and know what kind of environment they collected their seed from.
Unfortunately this confusion about the variability of hardiness within a species is probably one of the main reasons why many eucs are not considered "hardy" in areas where they could be. When people grow trees from seed of a species that was "supposed to be hardy" and it freezes because the seed was not collected in a cold area, it can give that species (or even the whole genus) a bad reputation for performance in their climate.
3. The nature of the freeze. This might not sound like a factor "determining" a plant's cold-hardiness, but it should be considered that freezes occur differently from climate to climate. Therefore this is a very important factor.
It is misleading to use exact temperature figures to indicate hardiness (but I cannot really avoid this because I have to give some idea of their relative hardiness and climate adaptability). Anyone who has experience growing marginally hardy plants can affirm this. Suppose we have a provenance of E. gunnii that is hardy to 5°F. If the temperature drops briefly to 0°F on a still winter night and then rises above freezing again without any more cold nights, the tree may come through completely unfazed or with only minor damage. However, if the temperature drops to 10°F and stays there for a week while the cold winds blow and the soil freezes, then it will probably die even if it never actually quite gets to 5°F. The problems of wind dessication and soil freezing (use mulch!) are even more harmful to eucs than most other marginally hardy plants.
Eucs are, of course, used to experiencing freezes in Australia, where some species even grow at ski areas. But freezes in Austrailia are not generally the same as freezes in the rest of the world, particularly the Northern Hemisphere. In Australia, the air is still during the coldest nights with no wind, and that is an important factor because cold winds can do far more damage than just cold by itself, and can even dessicate the tree if the soil is frozen. Where severe freezes occur in Australia, the ground is covered with snow, which serves to protect the roots from freezing. In more exposed areas where the snow blows away and the soil freezes deeply, eucalypts do not grow.
The underlying reason for the difference in climate is that Australia has hundreds of miles of ocean between it and the nearest cold land mass. In the United States and Europe, frigid winds can blow out of the north or east when an Arctic system comes. There is simply nothing like this in Australia: if any Antarctic system tries to get to Australia, it will lose its frigid Antarctic nature because the ocean will moderate it. Australia therefore has to produce its own cold, and it does this by way of radiational freezes on still, clear nights. Despite this important difference, eucs still have a certain degree of adaptability that enables them to grow in areas where arctic air can sometimes invade.
Precipitation patters and summer temperatures probably also affect euc hardiness, but it is not known to what extent. Generally the difference is much less significant than with such plants as palms and succulents, many of which can survive great cold if they are kept dry and have a lot of summer heat, but fail in mild rainy coastal climates. Most eucs, except for those originating from desert or subtropical climates, will not have much greater cold-hardiness in a region with hot summers and dry winters than in a cool maritime climate.
4. Siting and protection. Here is where you, the grower, come in. If your euc is marginally hardy in your climate zone, there are some things to can do to better its chances of survival.
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