Plant Hardiness

'Is this plant hardy?’  A relatively straightforward and reasonable question, you would think.  If only there was a straightforward and reasonable answer!  Whether a plant is going to be hardy or not for any given location depends upon an extremely complicated cocktail of interdependent factors. 

Starting simply:  for any given site there will be;

  • plants that are reliably hardy in any winter
  • plants that are reliably hardy in some but not all winters
  • those that definitely aren't but adorn the garden during the frost-free months.
  • those that are untried but could be

You can have a lot of fun with the last two categories – they are pretty well self-explanatory and the only limit is your imagination, glasshouse space and bank balance.

However, the first two need exploring and the following is a discussion of the various factors, rather than an answer!


1. What does ‘hardy’ actually mean?

Investigate just this one word and you will soon discover it means different things to different people.  A statement along the lines of ‘able to withstand year-round climatic conditions, including frost, without protection’ seems a reasonable consensus.  But evaluated over what period of time – and do we include the record breaking winters or just average years?  This is an issue that only the individual can resolve.  If you decide a plant that scrapes by in a mild winter with some protection is ‘hardy’ for you, then that is fine.  Conversely, you may only consider a plant to be hardy if it can survive the lowest historically recorded temperature in your location – again, that is fine.  The tricky bit is to always know what someone else means – like any review, knowledge of the reviewer is the only way to make sense of it.


2. Hardiness Ratings

Various attempts have been made to categorise either how cold hardy plants are, or how cold geographical locations get, by ‘ratings’ or ‘zones’.  None give a complete picture; at best they can be used as a very rough guide, at worst they are downright misleading. 

Here are the three commonest zone ratings tabulated together to give a rough comparison:

Hardiness Ratings

The RHS system has recently been undated and pertains to plants grown in the UK (rather than places). So a plant described as H5 would be hardy to between -10°C and -15°C

European Garden Flora (EGF) Zones again pertain to plants, this time as grown in central Europe. A plant described as H5 would be hardy to between 0°C and -5°C

USDA Zones represent a system devised by the US Department of Agriculture which describes all parts of America by taking the average annual minimum temperature reached there. These temperatures are then divided up into ranges or bands which are then transcribed onto a map, rather like contours, linking areas that experience similar figures for the mean low temperatures each winter. So for USDA zone 5 a plant must therefore tolerate between -23.3°C and -28.9°C

Clearly the systems are not comparable – they were not designed to be. Nor is it possible to relate climate in the UK and Europe to climate in the USA. To use an extreme example, the Shetland Isles and southern Alabama are both USDA z8b/9a as they share historical winter minimum temperatures. In summer Alabama is about 20C hotter!


3. Recent winters and average winters

The last few years have seen a down turn in winter temperatures following a period of exceptionally mild weather. Up until the winter of 2008/09 the previous ten winters had all been milder than average! Many growers were seduced by this and began filling their gardens with absurdly tender plants, believing this to be the new norm. Certainly the reality check of winter 2010/11 is fresh in many peoples’ memories and perhaps puts us back onto a more sensible path in terms of expectation of winter temperatures. The Met. Office work on a rolling 30 year period, taking the lowest temperature recorded each year for any given location, add them together then divide by 30 to get the average low temperature.

Normal Distribution

For most areas within the UK these figures, if plotted an a graph, would give a ‘bell curve’ or ‘normal distribution’ pattern with warmer and colder temperatures being recorded at evenly diminishing rates either side of the average.

One reason why the USDA system doesn’t work over here is the way the winter weather patterns differ on that side of the Atlantic. Rather than an even distribution around an average figure, there is a cyclical period of extreme cold when an arctic pulse sweeps down from Alaska, trapped within high mountains ranges– about every 10 years or so, followed by a series of much milder winters. These cold pulses effectively drag down the average.


4. Geographical location & Climate

Facts and figures - get to know the weather where you live. The Met. Office has its spies in most areas and the information is freely, if not easily, available to personal callers at their library in Bracknell. There are also weather stations all over the country that feed data to online weather websites – Weather Underground ( is one such. Unless you have a realistic idea of how cold your locality is likely to get, you are always going to be heading into winter blindfolded with fingers crossed.


5. Microclimates

A microclimate is where a local climate differs from the surrounding area. The term can refer to something as small as a single border or as large as an area of several square miles. For example, a small border in front of a south-facing wall is said to have a warmer microclimate compared to the rest of a garden as it will receive more sunshine, heat will be absorbed by the brickwork and then be radiated slowly. Multiply this effect many times over in a large city, with all the concrete, brick, asphalt and heating, and you get a microclimate often called an Urban Heat Island. There are many factors that can affect and create microclimates.


6. Length and depth of cold

As we have discussed, quoting a figure to which a plant is hardy can be misleading. Some plants can withstand a brief dip down to, say, -10C if it is only for an hour or two one night and daytime temperatures the following day rise to above freezing. The same plant might die if exposed to prolonged freezes of -6C over several nights with daytime temperatures staying below freezing for several days.

Mild winters, such as currently 2011/12, have their own problems in that during unseasonably high December and January temperatures some plants are inclined to remain ticking over only to face a sharp downturn in temperatures in February and the possibility of damage to weaker winter growth. This can damage or even kill a plant at a much higher temperature than the same plant exposed to a ‘normal’ winter.


7. Sun and summer temperature

We often hear of hot summers ‘ripening growth’ but what does that mean and how can it affect a plant’s hardiness? Many of our ornamental woody trees and shrubs come from regions of the world that have long hot summers, even if the winters are punishingly cold, so a long, hot, growing season means that the tender new shoots made in spring have a greater opportunity to mature into tougher woody stems, and hence more able to cope with winter. There is in fact a marked difference as you go from the south eastern tip of England and move northwest in the types of plants that thrive – many only perform best in the hotter southeast, some prefer the cooler northwest.

Higher temperatures can also often induce changes to the internal chemical composition of the sap within plant cells that effectively turn its insides into ‘anti-freeze’. This is especially the case with many succulent plants that, when grown in a hot summer climate, can withstand far lower winter temperatures than they can in a temperate climate with a milder winter. They just don’t get enough warmth in summer to change.


8. Orientation explained

‘South facing garden’ is a term frequently bandied about but what exactly are the advantages and why? Well, to turn it around, it is all to do with the length of shadow cast by a house in a north facing garden at different times of the year as this diagram demonstrates.

Winter Shadow

In summer the sun is high and the shadow of the house is short, only really covering the immediate back of the house. But in winter the sun is low and the shadow much longer. Without the sun’s rays the area in shade remains cool, frost and snow take much longer to melt and frozen plants take much longer to thaw. In a few cases, such as with early flowering Camellia, for example, this can be an advantage buds that thaw quickly can be aborted whereas those that slowly defrost remain fine.

South and north facing slopes can make an enormous difference as it changes the angle at which the sun’s rays hit the ground, and therefore the effectiveness of how the ground is warmed up.


The key thing is to identify such microclimates and make best use of them.


9. Overhead cover

We are all aware that frost is much greater on a clear, cloudless winter night. With a good covering of cloud temperatures rarely drop too dramatically. This is because the layer of cloud acts as a blanket to insulate the ground, preventing the heat gained from the sun during the day from radiating out into space. This same principle works on much smaller levels. An evergreen tree with a high canopy acts as a barrier to heat radiation, so tender plants are effectively insulated by the tree leaves. A simple layer of horticultural fleece does exactly the same job – prevents heat from radiating away at night. This can be particularly effective against late frosts, when new leaf growth or developing flower buds in spring are most vulnerable. A judiciously applied layer of fleece can make all the difference.

Overhead Cover

10. Moisture and air movement

Many plants resent their roots sitting in cold damp soil for winter. Many of our arid-loving desert and Mediterranean plants are extremely hardy to cold but if they sit with their ‘feet in water’ they will rot and die. In these cases it is particularly important to pay attention to drainage, adding material to the soil to improve water movement through the root zone. This is not usually a problem for sandy soils but for rich loam or clay soils they need ‘lightening’ with grit, gravel or a fibrous organic material like composted green waste. For some particularly drought-loving plants such as the hardier cacti, agave and other succulents, a mix of pure gravel is often the best medium, so that water is moved away from the roots as quickly as possible. If it can be done, a raised bed back filled with gravel or all-in-ballast is the idea solution.

In addition, some plants are also resentful of water sitting on their leaves for prolonged cold periods, especially if they are then exposed to a cycle of freeze/thaw as happens with snow cover in this country (we get, as the rail companies are fond of reminding us, the ‘wrong type of snow’!) when the melted snow penetrates right into the centre of a plant then freezes again overnight. For such instances it is beneficial to provide a physical cover such as a plastic cloche or polycarbonate shelter.


Air movement is something that largely gets forgotten but can be equally important. Stagnant, humid air can cause all sorts of rotting to develop, so if possible keep any plastic covers ventilated. Try not to plant too densely to avoid crowded neighbouring plants effectively trapping damp air around the base of the more susceptible plants. It is sometimes possible to grow a plant exposed to the elements outside with unimpeded air movement and soil drainage that fails when grown under glass because of the closed atmosphere!


11. Positioning

As mentioned, a border at the base of a south facing wall is going to have a slightly warmer microclimate and therefore an obvious choice to position a slightly tender plant. Similarly, tender plants are sometimes best sited under an evergreen tree canopy – often lower branches can be removed to ‘raise the crown’ allowing access to the relatively sheltered growing position beneath.

Camellia flower buds are resentful of quick thawing and will often abort if growing in full sun. A position against a north wall is ideal.


12. Provenance of the plant

Provenance means the ‘place of origin’ and, in many cases, can be a clue as to whether a plant will be hardy or not. As a general rule, plants that come from a similar climate to that in which you are trying to grow it will be hardy. And as a general rule, plants that come from a much warmer climate will not be hardy. But of course there are always exceptions,

Sometimes the question of provenance makes a huge difference. An example is in the recent trials of eucalyptus and acacia trees. Many species of both have an extremely wide distribution in the wild, often encompassing many different climate zones. It was found that plants raised from seed collected from populations found in the coldest edges of the species’ natural range exhibited more cold tolerance when grown elsewhere than plants raised from sub-tropical provenances. They might be the same species but not all can be grown successfully in the UK.

But plants can be surprising and frustrating things and not behave in a way that can be predicted. Occasionally plants are encountered that come from a tropical or subtropical place and yet grow happily through much of the UK. Sometimes for reasons that can be explained and sometimes, well, just because they are plants and plants don’t read the rules.


13. Acclimatisation and size

In general, and as far as plant hardiness is concerned, size matters. For example, a Trachycarpus fortunei with 2m of trunk will be hardy to -15C or lower. A 2 year old seedling might only be hardy to -10C but it will adapt or ‘acclimatise’ to its growing position producing tough, weathered and compact growth that is suited to its locale.

A 2m trunked specimen of Trachycarpus fortune that has spent its entire life in a heated glasshouse or grown at a nursery in considerably warmer climate will not be as hardy as one that has been UK grown outside to that size and may take several years to develop its potential hardiness. Often, newly planted Chusan palms will initially produce what appear to be stunted leaves, certainly much shorter than those on it when it was loaded off the truck. It is part of the acclimatisation process and, after a couple of seasons and removal of the older, larger, softer growth, gives an indication that the palm has settled into its new position happily.