THE GARDEN AT SOUTHERN COMFORT
Hardiness and climatic zones
If a plant grows well within a particular locality it can be said to be hardy in that locality. The hardiness of a plant is therefore defined as being relative to a locality and, as we all know, each plant must be hardy somewhere because, otherwise, it would already be extinct.
The conditions affecting the winter hardiness of plants are various. Temperature and variations of temperature alone, despite popular views, are not the only or even the most important factors affecting a plant's success in a particular locality. Other conditions include the following: soil type (acid/alkaline, well-drained/waterlogged, damp/dry), wind, air type (humid/dry, clean/smoky, salty etc), appropriate rainfall, conditions during the previous summer (hot/cool, dry/damp), the degree to which it may have been 'clobbered' during the previous winter, etc.. It is therefore clear that the winter hardiness of a plant is its reaction to the full gamut of conditions in which it grows.
In many cases we can 'increase' the degree of winter hardiness of plants by carefully siting them and providing them with the right type of soil. For example, an Agave americana poked into muddy soil next to a north-facing wall is unlikely to flourish. It may, however, succeed in the the same place but planted in very gravelly soil or even in gravel itself (provided it is appropriately fed during the growing period). It is better still to plant it next to a south-facing wall where it will happily grow and flourish even in relatively heavy soil - muddy soil may be pushing luck to its extreme.
Generally speaking, so called 'tender' plants from areas of low winter and high summer rainfall stand a very much greater chance of success if grown in sharply drained-soil where they will keep as dry as possible during the colder months. This is recommended for cacti and agaves. Even better if they can also be placed in the rain-shadow or an overhang of a house roof or even a wall.
It has been observed by many gardeners that hot, dry summers 'ripen' some plants in such a way that they become more resistant during the following winter. This is particularly true of 'woody' plant material which during cool, moist summers tends to become 'watery'; these watery stems and shoots are more prone to freezing and rotting especially during mild damp winters. It has also become clear to me that over time occasional harsh winters give plants at least an opportunity to recover before being 'hit' again. This is because some plants need more than one season to make up damaged and lost growth - such plants are bound to fail after a succession of moderately harsh winters even when they may not fail after only one possibly quite harsh winter.
For some thirty or so years I have gardened first in West Sussex (on London clay) and then in Torbay. In both places it has not escaped my notice that during winter a greater enemy than low temperatures is rot caused by mild and humid winters - cold winters are generally associated with drier air and therefore are often kinder to plants. In fact, alpine houses (cool greenhouses) were designed to keep plants from rotting and not to keep them warm. In their natural habitat such plants remain bone dry all winter under a protective layer of snow in temperatures well below zero degrees centigrade.
When talking of the hardiness of plants it is easy to to lose sight of the plant itself and think only about the conditions the plant requires. In other words, gardeners ought to be aware that there may be different strains of the same species with differing levels of hardiness. This is true of succulents such as aloes and agaves and probably of a good many other genera. The provenance of a plant is probably a good indicator of the type of strain; eucalyptus trees naturally growing in harsher, mountain areas are reputed to be more resistant to cold than the same species growing at lower, milder altitudes. Another factor here is age of plant. The older the plant the more resistant it is to cold. The explantation for this is schoolboy physics: the larger the mass the longer it takes for its temperature to drop, and often before the crucial tissues become damaged by a severe night frost the temperature begins to rise again the following morning.
Taking climatic zones into account when choosing plants is a wise thing to do but, as I have explained, this is not enough. It is just as important, perhaps more so, to look carefully at soil drainage and winter wet in particular. When water freezes it expands significantly and in doing so can both rupture plant cells and cause movement in the soil which in turn can damage delicate roots and stems. If this does not send a vulnerable plant on the path to oblivion then the subsequent attack by mould and fungal spores when the air warms up again most certainly will. In dry, well-drained soil these effects are greatly reduced.
Paradoxically, it is also true that water may protect certain plants. Arum lilies, Zantedeschia, are an example. When these are planted well below the surface of a pond they are unlikely to be killed by freezing whereas planted in a bed of drier soil they might be more at risk. However, this explanation is not fully satisfactory as Christopher Lloyd points out in his account of the exceptionally harsh winter of 1962-3*.
(* Christopher Lloyd, The Well-Tempered Garden, Penguin Books, 1987 p.156)
The hardiness zone scale was introduced by the USDA (United States Department of Agriculture) with the aim of systematising the lowest temperatures at which given plant species survive. The temperatures within a particular zone are the average minima and not the real range of temperatures; this means that within that zone there will on occasion be lower temperatures. The scale is therefore only a guide.
It is often easy to forget that the hardiness zone scale does not indicate whether a particular species will thrive in a zone where it is winter hardy. In order for a plant to be totally successful it must have, in addition the conditions mentioned above, the appropriate degree of summer heat and a suitable light level and length of day. This is the weakest point of the climate in the south-west of the U.K., where temperatures seldom rise above 30°C in summer and latitude dictates lower light levels and longer days in summer (as well as comparatively low light levels and short days during the colder seasons). Put in another way, there are many plants which may be fully winter hardy in a particular zone but not worth growing because of their high requirements for summer heat and light.
It is accepted amongst gardeners in Torbay that the coastal strip falls into zone 9b. A short distance inland it falls to zone 9a. However, judging by the severity of the very short (one night) spell which in exposed sites may have reached as low as -6°C (almost 8b level!) it may be wise to apply zone 9a to the area in order to minimise losses. I for one, however, am happy to take a degree of risk and will continue to experiment with zone 9b plants.
Links related to plant hardiness
http://www.rhs.org.uk/Plants/RHS-Publications/Journals/The-Plantsman/2007-issues/September/halfhardytrees The first of two, this excellent article by Owen Johnson in 'The Plantsman' is about growing plants at the limits of their hardiness.
http://www.rhs.org.uk/Plants/RHS-Publications/Journals/The-Plantsman/2007-issues/December/halfhardytrees The second article about trees at the limits of their hardiness.
http://www.trebrown.com/hrdzone.html See a map of the UK indicating the hardiness zones - from Trebrown Nurseries.
http://www.trebrown.com/articles/blog/?p=24 Climate adaptedness in palms - from Trebrown Nurseries. A bold attempt at explaining why palms adapt to new geographical locations (this could be applied to all plants).