A Northumberland Alpine Gardener's Diary
This entry: 09 November 2008 by John Richards
Northumberland Diary. Entry 97.
What does 'hardy' mean?
This topic is one of the oldest chestnuts in gardening, but I have been mulling it over in my mind since reading a comment in Chris and Lorraine's excellent article on Rhodohypoxis in the latest issue of the 'Alpine Gardener' (76: 334). In this, they note with surprise a comment in an earlier Show Report that inferred that Rhodohypoxis might not be hardy. I have a horrid suspicion that I might have been responsible for this comment, not that I remember this, or have checked the reference, but just that it is the type of thing I might have written!
Last January I visited Rhodohypoxis baurii at its well-known location at the summit of the Sani Pass in the South African (well, Lesotho) Drakensberg. Even in summer this is a cold and dreich location, and I am sure that reports that it survives temperatures well below zero for many months, often without snow cover, are quite correct. However, this area suffers very cold, but dry winter conditions, and most rhodohypoxis are very intolerant of cultivation outside, at least in Northumberland. Even when plants are covered by a cloche, they often rot off. So here is a species that is undoubtedly cold-hardy, but is not winter-hardy in our conditions.
Incidentally, the same is true of many subjects from the Drakensberg. I think it is very significant that many good, hardy, garden plants from South Africa, Hesperis (Schizostylis) coccinea, Galtonia candicans, kniphofias, dieramas etc come from very wet habitats that never dry out even in Drakensberg winters. Accustomed to winter wet, they survive our winters happily without protection. Many others, diascias, suteras and jamesbrittenias, delospermas, crassulas etc, not to mention rhodohypoxis, will often survive in very well-drained spots, troughs, raised beds etc, or if cuttings are taken into a cold frame or under cold glass, but dislike our freeze-thaw cycles when unprotected.
Before I continue, here is a photo of Rhodohypoxis baurii at Sani Top.
There are of course many reasons why plants are cold-hardy, but not winter hardy in British gardens. Perhaps the commonest is that mountain plants are adapted to being covered with snow, or reliably frozen, during the winter months when they are dormant. In these conditions they remain relatively dry, even when encased in ice, so that they become susceptible to fungal rots if they become wet while dormant. In these cases, it is possible to protect pot-grown plants under cold glass or in frames, while in the garden they can be covered with frame-lights or cloches.
The South African Drakensberg is winter dry but summer wet. However, there are other mountains with a low rainfall throughout the year. Once again, plants from such areas may be cold hardy but not winter hardy in Britain, and have to be protected from the damp throughout the year. Dionysias are the classic example, but I am featuring here Primula simensis, one of the Sphondylia section of primula that are related to Dionysia. This is the only species of primula to grow in the mountains of Ethiopia. In common with its relatives, it is principally winter-flowering. It is closely related to the Arabian P. verticillata, with which it used to be classified. However it has whiter foliage of a different shape, and shorter corolla tubes. This species has certainly survived -10c under glass if kept almost dry in winter, but it would immediately turn to mush if grown outside.
Yet another class of plants are those that are winter hardy, but which are scarcely worth growing outside because the flowers rapidly become spoilt in our winter conditions. This is true of many bulbs in particular. For instance, I grow reticulata iris hybrids such as 'Kathleen Hodgkin', 'George', Harmony' and 'Cantab' in the garden and in pots under glass. The former grow and flower reliably over many seasons, and multiply, but often the flowers become spoilt by rain, mud, molluscs or birds almost as soon as they are through the ground. Under glass they last for weeks! Yet snowdrops, aconites and some crocus remain in good condition whatever the weather. These are truly hardy!
One of the ironic things about hardiness is how difficult it is to predict garden hardiness from the wild locations of plants. For instance, there are many examples of plants that never experience frost in the wild, but are perfectly hardy in British gardens. A classic example is the Brazilian megaherb Gunnera manicata, said to be the largest herbaceous plant in the world. Doubtless the herbaceous habit helps to protect this plant from winter cold, particularly if the giant leaf remnants are folded over the vulnerable crowns. A most unexpectedly hardy 'alpine' is the beautiful Paeonia cambessedesii, which never encounters frost on its Mallorcan cliff-ledges, often just above the sea.
Yet, the high alpine Gentiana depressa from above 4000 m in high rainfall regions of the eastern Himalaya is notoriously frost-sensitive, even when kept dry under glass. Coming from a high snow-fall area, it may never be exposed to subzero temperatures in its lofty locations.
Another very important point is that a plant that is hardy in one part of the UK, may not be hardy in a colder, or wetter, region. A good example here is Convolvulus cneorum, so popular in many southern gardens, which we have never been able to over winter outside. Also, we find that many plants that will thrive outside in the UK will be unable to tolerate many zones of the US, or central Europe. This can have benefits for growers there. Many plants that we cannot overwinter outside in the UK without protection, will do so in central Europe for instance, especially some of the aretian androsaces, and plants like Jancaea heldreichii. Here is a photo of the latter taken on Olimbos last spring.
To summarise, I think that the word 'hardy' should usually be qualified. Our transatlantic friends have long been used to saying that a plant is hardy to a particular climatic zone 'hardy to zone 8' for instance, and in a country that encompasses subtropical Florida, desert regions of Nevada, temperate maritime coastal Washington State or the highlands of Colorado, this is understandable. The Mid-West, with its baking summers and freezing winters, is particularly unsympathetic to the gardener.
I also think it is helpful to say if a plant is 'garden hardy' (i.e. you can leave it outside unprotected), or 'cold hardy' which infers that it needs to be protected under cold glass. To return to the original subject, rhodohypoxis are cold hardy, but they are mostly not garden hardy.
This is proving to be a long-drawn out autumn, which is posing particular problems for leaf clearing. Many leaves have now been on the ground for a month, yet there are still many to fall. As I write, a cold wet gale is promising to remove the remainder. I doubt if we will be able to plant the tulips today! Acer capillipes is good at the moment. The name means 'hair foot', perhaps for the long slender petioles to the leaves.
I have occasionally mentioned our 130 m beech hedge, usually to moan about the effort, or latterly expense, in having it cut. But a neat boundary makes a wonderful framework in which to set the garden, and when it colours golden as it has at the moment, it is a beautiful sight. Here is part of the hedge from the lane, outside the garden.