The Hardiness Myth

A reminder that winter is just around the corner, Gary sets a few myths about cold-hardiness to rest. Nice pics kindly provided by Helen & Wilko from Holland. Brrrrr!
Gary Parker, Surrey, UK
Chamaerops No. 19, published online 23-07-2002

Winter draws on. Pics by Helen Lewis & Wilco Karmelk, Holland

I used to worry about hardiness. I would turn pale every time I read the RHS Gardeners' Encyclopaedia of Plants and Flowers because the experts were quite certain that most of the plants in my garden could not be grown outdoors in Britain. Luckily, plants can't read.

I feel it is time to stamp out the myths surrounding hardiness. The relationship between plant survival and climate is complex and cannot be represented by notional minimum temperatures. Even if it could, the commonly-quoted hardiness figures for many plants are just plain wrong.

Hardiness for many garden plants was defined during the great plant-hunting era of the eighteenth and nineteenth centuries, which roughly coincided with the 'Little Ice Age' stretching from the middle ages to the midnineteenth century. During those years, the river Thames would freeze so solid that fairs were held on it, complete with tents, horses, carts and thousands of people. Clearly, those winters were in a different league to modern winters - yet decisions made then about hardiness are followed to this day.

Palms in many areas of the world blithely ignore the minimum temperatures assigned to them. For example, in the cool South Island of New Zealand, palms such as Phoenix canariensis and the striking native Rhopalostylis sapida grow in areas that have over 40 winter frosts each year (almost twice as many as my garden in Surrey). How do the palms do it? The answer is that although winter nights can be colder than British ones due to the absence of a warm 'gulf stream', winter days are slightly more likely to rise above freezing point since the average annual temperature is fractionally higher. Also, leaves toughened by the somewhat sunnier summers are full of 'anti-freeze' in the form of tannins and lignin and withstand frosts better.

For anyone wishing to grow Rhopalostylis sapida outdoors in Britain, the 'Oceana' subspecies, occasionally available in the UK, is worth a try. It grows on the Chatham Islands, closer to Antarctica than any other natural palm population. However, these islands are actually quite mild and frost-free due to the moderating effect of ocean -just like Britain's Scilly Isles. The very toughest specimens of Rhopalostylis sapida live on the South Island, near the town of Christchurch on Banks Peninsula. These palms experience more frost despite being fractionally closer to the equator. The records show that in climatic terms Christchurch is uncannily similar to London. I just cannot believe these palms wouldn't thrive outdoors in southern Britain, so if anyone happens to visit New Zealand - seeds, please!

Plants need to photosynthesize to grow, and photosynthesis is dependent on temperature. Grass, for example, only photosynthesizes when the temperature is above 7¾C (luckily for us, otherwise we'd be mowing the wretched stuff in winter!) Some alpine plants begin to photosynthesize at just 2¾C, but at the other extreme, plants in California's Death Valley only reach peak photosynthesis at a blistering 48¾C. That could explain why some tough desert palms that withstand European winters cannot be considered successfully hardy: they fail to put on adequate growth in summer because it is seldom hot enough for full photosynthesis.

Many supposedly non-hardy plants have tough wood or roots, so the plants simply become deciduous or perennial. Of course, palms can hardly afford to lose all their leaves, but even so, adult palms are tougher than young palms. Professional landscapers in California say that mature palms are far hardier than immature ones, and are able to survive frosts and snow in the mountainous regions of northern California because the cells in the outer layer of their trunks, if killed by cold, remain to insulate the living inner cells. So if in doubt - plant a big 'un!

Finally, if all else fails, consider this: is a plant really unsuitable for outdoor use simply because it might not survive indefinitely? I used to feel that a plant death would be a disaster, but my thinking has changed since talking to garden owners in New Jersey and Pennsylvania. There, they grow traditional English garden plants, but their winters are much colder than ours and so many English plants are not reliably hardy. Every gardener loses a shrub or two each winter, but these 'snow kills' are simply replaced each spring as part of the normal gardening routine. Perhaps we should think a little more along those lines, at least when it comes to the less expensive plants. Why not enjoy them for a few years (or a few decades, global warming permitting!) and then enjoy replacing them with something new when a bitter winter finally finishes them off?

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