Thu, 10 Dec 2015
Dahlia 'Bishop of Landaff'
The attempts by the Met Office to provide long-range weather forecasting have been about as successful as the infamous conference of clairvoyants scheduled to be held in San Francisco in 1979. (The conference had to be cancelled owing to unforeseen circumstances.)
It’s a platitude of urban folklore that the best way of predicting tomorrow’s weather is that it will be similar to today’s. While that might hold water in the Atacama Desert (if there was any water to hold) it’s not a good axiom for our fast-changing temperate, maritime climate.
Perhaps I should differentiate between weather and climate. The terms are not interchangeable and accentuate the difference between long- term patterns of weather in a particular area (climate) and the day-to-day meteorological occurrences (weather). Thus one could say that the Atacama has an arid climate, but on the day I visited, the weather was wet. Plants respond to both weather and climate. One frost (cold weather) can blacken the dahlia foliage, but a succession of milder winters (climate) can allow the tubers to survive in the garden soil. When, as a child, I used to help my father in his Hampshire garden during the 1950s, it was an annual ritual to dig up the dahlia tubers.
After the first frost had killed the flowers, the stems were cut down to six inches above the soil and the large fleshy tubers lifted with a prong. It was my job to gently remove the soil from these subterranean, fat-fingered hands and then, as a reward, squirt them clean with water from a hose pinched-narrow by my own small digits. The tubers were then stored upside down until they were dry and dusted with flowers of sulphur, before being wrapped in newspaper and secreted in the loft. The following spring, they were liberated from the dark, spider-infested vault of the roof and unwrapped. They emerged looking like a collection of desiccated body parts, such as might hang from a witch doctor’s waistband.
Even after all that trouble, some of them did not re-emerge from the soil after their careful replanting in the late spring. However, for the past four or five years, I’ve had spectacular displays of individual dahlias among my rhododendrons and in the herbaceous flower border. I haven’t done any of the things mentioned above. The tubers stay in the soil, overwinter and emerge in early summer to push up through the shrubs and herbaceous plants before flowering in the late autumn. So what’s going on?
Why were Dad’s dahlias so tender and mine so hardy? The answer is not that the plants have changed, although over many years a population of wild plants will show minor adaptations to local conditions. The answer is that lately we have not had the severe ground-penetrating frosts that we used to get regularly in the middle of the last century.
So we can deduce that our climate in southern England has changed a bit over a half-century or so; but that’s not to say that cold weather won’t kill my dahlia tubers this winter if we get a hard one. Indeed, it’s not the just the survival of a plant that weather can call into question.
It’s often its performance as a garden-worthy subject. Consider apple trees. They tolerate our climate well but certain cultivars require a sustained period of cold weather to induce winter dormancy. Thus a mild winter can actually reduce cropping, by persuading the apple trees not to open all of their flowers in the spring.
In places where harsh winters are followed by definite thaws and reliable springs, the temperature starts to rise and it doesn’t regress below zero again until autumn. Thus, any tender leaves and flowers know that they are safe to emerge as soon as the thermometer increases to their requisite value. Not so in the UK. Particularly cruel is the year which exhibits a warm spell in late March, followed by harsh frost in April or even May.
That is the year of poor crops of fruits and hedgerow berries, or lost runner beans and ruined courgettes. Even some native plants will be hit by this malevolent combination. It’s akin to inviting soldiers to surrender and then shooting them when they climb out of their protective trenches.
The beautiful Persian silk tree (Albizia julibrissin) is an example of an otherwise hardy tree that will not grow in most of the UK, because of our stop-start springs. A long, hot summer will induce the normally flower-shy Staphylea colchica to believe that it has been moved back to its native Georgia. The following spring will see it spectacularly floriferous, but in a normal year it is a refractory species.
So should we be tempted by our period of milder winters to try a few of the marginally hardy plants such as fuchsias in an outside environment? Once again it comes down to clever gardening. The most familiar method is to use the microclimate created by a wall. A south-facing wall will reproduce the hot, dry root conditions needed by shrubs such as Fremontodendron californicum and, in addition, the wall of a house will be slightly warmer than the ambient temperature with the central heating running during the winter.
Sometimes it’s those marginal differences that will allow your plant to survive and flourish, happy in its microclimate and protected from the occasional extreme weather. Shelter from cold wind is another valuable aid and the overhead cover of rhododendrons is probably what protects my dahlias, but I must accept the fact that one hard winter could be their downfall. So you pays your money to the garden centre and you takes your choice. One thing for sure, nobody from the Met Office is going to know the future.