Read previous diary entries here.
Crunching frost last night, -6°C and following a -3°C frost the night before. We are promised a frost-free night tonight which is just as well. When we get deep frosts this early in the winter they can be damaging, particularly if they last for more than a few nights, as was the case in 2010.
When the soil profile and the glasshouse plunges freeze, the roots do too, often with fatal results. I have given up heating the alpine houses but, if a deep frost lingers for more than a few days, I may submit and move some of the more susceptible subjects to the conservatory which never drops below freezing. We will see. The problem with this garden in particular is that it lies on a north slope in the north of England, with a wood on top of the slope. This means that the sun never really reaches the garden for three months or more. On the other side of the valley, they bask in the low rays on what is, in fact, a lovely day but here we remain stubbornly frozen until the weather itself decides to relent.
Doubtless, this frost will bring down the remainder of the leaves. The large Magnolia soulangeana and the pollarded Liriodendron are always the last to fall in what has been a lovely autumn for colour. A few leaves still remain on the Acer griseum, which creates a jewelled pattern on the ground where the leaves fall.
The white-berried Sorbus are always the last to be taken by the birds and some berries still remain on S. cashmeriana and S. glabrescens. This year, the latter has kept its lovely butter-yellow leaves longer, to blend with the berries. Curiously, of the two sister seedlings of S. microphylla (originating from near Khumbu), the berries went on one last month but the other still carries all its fruit, next to a lovely Fothergilla.
Talking of berries, this is perhaps the best place to highlight the merits of that increasingly popular shrub, Callicarpa x bodnieri. I know that many people heartily dislike the brilliant and distinctive colour of the berries. I think they make a marvellous foil for other colours of leaves and berries in an arrangement and would not be without it.
Talking of the 2010 early freeze, this was one of many subjects that I lost then, and I definitely put its replacement in quite the wrong place: in far too much shade and root competition in summer. I finally got round to moving it into new soil and better light two autumns ago, and this forgiving plant has now recovered and is showing its pleasure in its new surroundings.
Before we leave autumnal subjects, a brief paean of praise for our wonderful Parrotia by the front door. We planted this about 25 years ago and it is now a superb mature individual, mascular and craggy. We have to prune it to shape and have lowered the height by a couple of metres, but it is well worth any trouble for its long-lasting autumn display.
I want to devote the greater part of this contribution to a short essay on a subject on which I have given some thought, which may help me at any rate to view my plants in a slightly different way. The question ‘how long does a plant live?’ does not have a straightforward answer in many cases, as we shall see. To start with some definitions, possibly uncomfortable to the ageing cohort of alpine gardeners, a life-span starts with the germination of a seed which grows into a genetically unique individual and ends when that genotype no longer survives. We see this in its simplest form when an annual, or biennial, germinates, flowers, sets seed and dies. Such plants never live much more than a year, usually much less.
Regarding perennials, most are capable of vegetative reproduction of some kind, rhizomes, stolons, dividing corms, tubers or bulbs, so that if and when a flowering shoot dies, resting buds survive to live another year. By using these buds as propagules (division etc.), the gardener may well lengthen the life of the genetic individual (born from a seed) by planting into new soil, escaping pathogens and introducing nutrients. I have been trying to think of perennials which lack means of vegetative propagation, and there are some; some lewisias for instance and monocarpic saxifrages like S. longifolia. Such plants have a finite life which is often ended by an expensive reproductive event (S. longifolia). In other cases, death may result from disease, predation, stress, or accident so that longevity may be unpredictable. And in some cases it may result from old age, which is the central topic here (see why it is occupying my mind!).
The life-span of all multicellular individuals is finite and senility results from an accumulation in a cell-line of disadvantageous mutations in the DNA, telomere shortening so that chromosomes become physically damaged, pathogens and (controversially) ‘programmed cell death’.
All of these accumulated hazards are overcome by the miracle of meiosis, the reductional cell division that results in gametes which fuse (sex) to give a new, ‘clean’ individual, always (in higher plants) starting as a seed. The new seedling may carry recessive mutants but will have shed most, if not all, pathogens and major age-related damage to the DNA and chromosomes. Note, however, that not all new seedlings result from sex. They don’t in dandelions, hawkweeds or most brambles and whitebeams. In those cases, the new seedling has been ‘cleaned’ of disease, but not of mutations.
But what if the new ‘individual’ comes not from a seed, but from a resting bud, a stolon, a division, a bulb or a cutting? It will carry all the mutational and pathogenic problems that have accumulated during the life of that genetic individual. Almost invariably, we find that old, ‘heritage’ clones are less vigorous than newly developed clones. If you look at the history of primroses, auriculas, Porophyllum saxifrage hybrids or any other garden group in which clones have been maintained by cuttings or division over many years, the story is the same. The older clones suffer from ‘old age’.
Curiously, this effect is not even. We all know of a few old clones (Saxifraga ‘Gregor Mendel’ comes to mind) which have ‘broken the glass ceiling’ and still flourish in our gardens, more than a century after the cross was made (the individual is more than one hundred years old). What allows such old clones to flourish when so many thousand have fallen by the wayside and fulfilled their life-span (as the asexual offspring become weaker and weaker) is not clear. However, they are very much in the minority.
Perhaps it is helpful to consider a plant such as creeping buttercup (Ranunculus repens) which is continuously producing a network of runners (stolons), each of which ends in a rosette. Every time a runner forms, it carries the possibility of having acquired a new mutation or pathogen, so that the population of rosettes which have arisen from a single original seedling will come to vary in the ‘load’ of disadvantageous inheritance that it carries. Some will become less fit than others so that, in time, natural selection will come to work on this population of rosettes which are still technically the same individual. In the end, all will die but some will be much more long-lived than others.
If we now address clonal trees such as Populus alba (White Poplar) or Tilia cordata (Small-leaved Lime) which are known to live to a great age and can, in the case of the poplar, cover many hectares of ground, we see the same effect working on a much larger and longer scale. However, surviving parts of an individual which came from a seed thousands of years previously will probably represent a tiny minority of the myriad shoots which over time have acquired their own ‘load’, finally with terminal results.
The same argument applies to less obviously clonal trees such as yew (Taxus baccata) or Bristle-Cone (Pinus longaeva). These can also live for thousands of years but if you visit these impressive survivors, it becomes clear that almost all the tree has died over its lifetime. Only a few, genetically fortunate, chunks have persisted.
Finally, let’s consider how these principles apply to alpines. Clearly, whether in the wild or in the garden, many are programmed to live a fairly short life e.g. members of the Androsace carnea group. However, there is no doubt that reproduction plays a very important part in this. In the wild, small plants of Primula scotica that were born from small seeds may never flower, or take many years to do so, but may survive for many decades. A larger plant, germinating from a large seed, may flower in its second year, set lots of seed and die. If you want your blue poppy (Meconopsis baileyi or M. grandis) to perennate, stop it setting seed. If your blue poppy is a hybrid (‘Sterile Blue Group’) it will set no seed and be much more likely to overwinter successfully. Selfing species of Epipactis (helleborine orchids) live several years before flowering for the first time, after which they almost always die having set every capsule. Outcrossing species such as E. helleborine may flower several times if they fail to attract a wasp and set seed.
If we turn to those immensely old cushions of New Zealand; Raoulia, Argentinian Verbenaceae or Himalayan Arenaria, it is likely that they will collect fewer mutants over unit time because they grow so slowly, and the severe climate will be inimical to many pathogens. Nevertheless, the same principles apply and the oldest plants show patches, gaps and die-back as some parts of the individual are genetically fitter and survive better than others.
A final illustration as a bone for those who struggled this far! Meconopsis paniculata, more lovely in winter than summer, and a good eample of a plant which is programmed to die as soon as it reproduces.
John Richards has lived in south Northumberland for 50 years and has been a keen grower of alpines, and visitor to the mountains, throughout the half-century. He and his wife Sheila have been in their present garden 30 years and John has been writing this diary about the garden since 2006.
John is Emeritus Professor of Botany at the University of Newcastle, where his research interests centre on the evolution and genetics of plant breeding systems. He is an authority on the genera Primula and Taraxacum (dandelions!). John is an AGS judge, exhibitor and Vice-President and previous President (2003-6).