Noel Kingsbury: how my PhD started my passion for perennials

In 2007 I completed my research doctorate at Sheffield. When I had started, sometime five years previously I remember very clearly being asked (in fact by the garden historian David Jacques) why I was doing it. I can’t remember the answer, but I probably said something about the need to give myself a bit more credibility (as I had no formal qualification in horticulture or landscape or anything relevant) and almost certainly something about the need to apply some basic ecological science to horticulture and planting design.

Here, I would like to tell something of why I did the PhD at Sheffield and put a personal story in the context of a wider movement (in using naturalistic planting) and an on-going story (how we make naturalistic planting that works).

Some time in the mid 1990s I had read Plant Strategies and Vegetation Processes by J. Philip Grime, at Sheffield in the Unit of Comparative Plant Ecology and so was introduced to CSR theory - CSR stands for Competitive/Stress-tolerant/Ruderal; and (in a nutshell) refers to a way of looking at the ecological behaviour of plants in terms of evolved strategies for their survival and reproduction through the expression of genetic traits. A bright light dawned. So much of what happens in gardening made sense, and also crucially, why so much of what happens in conventional gardening does not make sense. This was at a time when a loose group of people was just starting the ‘new perennial’ movement, and it was a particularly fruitful time to be questioning anything and everything in the garden and landscape world. The concept of the ‘ruderal’ for example - pioneer plants which start to grow from seed in huge numbers when ground is disturbed – which of course happens every time you hoe it; in other words this key, old-style, garden maintenance technique was providing a perfect environment, again and again, for weeds. The clear lesson was - old-style gardening was actually making work for itself. Instead, we needed to learn new techniques which used plant knowledge to reduce maintenance.

The selection of appropriate stress-tolerant species for difficult habitats had been made familiar by Beth Chatto and others, but the concept of ‘competitor’ however was a more novel concept, especially in terms of gardening. Looking around at planting design in the 1990s, it was clear that competitors, stress-tolerators and ruderals (which from now on I will call by the much more easily-understood term ‘pioneers’) were being mixed and muddled. If you were interested in what the then very influential Christopher Lloyd unapologetically called ‘high maintenance gardening’ then that was no problem, but for larger scale planting, it was clear that if we had a basis for choosing and combine plants other than the purely aesthetic or the rather basic sun/shade, moist/dry information of the standard garden reference book, then we might be able to reduce and simplify management. This was a time at which public planting in Britain was being decimated by government cut-backs and privatisation – all a part of the Thatcher government’s wider neo-liberal agenda of cutting back on public services. The dream was that, if we could produce simplified, science-led, planting design we could help do something for hard-pressed local government parks managers, as well as for ambitious and innovative private gardeners and designers.

So, an initial impetus behind my PhD project and a key part of its early stages, was undertaking the array of tests on garden perennials which the Grime work had used on wild plants. Leaves were scanned on a flat-bed scanner and their surface area calculated using a formula available online, plants dried in an oven to weigh dry matter content, etc, etc. All rather bizarre by the standards of some fellow garden people – although much came from my own garden or those of clients’, I had to beg some plant material off friends and colleagues, and there were a few raised eyebrows about the fate of material they gave me.

The hope of using the outcomes of a CSR evaluation for an advanced system of plant selection proved illusory. The real stress-tolerators were all pretty obvious to any moderately experienced gardener anyway: the silvery leaves, slight succulence, or the dark glossy evergreen foliage of plants from resource-poor environments were clues which were easy to recognise, and looking to the future of my work, easy to teach. Most garden perennials though are generalists and come out of the CSR tests as competitors, and this evaluation told us nothing more. The concept though has proved a very useful one in a more general sense in recognising that certain habitats are best planted with certain suites of plants, and that plant combinations based on selecting on a broad CSR basis can be looked after with a simplified maintenance regime. Particularly useful was the flagging up of R – ruderal/pioneers, as being fast-growing, short-lived, and often reproducing prolifically from seed; combinations made with them needing to take into account these factors, which have a marked impact on the longevity of any planting done with them.

My PhD research went on to look at competition and to map plant growth over the course of a growing season. I developed an idea that I could produce a set of categories for the perennials used in gardens and other designed landscapes. Whereas trees and shrubs show a relatively limited range of growth habits, perennials show a veritable miasma of very varying forms and habits. With a rapid growth in interest in using perennials in gardens, it seemed to me that a set of categories could be useful, particularly to professional landscape designers.

Clear, or even not-very-clear-but-workable categories did not appear during the research however. Nature after all, does not like hard and fast distinctions, the clarity with which we humans need to divide the world for the purposes of language being an alien imposition. What was more useful was the recognition of a series of gradients, a concept which the Dutch philosopher Rob Leopold promoted; Rob was a prominent part of the naturalistic planting movement until his death some ten years ago; his work on annual mixtures inspired Nigel Dnnett to begin work on his.

A key gradient I decided, was longevity. One thing had became very apparent in my research – that the words (words, you see: crude, oversimplifying) used to describe herbaceous plants lifespans: annuals, biennials and perennials, were grossly inadequate. There appeared to be a whole gradient of lifespans, and that these were the outcomes of genetic traits. In particular ‘perennial’ does not necessarily mean that a species goes on for ever (as the RHS Dictionary seems to think). Some perennials can clearly spread themselves in such a manner that they could, if circumstances are right, go on to infinity (a fact which some more nervous amateur gardeners easily recognise). Many others however showed no such tendencies and in fact would be more accurately described as ‘short-lived perennials’. Many are very popular as garden plants, such as Echinacea purpurea.

The lifespan gradient is in fact key to the gradient between ruderal and competitor. Both of these strategies involve a variety of other gradients, largely concerned with growth habits and vegetative propagation, the appreciation of which I believe is the key to understanding plant performance and which can help a great deal in making improved decisions in plant selection and management. I was lucky, in that after I finished by PhD, I was able to get some funding from a European Union funded research project the Department of Landscape were involved with: Interreg IVb Making Places Profitable – Public and Private open spaces. This neatly triangulated my research, as I was able to undertake a survey of experienced gardeners (amateur and professional) which confirmed much of the outcomes of the PhD work and did not (thankfully) throw up any surprises.

The ‘output’ of this work has been that I now have a much better knowledge of the behaviour and performance, which I have been able to disseminate through books and magazine articles, as well as a series of workshops run for all sorts of plant users. The workshop’s reputation as unlocking a better understanding of plants as materials for design professionals has spread far and wide, over the last year for example I have delivered it in Kiev, Prague, Upper Austria, Edinburgh, Longwood Gardens (Pennsylvania) as well as several locations nearer home. Most importantly perhaps, the workshop’s methodology is designed to enable participants to look at the plant species they use with a critical eye, and make their own discoveries about performance.

In the future I would like to extend my work on perennials to look in more detail at shrubs, in particular at sub-shrubs, which are so important in designed landscapes, and to work with colleagues in very different geographical and climate zones, where the range of species (often locally native) may offer very different variants on the themes we are familiar with in Europe. Some practitioners in Argentina and Uruguay have proved particularly receptive and it has been immensely satisfying to help mentor colleagues there. Grime’s work, initially aimed at understanding natural and semi-natural plant communities, is proving very useful in some surprising ways and places.