Plants and Planting on Landscape Sites

Selection and Supervision

Peter Ralph Thoday

• Publisher: CAB International
• Year: 2016
• ISBN:
  9781786390752
• Topic: Biological Sciences
• Subtopic: Horticulture
• Index: Biological Sciences

Landscape architects, design professionals and contractors alike require a good working knowledge of how to achieve plant establishment under a variety of conditions and situations. Overlooking the physiological needs of plants can lead to potential problems that can have negative financial and design impacts. Plants and Planting on Landscape Sites is a practical book giving practitioners in landscape design the essential horticultural knowledge and concepts needed to understand the limits of the material they are working with and make informed decisions.From specification to supervision, this book provides concrete advice along with practical examples for each stage of a typical project. It contains sections on: the landscape site; selecting, assessing and purchasing plants; understanding nursery practice; forms and types of transplant traded; seeds and direct seeding; pre-planting site work; transplanting; and care in the establishment phase. Specially commissioned high quality line diagrams and full colour photographs are used throughout to demonstrate meaning and give examples.Peter Thoday is an experienced consultant, international lecturer in landscape management, and past president of The Institute of Horticulture, who has had numerous roles in high-profile projects, such as Horticultural Director of the Eden Project. Written by an expert, this book is as an essential tool for landscape architects, project managers, contractors and nursery managers.

Introduction 1

Plants play a vital part in the ambience of the vast majority of landscape schemes, ranging from roads to business parks, schools and hospitals. The ‘green component’ is, at times, seen by developers primarily as a planning requirement but by the public as contributing to the attractiveness of the scheme, a quality future occupiers hope will enhance their image.

In spite of such approval, there remain many examples of poor development site plantings that fail to achieve their potential, many of them set within excellent overall architectural and landscape designs. Failure in this context very rarely means the death of all the plants. It is usually either the death or very poor condition of sufficient individual plants to ruin a planting’s composition, or that growth is so compromised that it takes years, if ever, for the specimen to achieve the size and form intended. Richmond Dutton (1991) in his research at Liverpool University found that while a few such failures were due to poor design detail, and rather more to inappropriate plant selection, the majority resulted from poor work at either the capital works or the maintenance stage. Clearly, gross malpractice leads to failure but this is rarely the cause; usually it is the omission or poor execution of husbandry tasks that, while apparently very simple, are in fact quite critical. Often this can be traced not to the ignorance or incompetence of the workers on site but to horticultural work and, in particular, soil handling being attempted in conditions that no one in any other branch of agronomy would contemplate working under. These circumstances are usually attributable to the dictates of the construction programme.

Concerns and responsibilities for the success of plantings start long before work on site and, as one senior landscape architect put it, can even include ‘the need to communicate what plants ‘are’ (i.e. that they are alive) to others within a design team’. It is equally important to ensure that the client understands what can be expected from the plantings – in terms both of their appearance at the completion of capital works and of how they will grow towards achieving the final design intent.

This book addresses those matters that arise during the preparation and planting of landscape sites. The success of such undertakings rests upon the creativity of the designer combining with the craftsmanship of the contractor. Both contributions are essential, and while the contractor is bound by the terms of the contract and the designer, in the guise of the supervising officer, is committed to act on behalf of the client, they share the common objective of creating a successful landscape. Within that challenge falls the selection and quality of the plant material, the site and its preparation, the planting operation and the management of the site during the establishment or ‘defects liability’ phase. It follows that when dealing with the soft landscape, those areas of soil within the design, designers must be sufficiently knowledgeable to recognize the advantages and limitations of the site’s immutable features together with both good-quality (and bad-quality) plants and good and bad horticultural practice.

In these pages an attempt is made to present a balance between providing some background insight through reference to plant and soil science and more directly applicable information on aspects of plant husbandry. In some cases decisions must made on the basis of local conditions and may even run contrary to what would otherwise be best practice; nevertheless it should be remembered that the fundamental aspects of plant biology remain both in play and unchanged.

Creating a living landscape requires at times a multidisciplinary approach, calling on the expertise, whether field or laboratory based, of, among others, drainage engineers, soil scientists, horticulturists and foresters. The more challenging the site the more such experts should be involved. Plantings are unique in that they are not, and cannot be, at their best when handed over to the client. A planting, unlike a hard landscape, takes months or years to deliver the designed intent. On completion of the capital works the responsibility to achieve and sustain that development passes to those who manage and maintain the planting.

Plants as Living Organisms

Basic requirements

Before considering the specific qualities required of landscape plants (see Chapter 5) we should recognize those characteristics that all plants share and that set them apart from all the other materials used in constructing a landscape or garden. To most readers it will seem unnecessary to point out that plants are living organisms and must be treated as such. It is not unknown, however, to find plants on site, both before and after planting, treated rather as if they were a stack of bricks; indeed the perishability of bags of cement may receive more understanding and care.

In common with all other life forms, plants require a number of conditions to support their basic metabolic processes and hence keep them alive, namely sufficient heat, light, water and nutrients. A plant’s tolerance of a shortage of such requirements may be more or less critical and varies from species to species.

•Heat: All plants require heat for basic metabolic processes but the level of high- and low-temperature tolerance varies greatly between species, producing various degrees of hardiness as discussed in the Hardiness section, page 48.

•Light: Light is the source of energy for photosynthesis upon which all green plants depend; however, there are big differences between the needs of shade-tolerant and sun-demanding species. Also, planting site light levels may be influenced by the shade of buildings and overhanging vegetation, a topic considered more fully in the Shade tolerance section, page 49.

•Moisture: Plant tissues dry out when water loss by evapo-transpiration exceeds water uptake. Impeded uptake occurs as a result of any one of the following: the root system is insufficient or dead; roots are exposed to the air; or the soil within the root zone is dry or not in contact with the roots. The application of water by irrigation is discussed in Irrigation at and after planting section, page 129.

•Nutrition: Although the availability of the elements required for plant growth is essential in the long term, their level is unlikely to influence transplant establishment. The timing of any additions will depend on the fertility of the site soil and the season in which transplanting takes place (see page 16, section on Soil fertility).

Requirement or tolerance?

Most cultivated plants have retained many of their evolved ecological responses to conditions that prevail in their natural habitats. Each environmental factor has an optimum level for the plant, at least in theory, although plants can tolerate some excess or deficiency albeit with reduced performance. Because few sites provide optimum conditions for a wide range of plants, the selection has to be made from those plants whose tolerances to such factors as water, light, soil pH or salinity fall within the ranges of local levels.

There is often a confusion between a plant’s essential requirement for various environmental factors and its tolerance of them. We may refer to a plant needing some environmental component, such as a chalky soil for beech trees, whereas in fact it simply tolerates it. Tolerance confers a significant ecological advantage because it allows a plant to colonize a site free from more sensitive or demanding species.

Strictly speaking, the environmental need of a species implies a condition required at a specific level to sustain a physiological process. It follows that in the wild the plant will be found only in locations that satisfy such a need. Obviously the same vital requirement holds true when the plant is in cultivation.

In the great gardens of the 19th century every effort was made to provide plants with ideal conditions, while the hoe looked after competitors. On today’s development sites we are more likely to utilize the ecological concept of tolerance of adverse conditions exhibited by what have become known as ‘tough’ species. Modifying a site in one or more ways prior to planting is, however, part of the programme required to achieve many designs (see page 108, Cultivations before and at the time of planting section).

Geographic origins

Today there is a heightened awareness of the possible ecological impact of amenity plantings, not all of it supported by research. As a result of such well-intentioned objectives, those involved may find that they are requested or even required to select plants with reference to one or more of the terms ‘population’, ‘native’, ‘exotic’, ‘provenance’ and ‘accession’.

The spread of a species in the wild is known as its natural distribution. Plants whose natural distribution falls entirely outside any given area are termed ‘exotic’ or ‘alien’. In Britain there are around 2400 ‘wild’ or ‘native’ species, the majority of which are also found wild in mainland Northern Europe; the remainder grow only in Britain and are said to be ‘endemic’ to this country. If a site falls within the natural distribution of a species that species is considered native even if it does not grow wild in the local area. For example, in Oxford, seakale (Crambe maritima) is technically a native (i.e. a member of the British flora) although it grows wild only on the coast. Species that grow wild in Oxfordshire are termed ‘locally native’. The distribution of a species is of course not controlled by national frontiers; however, there is a convention to speak of, for example, a ‘British’ or ‘French’ native species.

The term ‘provenance’ is used to indicate the geographical origin of a population of plants. Occasionally it is used to indicate the conditions under which the specimens were raised. When used by ecologists the term pinpoints the location from which the specimens, or more likely their wild ancestors, originated. A plant species from a particular location may have a distinct gene pool, in which case we may say that the population of the provenance forms an ‘ecotype’; see page 41, Ecotypes section.

It is commonly considered among ecologists that on rural sites only native species should be planted, even to the exclusion of their cultivars. The latter ban is particularly pertinent with agricultural crops such as white clover (Trifolium repens) or perennial rye grass (Lolium perenne), the man-made forms of which differ so greatly from the wild species or ‘type’ as it is termed. The avoidance of planting some non-native species is linked to two somewhat different concerns. One is the risk that they may produce ‘escapes’, i.e. feral offspring that become established in the surrounding area, as has Japanese knotweed (Fallopia japonica) in parts of Britain, whereas Buddleia (Buddleja davidii) has spread in North America, Australia and New Zealand as well as Europe. Pampas grass (Cortaderia spp.) has become a pernicious weed in Southern Europe and other warm temperate zones such as California and South Africa. The other concern is that escapes may pollinate the flowers of closely related wild species to produce hybrids and so ‘pollute’ the native flora.

The term ‘accession’, applied as a number or code, is used to identify any specific batch of plants or seed of a species collected at one time in one location.

Getting to Know Plants and their Cultivation

While some designers have a deep knowledge of plants, others, though they excel in wider aspects of landscape design, may have only a rudimentary understanding of plants as living organisms and the consequent demands that places on their care.

The plants that bring life to our public, institutional, commercial and industrial landscapes are at their most vulnerable between leaving the production nursery and becoming established within the landscape. The substantial plant science literature upon which much contemporary crop agronomy is based addresses such basic topics as soils, plant-breeding, taxonomy, arboriculture, ecology and plant physiology. This book cites appropriate research findings and recommends their incorporation into ‘good practice’ where they are transferable to amenity plantings.

Unfortunately there has been little in the way of experiment-based investigation or trials to assess plant behaviour under landscape site conditions. Such as there has been has focused on trees and as a result we have a good understanding of the common causes of poor growth and death among their transplants. Inevitably these studies have identified, brought into focus and blamed commonly encountered site conditions long known by plant scientists and agronomists to be detrimental to plants. For a detailed study of soil–plant interactions, the standard reference remains Soil Conditions and Plant Growth, originally from the pen of Sir John Russell (1912) but in its current 11th edition by Gregory and Northcliffe (2013); for an historical perspective, see Thoday (2007) Two Blades of Grass.

Anyone wishing to improve their understanding of the challenges faced in establishing amenity plantings will find their time visiting mature landscape schemes well spent. It may be more heart-lifting to visit a great garden, but peruse the local supermarket car park or industrial park and see which plants really succeed, which merely survive and which have failed – if you can find their remains! Such sojourns help one to recognize the quality of stock as it arrives on site and to judge its progress at each subsequent stage. These observations may reinforce the argument that under the conditions the opportunity should be taken to use a richer palette.

Great gardens may seem too mature to serve as a guide to new plantings, but they remain one of the very best ways of becoming familiar with a wide range of plants and to see their development as they mature. Many contain refurbished areas in which the date of replanting can be found. Botanic gardens, arboreta and the gardens of the Royal Horticultural Society (RHS) have the added advantage that specimens are labelled; in addition most carry an accession date indicating the age of the specimen. It is wrong to think of our heritage gardens as being full of only old and unobtainable plants; all the best-known gardens have recently introduced new and in many cases ‘landscape-useful’ plantings.

Nurseries, seed companies and specialist bodies such as the Royal National Rose Society have demonstration areas and/or open days that allow a wide range of plants to be compared and assessed.

Photographic libraries held on the Internet are valuable but their plant portraits tend to emphasize seasonal display and a close-up of flowers, whereas the designer is primary concerned with year-round plant appearance, form and stature. The pictures of pests and disease symptoms, however, are very helpful in enabling users to make an informed diagnosis.

The number of books on gardening is equalled only by those on cookery but few are written for a professional readership, either horticulturists or designers. The further reading which indicates publications that provide an insight into specialist subjects not covered in detail in this boo...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Dedication
• Contents
• Foreword
• Preface
• Glossary
• 1 Introduction
• 2 Landscape Sites
• 3 The Plants
• 4 Background to the Plants We Use
• 5 Selecting, Assessing and Purchasing Landscape Plants
• 6 Understanding Nursery Practice
• 7 Forms and Types of Transplant Traded
• 8 Seeds and Direct Seeding
• 9 Site Work Before Planting
• 10 Transplanting and Seeding
• 11 Care in the Establishment Phase
• References
• Index
• Back Cover