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Special report: landscape This month's report considers the landscaping of specimen trees, from planting issues and procurement to aftercare

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technical & practice

One of the most significant factors in the creation of a successful and memorable landscape framework for a new development is the design and procurement of the tree layer. Some of the technical issues that the landscape architect will consider include the specification of tree size and form; the construction programme; selection and procurement; installation; and finally, aftercare.

Retention of existing trees

The evaluation of the existing tree structure (if any) is one of the first actions undertaken in the appraisal of the opportunities that a new site offers. The benefits of retaining existing trees can include the creation of structure from day one, and a continuum with the past. Native trees will also provide wildlife value.

However, it is important to maintain existing ground conditions such as levels, surface finishes and hydrology, or the trees will suffer - particularly if they are mature and already in declining health.

The practicalities and constraints of construction, and providing construction access, can be difficult to reconcile with tree protection measures. In the long term, the creation of a new robust and vigorous framework, with species appropriate to the new development, may be preferable to the retention of fragments of the original tree groups.

New tree planting

Trees are generally specified by girth (circumference of trunk at 1m above ground). Nursery stock is between 12-20cm girth and semi-mature stock is generally considered to be trees of between 20cm-120cm girth. However, trees of between 20-40cm girth are the most frequently specified semi-mature trees, with the larger sizes only justified by exceptional requirements.

The maximum size of semi-mature stock is limited by transportation: the most that a low loader can accommodate is an 11m tree with a 1m rootball. A tree of this size could weigh up to two tonnes. Stock of 25cm girth or more will need to be planted by cranes and skilled operators.

Among the factors affecting the choice of the size of material will be whether there is a need for a dramatic impact from day one, and the expectations of the client.

The character and context of the project will also influence any decisions. In urban situations, larger material will clearly present a better relationship with the scale of buildings. They also have a far more robust structure which is less vulnerable to damage by vandalism. Semi-mature stock is particularly valuable on brownfield sites lacking any tree structure.

Ease of establishment is also important. Some species do not establish well as very large trees, and most large trees will struggle in difficult sites with a harsh climate and/or thin soils. All semi-mature stock will suffer some stress after planting and will show negligible growth in the following year.

Invariably, a mix of sizes of trees, located accordingly to function and visual impact, is the most appropriate solution. This also creates a more varied age structure.

Trees can be sourced in a variety of forms to suit the character of a project. Standard, clear-stem trees suit formal avenue planting and allow clear views beneath the canopy. Multi-stem and feathered trees have a more informal appearance and can provide low-level screening, if required. Highly cultivated and clipped trees can provide sculptural forms for special areas. Trees can also be sourced with pleached forms to create an immediate aerial 'hedge' from day one.

New stock

Planting season: October to March is the dormant season when rootballed stock can be planted. April-September is the period when most trees are in leaf and can only be planted if they have been container-grown or containerised.

Containerisation: trees up to 40cm girth are generally available from reputable nurseries in large plastic buckets. If trees are to be containerised, they need to be lifted from the field before the end of the dormant season and held in containers during the summer growing period to allow a dense and fibrous root system to establish. If the trees have insufficient time in the container the rootball will fall apart on removal from the container.

Larger stock can be containerised in demountable steel cages.

Cost: containerisation can add up to 40 per cent of the cost of rootballed stock. However, this should be offset against the programming flexibility this provides, and the invisible savings sometimes generated by not having to adjust the sequence of building construction works to accommodate the planting season.

Sourcing tree stock

It is essential to buy trees that have been properly prepared and cultivated to develop well-balanced crowns, whatever the size. The tree stock needs to have been transplanted every three years to encourage the natural development of a compact rootball. Transplanting also allows the trees to be set out at increasing spacings to allow the canopies to have unrestricted growth and develop to a natural form.

There only is a handful of reputable tree nurseries in the uk supplying quality trees of between 18-35cm girth, and even fewer that are beginning to supply the larger sizes.

German nurseries, however, have the benefit of many years of sound investment behind them and are consequently able to offer a superb range of well- prepared stock. If large quantities of semi-mature stock are required with matching forms for avenues and other forms of infrastructure planting, German nurseries are still the best choice.

Trees can be sourced from Belgium and Holland, but they tend to be grown at closer spacings, resulting in trees with narrow, less natural canopies.

Italian stock is rarely suitable for uk schemes as it can be too tender for our climate.

It is helpful to liaise closely with reputable tree nurseries during the detail design to check availability of species at the desired sizes, particularly where unusual trees or large quantities where matching forms are required. At the early stages of the Eurodisney project, the Derek Lovejoy Partnership project team travelled throughout Europe to source bizarre and wonderful plants including 'depressed' trees with drooping forms for the haunted mansion, and square headed trees for Sleeping Beauty's Castle. The latter started life as cone shaped Yews furnished to the ground, but after severe pruning and careful preparation, they achieved the form conceived by Walt Disney in his original depiction of the castle.

Procurement

It is worthwhile securing tree stock for a project as soon as the scheme has been defined to ensure availability of the stock at the desired size and quality - even up to five years in advance of the installation date. There are also savings to be made if stock is bought in advance. The nursery would normally require 55-60 per cent of the discounted value to be paid once the order has been confirmed, with the balance due on delivery to site.

The stock needs to be clearly identified as the property of the client in the unlikely event of the nursery falling into financial difficulties. It is essential to inspect and individually tag all the principal trees at the source nursery with security tags. At dlp, the client representative and architect are encouraged to participate in this process. The range and quality of materials is stunning, and it is not unusual for design changes to be made in the field in response to a client request.

These specimen trees will represent a significant proportion of the soft landscape budget. It is helpful for the client to 'touch' the trees and understand the production process to gain an appreciation of the reasons why a well prepared semi-mature tree is a justifiable investment.

On large, complicated projects, such as Eurodisney, the serial numbers on the security tags can be transferred to the respective tree stations on the construction drawings. This allows the contractor to call off the precise stock required as and when needed on site.

Aftercare

It is important to ensure that the investment in quality stock is recognised by the creation of appropriate ground conditions during installation, and the provision for maintenance regimes in perpetuity.

Unless ground conditions are particularly free-draining, tree pits should be drained to prevent waterlogging. Trees suffering from waterlogging will not necessarily show signs that a problem exists until it is too late to remedy it.

It is a sad fact that some clients will buy good quality landscape schemes, but make inadequate financial provision for the maintenance that will allow the scheme to flourish and reach the full potential of the initial investment.

Jessica Beattie is associate director at the Derek Lovejoy Partnership. Tel 020 7828 6392

ORGANISATIONS

Arboricultural Advisory and Information Service (AAIS)

Alice Holt Lodge, Wrecclesham, Farnham, Surrey, GU10 4LH, tel 01420 22022, fax 01420 22000

Arboricultural Association

Ampfield House, Ampfield, Romsey, Hants, SO51 9PA, tel 01794 368 717, fax 01794 368 978

Horticultural Trades Association

19 High Street, Theale, Reading, RG7 5AH, tel 0118 930 3132, fax 0118 932 345, e-mail hta@martex.co.uk, website www.martex.co.uk/hta.

Landscape Institute

6/8 Barnard Mews, London, SW11 1QU, tel 020 7738 9166, fax 020 7738 9134, e-mail mail@l-i.org.uk, website www.l-i.org.uk

GUIDANCE

BS 5837: Code of Practice for Trees in Relation to Construction.

BS 3936: Part 1: Specification for Trees and Shrubs.

BS 4043: Recommendations for Transplanting Rootballed Trees.

NHBC: Chapter 4.2 Building near trees.

NJUG: Guidelines for the planning, installation and maintenance of utility services in proximity to trees.

EVENT

'Landscape 2000'

Earl's Court 13-15 June

The Exterior Design and Outdoor Environment Show

PUBLICATIONS

The Hillier Manual of Trees & Shrubs ISBN 0-7153-9942-X

The Hillier Book of Tree Planting and Management

By Keith Rushforth. David & Charles. ISBN 0-7153-8589-5

Trees in Britain

By Roger Phillips. Pan Books. ISBN 0-330-25480-4

The Royal Horticultural Society Gardeners' Encyclopaedia of Plants and Flowers

Dorling Kindersley. ISBN 0-86318 386-7

Trees

By Andreas Feininger. Thames & Hudson

Meetings with Remarkable Trees

By Thomas Pakenham. Phoenix Books. ISBN 0-75380-237-6.

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