Architecture: Natural palette for an urban landscape: Dan Kiley (left) uses trees, shrubs, water and light to soften the blow of the American city. Robert Holden reports

DAN KILEY, born in Boston in 1912, is arguably the leading American landscape architect of the 20th century. From small gardens to grand public places, he has planted some of the most remarkable meeting grounds between the rigours of architecture and the ramblings of nature. A witty, apparently light- hearted, yet passionate man, his career has encompassed the design of the spectacular Fountain Place, Dallas (1986-87) and the courtrooms for the Nuremberg trials that he made when working with the US Army Corps of Engineers at the end of the Second World War.

Kiley was in London last week for the opening of an exhibition of his work at the Architecture Foundation. This comes at a particularly appropriate moment. As grand and rival building projects designed to coincide with the millennium celebrations are weighed in the balance, Kiley's work encourages us to think less about rock-solid, four-square buildings and more about nature, gardens and, above all, public spaces. Perhaps one millennial project will be a civic space on the scale and of the calibre of Fountain Place.

Since 1940, Kiley has been developing an approach to landscape design from a basis of Modernism and Classicism. He is a Modernist in the sense that his design is based on function, and a Classicist in his selective use of regular geometry - the circle, the square - and increasingly a belief in meaning and allegory. Though his designs may, at a glance, appear finite and fixed, they are settings for gradual and continual change. 'Dance is the embodiment of what we're trying to do in landscape designs,' says Kiley.

'Structural spatial movement - it can lead you anywhere.'

Kiley is an architect of space, dealing with ground as form, trees as sculpture, and shrubs, vines, groundcover and water jets as textures and shapes that articulate the surface of his forms. His is not garden design in the established English manner; his palette of plants is austere, his effects grand, noble and rigorous.

His first garden design was in 1940 for Charles and Nina Collier at their home near Alexandria, Virginia. The Collier garden gives clues to his later development: there is the use of informal, linear gestures and wedges, the regular grove or orchard, the importance of space-defining hedges.

Progressively, the regular geometry of these gardens has become stronger, though nearly always offset with touches of informal geometry: the garden at Columbus, Indiana (complete with draped Henry Moore bronze) is a good example. The distance between Modernism at its purest and the Classical is not great, and Kiley's Classicism always grows out of the Modernist tradition; there is no hint of sentimentalism in his work. Kiley is never Neo.

Currently, he is creating a garden in England with Kevin and Angela Dash at Woodend near Chichester. The approach is subtle, just a single bar gate in a hedge at the end of a winding gravel path. Through the gate you descend into a small court with a central path of stone flags patterned by grass joints.

The rest of the court is filled with knee-high box hedges forming square compartments around single evergreen shrubs and the typical Kiley signature of a regular grove of small fruit trees.

This is a very modest garden compared with the large estates he has worked on in the US, but even on a small scale he is able to create a place that is memorable, composed and controlled.

As a landscape architect working on a big scale in urban settings, Kiley never loses touch with the garden. Garden design is his beginning, yet it is as a maker of major public and institutional spaces that he has made a special contribution to landscape design and to the way we live in city centres. In the US there is an active, living tradition of public and private patronage of public space design. Kiley has been responsible for the setting of some of the country's major cultural and commercial institutions, including Rockefeller University and the Lincoln Center in New York, the National Sculpture Garden in Washington, the Chicago Art Institute, the Dallas Art Institute, Independence Mall in Philadelphia and Fountain Place.

Outside the US he has designed the Grande Dalle leading to La Defense in Paris, and he is working on the surroundings of Richard Rogers' European Court of Human Rights in Strasbourg, due to open next year.

Kiley has spent his professional life working with the leading architects of American Modernism, including Eero Saarinen, Skidmore Owings Merrill, IM Pei, Harry Cobb, Louis Kahn, Harry Wolf and Kevin Roche. His work with them mirrors the key concerns of the greatest Modernist architects - a play of light and shadow on pure form, but in Kiley's case modified and softened by nature.

Two constant themes in his urban gardens are water and light. And never more so than at Fountain Place, where the water gardens are a setting for two 60-storey towers by IM Pei and Harry Cobb.

'The minute I saw the site which was to become the Dallas Fountain Place,' Kiley says, 'I knew all six acres must be water and there would be bald cypress trees and people would walk flush with the water and there would be geometric waterfalls.' The result comprises a field of water: 'here, you appear to walk on the water,' says Kiley. One hundred and sixty jet fountains, 263 bubble fountains and 440 bald cypress trees in granite planters are the ingredients for this magical public space. Every component is lit subtly at night. Trees appear to grow out of the water, people appear to walk on it and the sound of water subdues the noise of the city. Foliage and running water cool and soften the blow of blazing Dallas afternoons.

Kiley uses water on all projects where it is appropriate. One of the most remarkable of his water-based projects is the Chicago Filtration Plant (1964), where he created a beautiful civic amenity from what would normally be an out-of- bounds public utility. Here, water shoots up to 100 feet from fountains clearly visible from ascending and descending aeroplanes.

All the time the genius of his designs depends on an acute and specific sense of place. His sensitivity to site and his ability to balance the geometric with the natural is very clear in his work in the countryside and in the many gardens he has designed for individual clients. Yet the urban spaces he has created are conversations between site and city, nature and the citizen.

'I don't draw lines between design and life,' says Kiley. His materials are light and shade, water and earth, rock and vegetation. His designs are stages for life. He is the great choreographer of the American civic landscape.

Robert Holden is a landscape architect.

Dance, Structure and Landscape runs until 27 November at the Architecture Foundation, 30 Bury Street, London SW1. Tel: 071-839 9389.

(Photographs omitted)