Flower Show: The excellent German federal garden exhibitions have produced some fine works but few as good as this, which complements the outstanding eighteenth- and nineteenth-century landscapes of Potsdam.

This year's German Federal Garden Exhibition (Bundesgartenschau) (1) is at Potsdam, Frederick the Great's town near Berlin and traditional headquarters of the Prussian army. Bornstedter Fields, which form the site, are near Frederick's Sanssouci, with its magical sequence of gardens that are reinforced by nineteenth-century picturesque pleasure grounds and buildings by designers like Schinkel and Lenne. Potsdam at the end of the eighteenth century and the beginning of the nineteenth was paradoxically the home of both a ferocious military regime, and of the most delicious works of landscape architecture. The Fields were a parade ground used by the Prussians, the Wilhelmine army, the Nazis, and finally the Russians, who crisscrossed the flat ground with earth berms, thrown up as protective outworks for their barracks. The berms provided the basic framework of the new park, and the Flower Pavilion, by Barkow Leibinger Architekten, is its focus. They extended the armature of the berms to form an artificial valley running east-west. Building height could be limited as it was cut into the ground, and displaced earth was used for new mounds. The strategy kept the Biosphere down to the general height of the berms, and prevented it from competing with local historic landmarks, but it is carefully aligned on axis with the Pfingstberg monument.

The continuous ground plane is shaped into a 200m long space, simply enclosed by a steel and glass curtain wall. The roof is supported on precast concrete beams and has large rooflights. Rainwater from the roof is collected in a large concrete gutter and is collected in a central cistern from which it is used to humidify the building's atmosphere. The conservatory is framed by two anchor points, the entrance at the east end and the orangery to the west. The architect's aim in the central botanical garden has been to abstract geology, and you go into the great central space from the foyer through a slate-clad cliff to be engulfed by terraces and basins covered in tropical trees and plants. Frederick the Great, with his passion for the exotic, would have been proud of the place which, after the close of the exhibition, will be run by the Cinemax company as a commercial attraction for 20 years under a lease from the city.

The jury was very impressed by the way in which a large building was handled with great sensitivity to the form and history of its site and its relationship to the little city of Potsdam. And by its many suggestions about making big buildings sustainable.

The regular sequence of German garden exhibitions is admirable. Each is held in a different city, and each is funded by public and private money. Their aim is to take pieces of run-down land, use them for the show duration, then convert them to permanent parks.