Columbia subject headings:
Harvard subject headings:
Horticulture--Europe--History--Congresses.
Urban gardening--Europe--History--Congresses.
Urban gardens--Europe--History--Congresses.
Princeton subject headings:
Urban gardening--Europe--History--Congresses.
Urban gardening--Germany--History--Congresses.
Urban gardens--Europe--History--Congresses.
Urban gardens--Germany--History--Congresses.