1958 that the park, which was managed by the (Van Hoey) Smith family for generations, gained wider attention. The park, approximately 20 acres in size, contains approximately 4,000 different types of trees, shrubs and perennials, amongst others the national plant collections of conifers, Quercus, Fagus, Rhododendron, Ligustrum, Rodgersia and Hosta. The Park (70 acres) on the Maas south of the Westzeedijk at the Euromast. The eastern half was constructed between 1852 and 1863 to a design by the firm Jan David Zocher. The western part was added in 1866 with some modifications. The first Floriade in 1960 was held in Het Park with the Euromast observation tower being erected to mark the event. National Heritage site since 2011. Park Schoonoord (3 acres) is located in the Scheepvaartkwartier and was designed in its current form in 1860 by Jan David Zocher. The Kralingse Bos (500 acres) with the Kralingse Plas (250 acres) is located in the Kralingen district and has been based on a design by Marinus Jan Granpré Molière since 1928. In 1953 the Kralingse Bos is finally officially opened. The Vroesenpark in the district Rotterdam-Noord was laid out from 1929 to a plan by city architect W.G. Witteveen. The Zuiderpark (780 acres) is located in the district of Charlois. The park was laid out as a utility park from 1952 and not as an ornamental park. Since 28 May 1994, Rotterdam has had the phenomenon Opzoomeren. 15% of Rotterdam residents (about 100,000 residents) say they participate in this phenomenon. At the end of 2020, the city has a record number of 2,503 Opzoomer streets, which is mainly reflected in the construction of facade gardens. The municipality of Rotterdam is encouraging the construction of green roofs. There is an attractive subsidy for roof owners and the city has now provided a number of municipal buildings with a green roof. As of January 1, 2020, the water storage capacity requirement has been increased to 30 liters of water storage capacity per square metre. This reduces the burden on the sewer system during heavy rainfall and reduces the risk of flooding on the street. The city's largest green roof is located on top of the Groothandelsgebouw next to Central Station. The Dakakker is the largest roof farm in Europe on top of the Schieblok. The Dakpark is an elongated, narrow park in the district Bospolder-Tussendijken in Rotterdam-West. It has been built at