New Haven has a long tradition of urban planning and a purposeful design for the city's layout. The city could be argued to have some of the first preconceived layouts in the country. Upon founding, New Haven was laid out in a grid plan of nine square blocks; the central square was left open, in the tradition of many New England towns, as the city green (a commons area). The city also instituted the first public tree planting program in America. As in other cities, many of the elms that gave New Haven the nickname "Elm City" perished in the mid-20th century due to Dutch Elm disease, although many have since been replanted. The New Haven Green is currently home to three separate historic churches which speak to the original theocratic nature of the city. The Green remains the social center of the city today. It was named a National Historic Landmark in 1970.