Take the Flower District in Manhattan, which once stretched from Broadway to 6th Avenue between 23rd and 33rd streets in the Chelsea neighborhood, but has since largely been reduced to 28th Street between Sixth and Seventh Avenues. As in many other parts of the city, typical lots in Chelsea go 100 feet deep, but the majority of these buildings — a mix of pre-war townhouses dwarfed by new glass towers like the hip Moxy hotel — don’t stretch back that far because they require light and air at the back. That leaves secret courtyards inside many city blocks, many of them overgrown, inaccessible spaces, notwithstanding the occasional private garden.