The story goes that the Liz Christy Houston-Bowery Garden was the first community garden founded in NYC. Located on the Lower East Side on the corner of Houston and the Bowery, this garden was formed in 1973, when members of the Green Guerillas threw seed bombs over the fence of what was then a vacant lot. (There is one older surviving garden in Prospect Heights, Brooklyn–a WWII-victory garden that survives today as a community garden.)
The Liz Christy garden thrives due to the agreement between the City of New York and State Attorney General Spitzer forged in 2002 (he declared a moratorium on the destruction of community gardens on city-owned property). It is one of the rare older gardens that still exists on the Lower East Side. So, go visit it yourself!
The Green Guerillas has grown since then from a small neighborhood organization dedicated to green space and urban farms to a City-wide advocacy and educational non-profit organization dedicated to supporting gardens.