I came across this blog:
How does nature overtake a cemetery? Maybe the last living members of the group in charge die, or the families with relatives buried there move away. Perhaps a few people — or even one individual — cared for the site and then one day stopped showing up for who knows what reason, like in the case of Philadelphia’s B’nai Israel/Hebrew Mutual Cemetery. Then there are those cemeteries under the care of an organization whose age simply means its gravestones have weathered centuries of, well, weather.