Why is it that when we see images of major new urban developments today - whether housing units, office blocks, or shopping malls - those developments are as likely as not to be covered in a thin canopy of trees? In this paper, I approach this question by thinking about the long history of the idea that cities are somehow bad for human beings, and especially that they are bad for our mental health. From Shanghai to Paris, from Biophilia to "cheap nature", from Donna Haraway to Thomas Heatherwick, I examine the growing entanglement between the psychologically restorative city and the green city. I ask: what does it mean, for historical cultures of both madness and the city, that we now ask our urban spaces, recast as centres of nature and biological life, to have even a reparative relationship to mental health?.