I see a lot of container and possibly raised bed gardening at first, until the concrete is pulled up from the sidewalks. Hydroponics may be a good standby, if nutrients can be obtained.
I wonder if anyone has thought to raid the sewage treatment plants on Manhattan to haul away the processed, digested sewage sludge, which is used as a soil amendment. This could produce another rumor to the effect that the Duke's lost his freakin' mind: "...And they were willing to _pay_us to get that processed human _crap_!
What the hell is the Duke gonna DO with that stuff???"