CARACAS, VENEZUELA - Unemployed bus driver Rafael Lira recently landed an unlikely job amid the high-rises of downtown Caracas. He's now a farmer. Mr. Lira and eight others tend an acre plot wedged between the Hilton Hotel and a pair of busy avenues, where lettuce, mint, beets, and other crops thrive. Lira says he has become self-sufficient from his steady share of vegetable sales. "[This farm plot] shows that this is viable," he says. "That one can live from this and obtain independence." Urban farming's success here is rooted in an unlikely source: visiting agronomists from Cuba. But in a...