A two minute walk separates Tanya Tolchin and Scott Hertzberg’s little house in Upper Marlboro, Maryland from the field they cultivate. The shady and muddy path then opens to a green wide field surrounded by tall trees, a greenhouse on the left, tomatoes on the hill to the right, and tall bushes of eggplants with violet flowers to the far side. “Scott is right there, harvesting the eggplants,” Tanya...