TRENTON, Mich. – Using heat from a forge that turns orange-hot metal into everything from car parts to hand tools, a Michigan manufacturer is developing an energy-efficient way to warm a year-round greenhouse on the company’s roof.

Trenton Forging President David Moxlow started growing fruit and vegetables atop the plant southwest of Detroit in November and has already harvested greens, peppers, broccoli, strawberries and tomatoes that are shared with employees and visitors. The company is among a number nationwide that are developing technology and techniques for rooftop gardening as interest in local and homegrown food grows.

Sky Vegetables Inc. aims to build hydroponic farms on roofs in New York,…