They include City-Hydro, a farm built in a spare bedroom on the second floor of Larry and Zhanna Hountz’s three-story rowhouse in Baltimore. Mr. Hountz came to urban farming out of necessity. After a serious car accident, he was unable to leave his house for two years and had trouble concentrating. He couldn’t go back to his previous job as a digital security consultant. “Zhanna had gone to the grocery store and bought some heirloom…