In the city of Philadelphia, when weedy vacant lots were cleaned up and brightened with beautiful flowers and trees, nearby residents also brightened—in overall mental health. This proved especially true for those who struggled economically.

“There’s a growing body of evidence that green space can have an impact on mental health,” said Dr. Eugenia South, “and that’s particularly important for people living in poorer neighborhoods.” South, a faculty member at the University of Pennsylvania’s Perelman School of Medicine, is coauthor of a study on the subject.

The downtrodden people of Israel and Judah found fresh hope in the prophet Isaiah’s vision of…