A study published in Applied Psychology: Health and Well-Being, researcher, Frances Kuo finds that "green time" has a positive impact on children and adults with ADHD. 
"Those who play regularly in the same green outdoor settings do have milder symptoms than those who play indoors or in playgrounds. We also found that children who were hyperactive had less severe symptoms if they played in an open environment, such as a soccer field, rather than in a green space with lots of trees." - Frances...