Adolescent secondary school students are faced with multi-dimensional physical and psychological challenges that could impact on their mental well-being. They are at an adolescent development stage, and need to cope with abrupt changes in their physical bodies. They are often under inter-personal pressure in forming new social relationships . Academic pressure is also an important source of stress which affect mental health [2, 3]. Most local students seek to relieve stress through engagement in physical activities, seeking peer support or listening to or engaging in music . Yet, green space, gardening, horticultural and similar activities were found having positive effects on mental health.
In the literature, there were a number of evidence which show the association between green space and its related human activities and health. Urban greenery and water were deemed important elements to improve indoor air quality, temperature and protection against heat and cold, which have effects on both physical and psychosocial health . Research evidence with multiple angles in terms of neuroimaging data and supporting cellular and molecular animal studies indicated the interaction between social and environmental risk factors of mental health .
In the community, adults who lived closer to green space had lower perceived stress, better perceived health and related quality of life (QoL), and were more likely to use green space for relaxation . According to a review done in 2013 , adults with mental health issues had their depression and anxiety reduced, and benefited in physical, emotional, social, spiritual and even vocational domains after joining gardening activities. In Hong Kong, researchers found that horticultural activity could improve psychiatric patients' level of anxiety, depression and stress in a randomized controlled trial (RCT) . The above evidence imply that human activities associated with plants for aesthetics could have positive effects on mental health.
Longitudinal evidence indicated that health benefits from green area appeared in different life courses between genders. In men, benefit of more green space emerged in early to mid-adulthood . In women, but at older ages, moderate availability of green space was found contributing to better mental health. More benefits of green space might extend to physical health also. Previous study showed that cardiovascular disease and respiratory disease mortality rates decreased with increasing green space, but in men only . Yet, women were found more active, saw greater aesthetic value, and had higher self-reported wellbeing in urban green spaces than men . However, these gender differences were not confirmed in adolescents who would participate in green activities. Outdoor green space was found facilitating protective factors of health such as increased free-time physical activity level in early adolescents . Previous study also showed that neighborhood parks was associated with reduced stress among adolescents regardless of socio-economic status (SES) . Nonetheless, the mental health benefits of gardening, horticulture or similar activities on adolescents needed further studies.
Due to increasing urbanization and densification in Hong Kong, most students live far away from green space. Hydroponics is a method of plants growing in a flexible scale with the use of water-based, nutrient-rich solution. With proper care, the plants can grow fast and strong by using less water; and hydroponics plants are more environment friendly than soil-based plants [15, 16]. Hydroponics in the green roof systems are potentially applicable in Hong Kong building for example the schools in which adolescents spent most of their time .
Whether having green space at schools could provide them, however, a means for stress reduction or help them develop a green-living attitude and behavior remained unclear. Earlier studies investigated the effects of green space programs holistically, but the specific effects had remained unexplored to date. There was also limited research on the relationships between green space, gender differences on mental health in secondary school students. Thus, the aim of this study was to investigate the impact of a pioneer school green space program on selected mental health indicators in secondary school students after participating in either the designed program or control activities with follow up.
It is hypothesized that after participation in the green space programme, students' will have their green space exposure and mental wellbeing in terms of 1) green space use and satisfaction, 2) anxiety and stress symptoms, 3) level of happiness improved compared with those who joined control activities.