With limited space for parks and gardens, architects and city planners often find it challenging to include greenery in neighborhoods in cities. One creative solution is to grow plants on unused areas like walls and rooftops. It's a popular idea, and now rooftop gardens and green walls have been sprouting up（大量冒出） in cities around the world.

    There are many benefits of having green spaces to the urban landscape. Adding gardens to rooftops or walls can create a pleasant environment — what was once a gray cement（水泥） wall can become a colorful, blooming garden. The CaxiaForum art gallery in Madrid, Spain, is a famous example — one of its walls is covered with 15,000 plants from over 250 different species. In other cities, green walls are being used more functionally, to cover up construction sites and empty buildings and to decorate the lobbies of office buildings.

    Using plants to cover walls and rooftops can also keep cities cooler in the summer. Buildings and roads absorb the sun's heat and hold it, causing a building or neighborhood to stay warmer longer. Plants, on the other hand, provide an enormous amount of shade. There is evidence that growing a roof or wall garden can lower a building's energy costs.

    In New York City, public schools plant rooftop gardens that can reduce heating and cooling costs. In addition to saving the school money, teachers and parents love the gardens because of their educational value — it's a fun and healthy way for their kids to investigate the world around them. “For the children, it's exciting when you grow something edible（可食用的）,” said Lauren Fontana, principal of a New York public school.

    Rooftop gardens and green walls may require a bit more effort to grow and maintain. However, hard work always brings rewards, and with green spaces, the rewards are plentiful.