Where do you go for your salad greens Most likely,you go to the supermaket.
But soon it may be possible to go up to the roof.
More and more cities are encouraging building owners to cover their buildings with green roof - with or without gardens.
Not all green roofs have vegetable gardens or any kind of garden.
There are two types, in fact. The first is called intensive.
It has a deep layer of soil, and can be planted with vegetables or flowers and trees.
There require regular watering and care.
They are also quite expensive to build since the layer of soil is heavy and need strong support.
The other kind of green roof is the extensive green roof
This tyoe is actually more common.
It has a thinner layer of very light soil and only low-growing plants, such as grasses or sedums-desert plants that can gros in thin soil and with little water.
These roofs are less expensive to build and require less care afterward.
In North Americ, Toronto is the leader of the green roof movement, with laws that require green roofs on many new buildings.
Chicago is promoting green roofs as well.
There is now a green roof park on City Hall, where workers can go at lunchtime.
A green roof on top of the Uncommon Ground restaurant in Chicago is planted with vegetables that are served downstairs in the restaurant.
In St.Louis ,the children's hospital has created a green roof with a pond and walkways for patients and families.
In other parts of the world, green roofs are also becoming common.
In Tokyo,Japan,laws has been passed that require green roofs on all larger new buildings.
Some cities in Europe have gone even further, particularly in Switzerland.
In Basel,Zurich, and Luzern, all new buildings with flat roofs-large or small-must have plants on top.
Green roofs cost more to build than regular roofs, but in the long term, they allow building owners to save money.
In most cities,people have to use air conditioning to stay comfortable in the summer.
Building owners pay huge electricity bills to run that air conditioning.
With a green roof, much less air conditioning is needed.
Studies have shown that a regular black roof can reach 82c on a summer day, while a plant-covered roof will stay much cooler,not more than 30c.
This makes a big difference in the temperature inside the building.
Green roofs also benefit cities in several ways.
First,they help lower air temperatures in summer.
In fact, regular black rooftops collect heat by day and release it again in the evening.
Instead,the plants on green rooftops prevent the heat from building up.
Second, green roofs absorb a lot of rainwater during storms-up to 70%.
This can make a big difference in cities where heavy water running off roofs can cause flooding and pollution.
Furthermore,with a green roof,a new green space is created.
This can benefit both building owners and everyone in the city who lives or work nearby.
Whether it is planted with vegetables,grasses,or trees, it can provide people in the middle of cities with a green place to go.
Studies have shown,in fact,that spending time in green spaces helps reduce stress and lower blood pressure.
Finally,scientists have noticed that green roofs are also good for wildlife.
The roofs provide a home for many kinds of birds and insects,and much-needed resting place for birds or butterflies on their way north or south