[1] On a hot summer afternoon, two city blocks side by side can feel like different worlds. On one block, the sidewalk is bare, and the sun bakes the concrete. On the next, a row of trees throws shade across the walk, and people waiting for the bus stand in the cool shadows. Cities are paying closer attention to that difference. Street trees can make city blocks cooler and more pleasant places to live, but they do that job well only when people plan for them and take care of them.
[2] Trees cool a street in two ways. First, their leaves block sunlight before it reaches the pavement. Bare pavement soaks up heat all day and gives it off long after dark, which keeps some neighborhoods warm even at night. Second, trees pull water up from the soil and release it through their leaves. As the water dries into the air, it carries heat away, much as sweat cools your skin. When workers in one city checked ground temperatures on a summer afternoon, sidewalks under trees were often more than 20 degrees cooler than sidewalks in full sun.
[3] The benefits go beyond comfort. Buildings on shaded blocks stay cooler inside, so the people who live and work in them may run fans and air conditioners less often. Trees also catch rain on their leaves and slow it down, which eases the load on storm drains during heavy storms. Some shop owners say that shaded streets draw more people on foot, and those shoppers tend to stay longer.
[4] Planting a tree, however, is only the first step. A young street tree needs water every week during its first two or three summers, until its roots are established. After that, most trees can reach the water they need on their own. Without that early care, many new trees die before they ever give much shade. As one city tree worker put it, "Planting is the easy part. Keeping them alive is the real job."
[5] Cities must also choose the right tree for each spot. A tree that grows too big for a narrow strip of ground can lift the sidewalk with its roots or tangle with power lines overhead. Many cities now keep lists of trees that fit tight spaces, put up with salt from winter roads and survive long dry spells.
[6] Shade is not spread evenly. Some neighborhoods have far fewer trees than others, and they are often the ones that need shade the most. A number of cities now plant first on the blocks with the least shade, and they ask neighbors to help with the watering. One woman who waters the young maple outside her building said, "Three years ago it was a stick in the ground. Now my kids wait for the bus under it."
