Cooling Your Urban Desert

Whether global warming is truly the state of things to come, one thing we know for certain is that cities in the United States are heating up. According to "Planting the Urban Desert," a previous article in the Urban Forests magazine, cities today average 5 degrees warmer than they did at the turn of the century. There are many reasons why this is true, but none so obvious as the reduction in the number of trees.

Trees are beneficial in a number of ways. First, they cool the air with their shade. Second, there is an aesthetic cooling effect that is difficult to measure with an instrument. I often try to prove this point through an unscientific experiment. When I am in people's yards - or when people are in my yard - I often ask what they think the temperature difference is between standing in the sun or standing in the shade of several trees near the house. There is an actual physical difference, but the interesting thing is that people always guess the temperature under the tree to be lower than it actually is.

A scientific study conducted in Tucson, Arizona revealed that for every dollar spent on tree maintenance, over $2.62 worth of benefits was returned from air conditioning energy savings, dust reduction, and the slowing storm water runoff. Recognizing the beneficial effects of trees may reduce thoughts of "zeroscapes" - not doing plantings at all or the least possible to get by.

After the droughty summers of 2001 and 2002 and the dry early fall we had this year, zeroscapes may seem appealing, but in fact both aesthetically and economically, zeroscapes are a big mistake. As California Forester Harold Walt stated in a letter to mayors of cities in California, "During past energy crises, we didn't stop driving our cars or heating our houses; we built more fuel efficient cars and turned down the heat." According to Urban Forests magazine, he went on to say that this is not the time to stop planting trees, but a time to begin planting drought-tolerant types of varieties, and to conserve water to irrigate new and existing trees.

For example, if a water shortage were to develop as it has at some points in the past, one water conservation step would be to water your trees and shrubs, but stop watering your lawn. A drought-damaged lawn only takes about two weeks to a possibility of six months to re-establish, but an established tree will take 15 years up to an entire human lifetime to replace.

Remember also that a tree planted strategically on the south and/or especially a west side of your home will cut cooling costs dramatically as it grows. Trees also add a delightful area in your yard for outdoor relaxation - as well as a good spot to hang the hammock.

It is not only important to plant new trees; it is also important to maintain existing trees. A newly planted tree requires about 10-15 gallons of water a week. Don't use sprinklers because too much water is lost to evaporation; instead, use a drip irrigation system, a deep root irrigator, or a soaker hose. For a young tree, one to three years old, soak the area around and just beyond the root ball. On established trees, water out at the drip line: the "line" at which rainwater would drip off of the widest branches and leaves. This is the area of working roots. To water around the trunk does not help the tree at all.

You might feel silly watering the tree at the drip line if it runs through the middle of your yard, or an area near a swing set, but you want to be as efficient as possible with the water you use. I love to see green circles of grass caused by proper watering at the drip lines of the trees, but not the full lawn. Realize, too that as the water drains down through the soil, it spreads out. This aids roots' growth and, therefore, tree health.

People need green space. Not only for the economical reasons of reducing air conditioning costs, but also for the aesthetic pleasure that trees bring. As cities heat up, we must commit ourselves to planting and maintaining trees to cool our urban desert.