Desert Gardner:

Leaves on ground serve purpose for trees

Thursday, Jan. 8, 2009 | midnight

Angela O'Callaghan

It takes a certain fortitude to appreciate the winter, even in our part of the world. Southern Nevada winters are, of course, quite mild in comparison to some other places, where there is ice to scrape off windshields and snow to sled down. While we (mercifully) do not experience that kind of extreme discomfort, it is possible to see the winter's effects here in the desert southwest, especially in gardens.

Many landscape plants that tolerate blistering high temperatures and desiccating dryness will show distinct changes, such as turning colors when the weather becomes cool. It may not be below 32 degrees for long periods as in the frozen north, but freezing temperatures are not necessary to affect growing things. The outcomes of mere chilling, where temperatures fall into the 40s, can be dramatic.

In deciduous trees, for instance, the green of the leaves fades, and they ultimately drop off, crisp and dry. This may seem like a great loss, but trees have a greater degree of fortitude than we might first expect.

Plants' green color is due to the compound chlorophyll. This is the chemical that permits plants to produce oxygen (which we in turn breathe) and sugar, the raw material for all other growth. When temperatures drop below a certain threshold and fewer hours of light are available, the plant will not waste valuable resources producing compounds that will not be used. In fact, the green compound itself gets broken down and its components are pumped deep into the dormant plant. By storing these materials for later use, the plant protects itself against the stress posed by the nutritional demands of a new growing season.

Once the chlorophyll is gone, the leaves ultimately lose all their color and drop. "Fall color" is the glorious interim state preceding this, where leaves may turn orange, gold, red or purple. There is little fall color here in the Mojave Desert, but some trees, such as Chinese pistache, produce a lovely deep magenta before leaf drop.

Despite the fact that we look at fallen leaves as a nuisance, the leaves that became crunchy are not trash as far as the tree is concerned. The dead leaves serve as mulch, insulating roots from chill. Mulch acts much like the insulation in a ceiling, moderating temperatures from extreme highs and lows, and maintaining a level of moisture. The thicker the mulch, the more effective it is, as much as 3 or 4 inches.

While dead leaves are lying on the ground, they are also decomposing, releasing nutrients into the earth, which in turn increases its fertility.

It is a great pity, then, that homeowners are often forced to remove all these nutrient sources from their grounds. Even worse is when people are told to remove trees that have merely dropped their leaves in winter. Deciduous trees evolved to lose all their leaves and produce a new crop the following spring. Gardeners need to make sure that this information gets to people who make decisions to remove trees that are completely viable.

Angela O’Callaghan is the area specialist in social horticulture for the University of Nevada Cooperative Extension. She can be reached c/o the Home News, 2360 Corporate Circle, Third Floor, Henderson, NV 89074, or TheNews@hbcpub.com.