The tough tamarisk

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You can't kill it with drought. Fire only makes it stronger. Bury it in a sand dune and it thrives. Dig it out, and the roots come back with a vengeance.To early settlers in the West, this tree was a lifesaver, protecting farms and ranches against blistering desert sun and searing winds. Tamarisk was the tree from heaven as far as homesteaders were concerned, but since then has become a green hell.Before widespread irrigation, the seedlings did OK on minimal water. Livestock could consume the foliage safely and found shelter beneath the canopy. In many parts of the Southwest, it is the only tree to survive the rigors of a nearly rainless environment.The hardy, hard-to-kill, fast-growing plants certainly did fill a vital role in making the new land more inhabitable, but these same qualities made them thwart our best efforts to eradicate them later on.Out west, there are a variety of tamarisks -- the tree, Tamarix aphilla, used primarily for shade and windbreaks, plus a half dozen smaller species. Each impacts the environment in a different way.The tamarix tree is most often seen in windbreaks designed to control blowing sand in desert regions. It grows 30 to 50 feet tall at maturity, bearing fine needlelike foliage and fluffy pink flowers. Most of the older trees have naturalized and do not receive irrigation. They survive by a huge taproot that can extend 100 feet into the soil and lateral roots that can spread nearly double that. A single individual can consume up to 300 hundred gallons of groundwater a day.Consider miles of tamarisk windbreaks along a road or railroad track and then calculate the cumulative effect of all those taproots on the water table. In one case, a small spring that served wildlife over many square miles of waterless terrain dried up due to the demands of tamarisk roots.Smaller species of genus Tamarix in the West were imported at different times from varying sources, mostly as ornamentals. They may also have arrived serendipitously in ship ballast or animal feed. Some are smaller scrubby plants that can produce prodigious amounts of seed each year. They germinate in streambeds during brief rains and then take up residence to survive on water deep in the sandy soil.It is believed that a single tamarisk tree can produce a half-million seeds per year. In areas along the banks of the upper Colorado River, there are green lawns of tamarisk seedlings estimated to contain up to 15,000 seeds per square yard. In streambeds where these exotics thrive, more sensitive natives such as the smoke tree may be easily crowded out.The competition isn't just for water. The tamarisk, like so many other species from exceptionally dry climates, exudes natural toxins and excess salt from foliage or roots to deter other plants from becoming established within the tree's influence. This not only prevents native plant seed from germinating, it causes pre-existing plants to die out as tamarisks spread with age. In home sites shaded by older tamarisk trees, virtually nothing will grow within its sphere of influence, likely due to salinity caused by decades of leaf drop, which explains its common name, salt cedar.For those building new home sites in these arid regions, keep this example in mind when selecting plants for landscaping and shade. Nobody predicted the problems tamarisk would cause, and today's plants may prove to be tomorrow's pariahs.The best way to plant well and plant safely is to select regionally native plants to enhance your homestead. After all, if it grows well locally for Mother Nature, it will certainly do just as well for you.(Maureen Gilmer is a horticulturist and former host of "Weekend Gardening" on DIY Network. Her blog, the MoZone, offers a groundbreaking series of great ideas for cash strapped families. Read the blog at www.MoPlants.com/blog. E-mail her at mogilmer(at)yahoo.com.)