By Bill Ward
Published in The Boerne Star on September 17, 2002
Ever since people have been able to travel to foreign lands, they have brought home the exotic plants that fascinated them. Columbus supposedly returned from one of his voyages with the small cactus Melocactus.
Countless others must have followed his example of collecting cactuses from their New World native habitat. To look at the Mediterranean area today, you’d swear all those spiny succulents in the countryside are natives.
In some places around the Mediterranean Sea, our native Prickly Pear probably has become an invasive exotic (i.e., a rapidly spreading non-native plant that changes the natural habitat by replacing native species). That’s the danger of growing non-native plants.
Of course, the vast majority of plants traded around the world do not become invasive. The few that do, however, can wreak havoc with the natural ecosystem by crowding out native plants and drastically reducing biodiversity. Commonly the exotic plants have few natural enemies in their new home.
An unchecked invasion of exotic plants not only changes the native-plant population, it brings about many other changes in the system.
For example, the root system of a new plant introduces new soil microbes which change the soil properties. Insect populations also change with the disappearance of the native plants to which the bugs are adapted.
When that happens, the lizards and birds that need to feed on those particular insects either die out or migrate elsewhere. And so it goes, on and on up the food chain, sometimes all the way to us.
The invasion of exotic species is recognized as such a major environmental threat that President Clinton issued an Executive Order (EO #13112) directing certain federal agencies to take measures to reduce the hazard.
Various forestry and ecology groups have assigned certain exotic plants to lists such as the Dirty Dozen List or America’s Least-Wanted List.
The Boerne area is host to many species on those lists. A couple of the trees high on most lists, Chinese Tallow and Tree-of-Heaven, have been used in local landscaping for many years.
Both of these trees were introduced into North America from Asia in the 1700s and are desired for their fast growth.
In Texas, when post-WW II suburbs sprang up in old pastures, many people planted Chinese Tallow to get quick shade.
A local friend asks, “Why do you knock Chinese Tallow? If it weren’t for Chinese Tallow when I was young, our yard wouldn’t have had trees.”
The problem is that these trees have adapted so well that they have spread from yards into the countryside in many parts of the U.S.
A local example of invasive growth of Chinese Tallow trees is at the Cibolo Nature Center (CNC). The CNC is the downstream recipient of what goes into the Cibolo Creek in Boerne, and apparently some Boerne Chinese Tallow trees grow where their seed can wash into the creek.
Much of the riparian habitat at the CNC is being altered by rapidly increasing numbers of Chinese Tallow.
Volunteers at CNC will have to spend several weekends this fall and winter trying to control the invasion of Chinese Tallow and other exotic trees to prevent native riparian vegetation from being nudged out.
The incentive for all the work this will entail is the realization that huge expanses of natural vegetation on the Gulf coastal-plain are now completely replaced by Chinese Tallow prairies.
We also should be on the alert for the Tree-of-Heaven at the CNC. Large old specimens of that tree are found around Boerne, among other places, half a block off Main Street in the heart of town. Since its introduction into Philadelphia in 1784, Tree-of-Heaven has overrun native vegetation in forty-two states from coast to coast. Saplings of this tree in the wild may be confused with natives such as Sumac, Black Walnut, and Pecan.
We will discuss other invasive exotic plants in future columns.
At the Oct. 1 meeting of the Boerne Chapter of the Native Plant Society of Texas, Susan Sander will talk on her survey of exotic plants that are invading the Hill Country. She entitles her talk “Come to the Hill Country and See the World.”
-Chuck Janzow contributed to this story.
