Thank you for your eloquent comments regarding the use and appreciation of introduced species.
We strive to keep the tone of our answers as non-adversarial as possible, but because of our passion for our area of study, we're sure we fall short of our goal from time to time. Moreover, we do not disagree that the introduction of non-native plant species has benefitted mankind greatly and in numerous ways. It would be difficult to imagine the state of agriculture and horticulture without introduced plant species. The mission of the Wildflower Center, however, is to study and advocate for the appreciation and use of plant species within the areas of their native occurrence. So naturally, our answers will focus on the benefits of using native plants in horticulture and on protecting wild, native habitats.
Of course, we can and often do attempt to make a strong case for exercising utmost caution when transporting plants from their native lands and introducing them into other areas. Through introduction, any number of plant diseases, plant pests and problem plants themselves have dealt devestation of unimaginable cost to native plants and habitats. Just a few notable plagues or agents of plagues that can be traced to the introduction of plants or plant material in North America are Dutch Elm Disease, Chestnut Blight, Gypsy Moth, Emerald Ash Borer, Cactoblastis Moth, Kudzu, Hydrilla, Salvinia, Melaleuca and Saltcedar. By any measure, the cost of these introductions is staggering. Sadly, this short list is just the tip of a great iceberg of misery that we have unintentionally inflicted on ourselves.
Problem plants aside, another reason to consider the use of native plants -- especially in arid areas -- is their ability to thrive in adverse climatic conditions. For example, in central Texas our summers are hot and dry and seem to be getting hotter and drier each year. Fortunately, we are blessed in our region with a palatte of nearly 2000 plant species with which to paint our landscapes. Since every one of these species has been here for thousands or even millions of years, they're natural choices for use in our landscapes. These plants don't just somehow survive here, this is their home. Where water resources and chemical use are increasingly important issues, native plants excel by requiring the application of little or none of either.
Finally, we embrace a somewhat more subtle esthetic that Lady Bird Johnson aptly described with, "Each region speaks with its own vernacular." Commonly-grown horticultural plants are so widely used largely because they're beautiful. But when you see the same petunias, pansies and marigolds everywhere you go, American landscapes begin to take on a certain sameness that lose their ability to earn our admiration or even our attention. Mozart was a wonderful composer, but few would want to hear little other than his compositions everywhere they went. By carefully selecting and using native plants in our gardens, we give our landscapes a distinctive regional flavor, much like lobsters give Maine or chiles give New Mexico their own, unique regional cuisines.