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Exotic, introduced or invasive?
May 22, 2013 | 1132 views | 0 0 comments | 7 7 recommendations | email to a friend | print
Wild Petunia is a South Texas wildflower that blooms all summer and fall. It is as pretty as its relative, Mexican Petunia, but less likely to become invasive in our yards and gardens. Photo by Robert Benson.
Wild Petunia is a South Texas wildflower that blooms all summer and fall. It is as pretty as its relative, Mexican Petunia, but less likely to become invasive in our yards and gardens. Photo by Robert Benson.
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Mexican Feathergrass, Mexican Oregano, Mexican Oleander, Mexican Honeysuckle, Mexican Petunia and Mexican Flame Vine: these are all plants found in the Beeville Butterfly Garden. They are also growing in many of our own South Texas gardens. Did they all come from Mexico?

Well, it seems so, at least in most cases. Feathergrass actually grows wild in the mountains of West Texas as well as in adjacent Mexico. And the oregano is found in dry scrubby areas of the Trans-Pecos and along the Rio Grande, both on the Texas and Mexico sides.

The other four are all exotic (foreign) species that have been brought from the more tropical areas of Mexico and Central America. With adequate moisture and some protection from frost, all of these exotics thrive in our South Texas landscapes.

An exotic species that survives in its new home is usually called an “introduced” species.

If the introduced species grows or reproduces rapidly, establishes itself over large areas, and persists, then it may become “invasive.” An invasive species “causes or is likely to cause economic or environmental harm.”

Economic harm results from an introduced plant out-competing other plants, to the extent that it causes considerable expenditures of time and money. For example, King Ranch Bluestem, an introduced grass, grows extremely well in South Texas, but it was found out to be of poor nutritional value. Better quality grasses are squeezed out by the highly competitive KR Bluestem. Controlled burns, herbicides, and plowing are costly and are only partly effective .

Likewise, some introduced plants result in environmental damage. This is the case with the Chinese TallowTree. It was brought in to provide fall color here in the South, where very few native trees turn red and gold. Sadly, it spread rapidly into natural wetlands and crowded out much of the native vegetation, eliminating food sources for birds and wildlife. Tallow trees also sucked up much of the water.

Invasive species do not have to be plants. Among the top 10 invasive species in Texas are feral hogs, red fire ants, Africanized bees and zebra mussels. Most of us in South Texas have at least heard of these creatures, even if we haven’t actually had a run-in with them. (Who hasn’t had a run-in with fire ants?)

Most of us are pretty careful not to introduce animals to areas where they do not already exist. We know not to let exotic reptiles loose into the wild (think of those monster pythons in the Everglades of Florida!) We are aware that House Sparrows, introduced to the United States in the 1850s, have become pests. But plants are another story.

For decades in this country, we have brought in exotic plants for the horticultural trade. Those plants that do well, we propagate, we share and we sell. Unfortunately, a few of these plants have gone on to be harmful invasives in some areas.

A website called www.Texasinvasives.org maintains a database of plants in Texas that are economically and environmentally harmful. In the South Texas Plains eco-region the worst offenders include Salt Cedar, Chinese Tallow Tree, King Ranch Bluestem, Guineagrass, Water Hyacinth, Brazilian Peppertree and Giant Reed. If you know these plants, you are probably not surprised that they are considered invasive.

As I glanced through the list of species “suspected of causing invasive problems”, I found several surprises: Elephant Ears, Mimosa, Japanese Honeysuckle and Chinese Wisteria. (Not surprisingly, I also found Bermudagrass—the bane of every gardener!) But the biggest shock was seeing Mexican Petunia on the list!

Mexican Petunia (Ruellia brittonia or Ruellia caerula) is a horticultural superstar. It grows 3 to 4 feet tall, has daily blooms of purple or pink and attracts butterflies. It also reseeds heavily and spreads eagerly by rhizomes. And that’s the problem: It can get out of hand very easily and take over your garden—and the neighbor’s garden, and the woods, the parks, and fields.

But I helped plant it in the Butterfly Garden! What should I do? Happily, I found another website which has some tips for us gardeners. One tip is to contain it (or control it) by using root barriers, trimming regularly and by harvesting seeds and fruits before they are spread.

Better still, the website said, is to use non-invasive alternative plants. So I checked and there are other Ruellia species. In fact, in this part of South Texas, two native Ruellias grow. I even found one in my own Brush Country backyard! It is called Ruellia occidentalis, or Wild Petunia, and its flowers are almost exactly like the Mexican Petunia. The only difference I can see is that the leaves are a bit greener and rounder.

Wild Petunia is just starting to bloom here in South Texas. It will flower from now through the fall. Look for it in areas of part shade such as along the edges of brush and under live oaks. It can be seen along rural roads in most parts of Bee County. Enjoy this quiet but pretty native plant!
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