Publications (Paul's Blog)

August 24, 2009

INVASIVE SPECIES

Filed under: Uncategorized — Paul Dowlearn @ 9:03 pm

As gardeners we should strive to be aware of plants that may cause problems in our particular environment. To most people, the word invasive raises a red flag as a plant that could cause problems in the home landscape. For example, nutsedge in the lawn would be considered an invasive species. However, as defined by Texas Parks & Wildlife, the EPA, and other environmental groups, the term “invasive species” refers to foreign plants that have increased in the wild to the point of choking out or displacing our indigenous native species. Ironically, in this case the nutsedge being a native found in the wild would not be considered invasive. Bermuda grass (originating in Africa) would be considered invasive instead.

This example will likely raise a few eyebrows among homeowners as bermuda grass has long been considered a good choice of lawn grass and a staple forage crop in agriculture. In fact, it seems our Texas Department of Agriculture actually encourages the cultivation of bermuda as one of the easiest ways land owners can receive tax exemption for agricultural land use. Likewise, our local nursery professionals and extension agents continue to tout the virtues of bermuda lawns. Yet, if you research the “weed lists” or “invasive plants” listings on the A&M or TDA websites you will sure enough find bermuda grass listed as a noxious weed. Go figure.

In reality, as gardeners we all can testify to the stubborn invasive nature of bermuda grass when we try to keep it out of the flowerbed or when we attempt to eradicate a patch to plant something else in its place. This gardener has actually observed bermuda in road ditches out in the Chihuahuan Desert where rainfall averages less than 10″ per year. This is irrefutable evidence of the sheer tenacity and drought tolerance of this foreign grass that was brought here with every good intention but has spread itself into places where it should not be.

The term “invasive” is a scary one to most people. If you call a plant invasive it is almost sure to make your friends and neighbors think twice before using it in their landscape. But, when you think of it, most of the plants we use as groundcovers have an invasive nature. How else could they have that ability to get out there and cover the ground quickly? Just how invasive a plant may be is relative to your local environment.
invspcsA good example of this would be ground ivy or gill ivy (glechoma hederacea). Ground ivy is a fast growing ground cover that does not climb (hence the name), stays evergreen, and grows to only a few inches tall. Because of these obvious attributes, ground ivy comes highly recommended by several leading horticulturists here in Texas. However, if you contact a horticulturist from the milder climates of the east coast, you will certainly be warned that this European invader has caused all sorts of problems by choking out sensitive native plant communities in forested areas.

You can research invasive plants of Texas on the web by visiting our Texas Department of Agriculture (www.agr.state.tx.us/ <http://www.agr.state.tx.us/> ), Texas A&M (texasextension.tamu.edu/), and Texas Parks and Wildlife (www.tpwd.state.us <http://www.tpwd.state.us> ). One of the better sites for the entire U.S. is the Plant Conservation Alliance at nps.gov/plants/index.htm. As you peruse these various lists you will be amazed at just how many of the common plants we use as ornamentals are considered invasive in various parts of our nation. You will also find that the strict definition of an invasive plant is simply any non-native plant that has escaped cultivation. All native plants are exempted from these lists regardless of how invasive (trumpet vine (campsis radicans), for instance) they may be in the home landscape unless they are taken (presumably by humans) out of their native habitat and introduced elsewhere.

So what should a gardener who is also concerned about environment (that includes most of us) do? Should we refuse to plant any or all of these invaders? The choice is bewildering when you consider the fact that most of the plants we see in the traditional nursery system originated in Europe, Asia, or the Mediterranean.

The answer lies in understanding just what plants are capable of reproducing themselves in your particular part of the state. For instance, the Chinese tallow tree (sapium sebiferum) is taking over creek banks and low areas from Houston to Beaumont and points east. However, here in northern Texas, the tree cannot survive due to our colder winters. In fact, here in my hometown of Wichita Falls, Chinese tallow has a poor chance of survival even with the help of a diligent gardener. On the other hand, I have noticed Japanese privet (ligustrum japonicum), and Japanese honeysuckle (lonicera japonica) are gaining a foothold along our streams and rivers especially near populated areas. So the astute gardener discovers that what may be invasive for some might be benign in his or her local area.

The nursery industry could play an important role in educating the public by steering customers toward better plant choices but this, unfortunately, has not been the case. In fact, the nursery industry itself has been responsible for many of these plant introductions. If you are among those who have a local nursery with an “environmental consciousness,” please support their efforts and encourage others to follow suit.
Currently our state does have laws in place concerning the sale of some aquatic plant species. We could and perhaps should put some parameters in place regarding the sale of invasive dryland species as well. One of the fastest growing segments of land use has been the reclamation of indigenous plant populations. Efforts have been made by the Ladybird Johnson Wildflower Research, the Native Plant Society of Texas, The Nature Conservancy and a host of like-minded organizations too numerous to mention here. All of the people who have been involved in land reclamation projects will agree that it is extremely difficult if not impossible to return land to its pristeen state because of the number of introduced plants that are firmly entrenched in the environment. The first logical step must be to prevent the introduction of even more. Only then can the attempts at eradication have a positive impact.

Plants have no way of understanding where we want them to grow and where we want them to stop. They simply do what they do. At present there are no herbicides that can distinguish the invaders from indigenous plants. In some instances such as lawn grasses, ground covers, and vines used to cover unsightly areas, invasiveness can be very desirable. In other cases, introduced plants such as kudzu, hydrilla, and salt cedar have been devastating and are costing us as taxpayers untold millions in habitat loss. The best course of action for us as concerned citizens is to familiarize ourselves with potential problems and curb our own use of invasive plants then spread the word to others. We also should support legislation that will curtail the import of more invaders. Some states have already taken action. Texas will follow suit. If you have questions or doubts, contact your local extension agent, Parks & Wildlife representative, or botanist. Know what you are planting before you plant.

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