Invasive Plants

Texas Cooperative Extension,
Texas A&M University, College Station, Texas

November-December 2006

Invasive Plants on Texas Rangelands

by Dr. Barron S. Rector, Associate Professor
Department of Rangeland Ecology & Management
Texas A&M University, College Station, TX

Introduction:Livestock grazing has been one of the most important management tools for our natural grasslands and established pastures in Texas, but throughout time, a number of exotic, invasive plant species have gained a foot-hold on Texas grazing lands and all other associated land forms, riparian and aquatic systems. Daily it is discussed and much consideration is given to the impact of foreign and invasive plants. A number of perspectives and attitudes towards foreign plants emerge. Do these foreign plants cause a detriment to the natural system? Are naturally occurring processes negatively affected? Can society and humans survive in a human created environment without the use of foreign plants? Can the goals and progress humans state is important be reached with invasive plants present?

The word "invasive" is usually defined as meaning intrusive, invading and offensive. To be invasive may imply that the organism has never occurred on a specific site or niche before or prior to its first introduction. Invasive plants are those that have a tendency to spread and invade healthy landscapes ultimately causing some kind of negative impact. Invasive plants are often best defined as “plants that do not stay where they are planted”. Invasive plants occur all over the state of Texas and today are recognized as a serious environmental threat. An “invasive species” is defined by Executive Order 13112 as a species that is 1) non-native (or alien) to the ecosystem under consideration and 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.

Johnson grass
Johnson grass, now practically ubiquitous in Texas
Most often we tend to think of invasive plants such as honey mesquite, common broomweed, ragweed, huisache, dewberry, green briars, persimmon, prickly pear, and others. These plants will increase on the land and are looked at as pests which reduce livestock carrying capacity of the land and may contribute to economic decline of a farm or ranch. In reality they are plants that are native to Texas, and are actually responding to human management and/or changes in the prevailing climate. Problems with these plants may be most adequately tackled by changing our land management that created a suitable environment or place for them to grow. From an ecological perspective, these native plants often named “weeds” or “brush” are classified as “invaders”. As part of the natural ecology and change in the landscape, they are supposed to be here.

Exotic invasive plants are those that are and have been introduced to Texas from foreign countries. Plants that ha ve been moved from their native habitat to a new location (such as a different country) are typically referred to as “non-native”, “nonindigenous”, “exotic” or “alien” to the new environment. These exotic plants which may be invasive plants in Texas include plants such as Chinese tallowtree, musk thistle, Macartney rose, scotch thistle, yellow star thistle, Malta star thistle, blessed milk thistle, jointed goat grass, Christ-thorn, Camel thorn, Japanese honeysuckle, Japanese and Chinese privet, Jerusalem cherry and others. Although with some exceptions, these invasive plants thrive in disturbed niches and get a foot-hold on sites where naturally occurring processes have been weakened or eliminated.

Invasive species have been introduced into Texas in a variety of ways. For most invasive plant species in Texas, little is currently known how most made their way into the state. Many times the knowledge of who brought the plant into the state or why is now lost and forgotten. Initially, these kinds of plants could have been those whose seed stuck on the clothing of early pioneers or were contaminants in feedstuffs brought from Europe, Asia or Africa. This type of introduction occurred long before we had USDA APHIS port of entry inspectors, the Texas Seed Trade Law or the Federal Noxious Weed List or before there was any concern with what foreign plants could do in Texas. Many times, these plants were introduced intentionally for beneficial purposes, but later turned out to be invasive and did not stay where they were planted. Humans seldom knew the biology of an introduced plant or speculated on where a plant might be in the next 30 years. Plants from foreign countries are often planted without humans having knowledge of what the plant might do in the future or in the environment once established. Who would have guessed that Macartney rose from Japan, introduced in the 1850’s as the “living fence”, would now be a major invasive plant of pastures in the eastern half of Texas? Or that wonderful introduced livestock forage grass named “Johnsongrass” would cost Texas taxpayers a mere $53 million in 1998 for chemical control on Texas roadsides alone? Or that King Ranch bluestem, a noted grass useful in the recovery and protection of played-out farmland, would be the number one weed on roadsides in central Texas today? It is estimated that 100 million acres in the United States are infested with invasive plants and that every year, the invasion accounts for an additional three million acres infested.

A problem that we have as humans have relates to the fact that "the value of a plant is in the eye of the beholder". A potentially invasive plant such as Jerusalem cherry is pretty and a suitable potted plant or ornamental in the house or on the patio at Christmas time and the winter months. What humans fail to recognize is the "potential" for any exotic plant to be invasive if it is introduced to a suitable surrounding niche or environment. Those folks who thought the Jerusalem cherry plant growing in a nice clay pot was dead, threw it over the backyard fence into another niche. They did not know that the seed in the mature fruit on the dead plant would germinate, create surviving and reproducing offspring and cause a future invasion.

What Should Be Done? To be successful in land management and the livestock business in the 21st century, it is paramount that the manager of Texas rangelands and pasturelands (both natural and modified or disturbed lands) understands the impact of his/her decisions and what is planted on the land. Any new foreign or introduced plant that is planted on the land must be heavily scrutinized for what it will do after planting and establishment. You must ask, “Will the plant stay where it is planted?” It must also be remembered that the value of any plant species is often in the “eye of the beholder”. An introduced plant species might be useful in an agricultural or land restoration sense but could be devastating to the health and ecosystem functions of Texas natural lands or streams and rivers. What we do in land management and the impact our decisions cause must also be looked at in the realms of the larger landscape or ecosystem picture. The hardest impact to see is the influence of invasive plants on natural processes.

What is Happening in Texas? One of the major problems with invasive plants in our native and established pastures, is that the landowner can not identify a new species of plant when it shows up. If the landowner sees a new plant in a pasture and he can not name it, most often the plant is ignored. There is a certain amount of risk in ignoring the plant or believing that it will just go away and it is not a problem. The decision to not seek out the name of the new plant can lead to future problems. It is only through the correct name of a plant that we can look it up in a book, talk to an authority, verify its value on the landscape or seek methods of management that would control or eliminate the plant. Just one plant left to flower and set viable seed can lead to hundreds or thousands of plants present in a few short years. This is especially true for exotic thistles such as musk thistle, a native of Europe, which may have from 300 to 600 seeds per flowering head.

Once many of these plants set and shatter seed, the problem may not go away or be solved for years as seen on continually disturbed soils. Several years of good seed production will lead to an abundant amount of seed in the soil bank. Misinterpretation and lack of knowledge on these facts led to a recent article in the Austin American Statesman which stated, “that the land is destroyed if musk thistle were growing on the property”. Plants such as distaff thistle, a native to Italy and the Mediterranean region, are noted to have viable seed in the soil for up to 19 years. Most weedy species do not germinate all of the previous years' seed in one year, as we see with many of our domestic or row crop species. It is a survival mechanism to spread the germination of seed out over a longer period of time. Even the native Texas bluebonnet will germinate only from 10-15% of seed in any one year. If a manager begins to work on control of a new plant some years after the initial invasion, plants continue to come up for many seasons and require the landowner to address a larger problem than when the plant first showed up. The management cost is increased and the potential for further spread encouraged.

Stewardship Role: Each land manager or owner is the steward of the land managed, whether this is one acre or 50,000 acres. There is a certain amount of complacency that exists in Texas because land managers, owners and lessees are working only with short term goals. In other words, there may only be a concern for what can be gained today or the profit that can be made today. To maintain healthy Texas lands that will be productive for years to come, the role of stewardship must be accepted by all that deal with the management of land. It is incorrect to believe that the next owner or manager can or will solve the problems that exist today. In many cases it may be more expensive to solve today’s problem tomorrow.

New and present owners are constantly seeking to find the magic plant that will solve all grazing problems, grow every day of the year and produce premium cattle. This search may reflect the fact that we still do not understand how to manage the plants that already occur on the land; or it is an excuse to hide the results of years and generations of mismanagement. For example, kudzu was introduced into the United States as a forage legume for cattle. Cattle were observed to eat the leguminous vines readily and it was nutritious. But without an expectation of how or what this foreign plant could do here, the vines ultimately grew up into trees and out of the reach of grazing cattle. The kudzu did not remain short, in the grazing profile, or where it was planted, but has become the number one exotic invasive plant of the Mississippi River watershed. Now it covers trees and invades the lawnscapes of many urban regions in the area. This knowledge is well known but now kudzu occurs in Tarrant, Grayson, Colorado and several east Texas Counties and is noted to be invasive and a serious pest. Invasive plants like kudzu are not sterile and are not suitable forage alternatives when the negative impacts on the environment are reviewed. They produce viable seeds and because they are not native and act like weeds, they can spread rapidly.

How Did Invasive Plants Get Here? Many invasive plant species are introduced unintentionally. For example, during the drought of 1996, hay was bringing a premium price in the livestock industry. Many bales of hay were sold which contained just about anything that grew in the hay pasture or meadow. Goosegrass (Eleusine indica), an introduced annual grass with no forage value, occurred in some of these hay fields and was cut and baled. Unsuspecting buyers who just needed hay to get by, bought the contaminated bales and introduced goosegrass through feeding the hay into their pastures which had large areas of open bare ground conducive for a new weed to survive. This grass helped fill the weed niche after the next rain but also lowered the grazing value of the pasture. It is advisable to know your hay producer and what is growing in his/her hay fields. Successful invasive plants have strategies for survival or self-preservation. Exotic invasive plants are also successful on Texas lands because the biological or ecological factors and mechanisms that kept these plants under control in their native land are not present in Texas (disease, insects, biological predators, exact nutrient requirements and soils, prevailing weather conditions, etc.).

Changes in landownership in Texas are now becoming a further problem with invasive plants. A plant once planted by a previous landowner may now be looked at by a new landowner as a problem. Some new landowners may not know the name of an introduced plant and think it is native and natural. With so many bermudagrass cultivar releases, how can one tell them apart? The previous landowner may not have left records of what was planted on the land during the previous 50 years. Many times cultivars are hard to distinguish when they all key out to the same genus and species. A named cultivar does not mean that a taxonomic difference exists. Cultivars are human selected plants which have characteristics meeting a human vision or desired need. These characteristics include leaf length, time of flowering, size of seeds, temperature adaptability, time of maturation, period of green-up and the growth cycle. It is interesting to note, most exotic plant cultivars are not selected because they will not become invasive. Generally the qualities of ease of establishment, spreads fast, and sets a large amount of viable seed are used. Just what an invasive plant needs to be successful.

Is common bermudagrass an invasive plant species? Common bermudagrass (Cynodon dactylon) is one of the most successful invasive plants in Texas. In some situations it has great value to the landowner such as in a lawn or grazing pasture. Common bermudagrass will spread through the devices of stolons, rhizomes and production of viable seeds. Some landowners have tried to establish other grass species or cultivars to only end up with common bermudagrass because the seed of this grass is now in the soil over most of the state. In this situation, a valuable agricultural and landscaping grass can be considered an invasive plant. Common bermudagrass, a warm season perennial native to Africa, is a prolific seed producer and is also a weed of ditches, vacant lots, roadsides, gardens, and along streams, lakes and marshy swales. Is it of value in these situations or an invasive plant? The value is tied to the expectations that a person has for a given piece of land. Again, value is in the eye of the beholder. A safe exotic plant may be best described as one that has sterile flowers, produces no seed, has no means of vegetative reproduction and stays where it was planted. But what if the soil is bulldozed and carried or transported to a new site with this plant growing in the soil? It is difficult to find an exotic plant or example where the potential for spreading would absolutely not occur.

Invasive Plant Impact on Wildlife Most invasive plants do not provide food or much value to Texas’ native wildlife. Wildlife species are tied to a certain group of native plants which can serve as food, a source of water and shelter. Most native plants are also associated with a native animal or group of animals as part of their natural ecology. The planting of many foreign plants or invasive plants in large landscape patches can further the current problems seen in Texas with wildlife and plant habitat fragmentation. Numerous examples abound. Some invasive species can threaten the existence of native plants and animals and may even serve to cause human related problems. The Chinese tallowtree, first introduced as an ornamental for landscaping, is now invading much of the area in southeast Texas, from pastureland to vacant lots. Tallowtrees are changing the coastal grassland prairie into a wooded thicket of limited value. Although this plant, its leaves, flowers and stems are not eaten by any known wildlife species, some individuals declare that the flowers are a valuable food source for bees. When this is examined further, yes, the introduced European honey bee visits the flowers of the Chinese tallowtree, but it must be remembered that the European honey bee was introduced into this hemisphere to pollinate most of the food plants which we brought from Europe and Asia that are used today to feed humans. Remarkably it must be noted that the only native plants of Texas available everyday in our supermarkets to feed the human population are pecans and prickly pear pads in the form of nopalitos. All other plants available and eaten have been introduced.

Conclusion Invasive plant species cause a combination of economic, environmental and human health threats. Typically studies that document the harm from invasive species conclude that the United States needs to strengthen its legal authorities and existing programs. Determining whether a non-native plant is invasive, requires a context-specific analysis. A plant species may cause harm in one type of ecosystem, but not in others. Because ecosystems are dynamic and ever-changing, their vulnerability changes over time as well. Because of this, it is practically impossible to create a definitive list of invasive plants. It must be weighed to determine if the benefits of introduction of an invasive species outweigh the potential harm caused by an invasive species. This is the focus of the new national invasive species management plan set forth in Executive Order 13112. The management plan is focused on those non-native species that cause or may cause significant negative impacts and do not provide an equivalent benefit to society. To examine this document finalized in 2001, you should visit the following web site for more details, You should examine and even comment on Senate Bill 854 which directs the Texas Department of Agriculture to prepare a “noxious plant” list for Texas. The problem with lists occurs when we do not have the financial strength and man-power to enforce the rulings behind a list.

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