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Horticultural History Repeating Itself: Dispersal and the Invasion Lag Phase of Exotic Plants on the TAMU Campus

Edward L. McWilliams, Professor of Horticulture
Michael A. Arnold, Associate Professor of Landscape Horticulture

Texas A&M University, Dept. of Horticultural Sciences, College Station, TX 77843-2133

A paper from the Proceedings of the 10th Metropolitan Tree Improvement Alliance Conference held in St. Louis, MO, September 30 and October 1, 1998, co-sponsored by the Landscape Plant Development Center and the Society of Municipal Arborists.


The promotion of new plants in the nursery trade creates an increasing incentive to only briefly evaluate and then immediately introduce new plants. In general, as more plant taxa are introduced the chances are that one or more will become an invasive weed problem. It may be possible to effectively evaluate some herbaceous species in given region within a few years, but woody plants may require twenty, thirty, or more years to effectively evaluate them. This is due to the invasion lag phase which is the time between the introduction of an exotic (alien or non-native) species into a new environment until it naturalizes. Invasion lag phases can be significant for some species. This may be due to long delays between juvenility and sexual maturity, delays in production of large quantities of seed until the parental plant gains sufficient size, the number of years between climatic cycles favorable for seedling establishment, or a combination of these and other factors. Three landscape plants, Ligustrum quihoui É. Carričre, Pistacia chinensis von Bunge, Ulmus parvifolia von Jacquin, were investigated as examples of the variation in species responses during the invasion lag phase of naturalization on or near the Texas A&M University (TAMU) campus. Each species exhibited a unique invasion lag phase; extremely rapid for L. quihoui, intermediate for U. parvifolia, and extended for P. chinensis.


In recent years there has been an increased interest in foreign and domestic plant exploration. Development of new cultivars of ornamental species, or the resurrection of overlooked taxa from the past to supply the public's demand for new and different landscape plants. Several coordinated regional or state-wide testing programs including CEMAP (the Coordinated Marketing Assistance Program, in Texas, the Plant Select Program in Colorado, the Gold Medallion Program in Louisiana, the SERA-IEG 27 (Southeast Regional Agriculture Information Exchange Group 27 project and others have been formed. Their goals are to promote the use of adapted while environmentally responsible plants for their regions, and increasing the genetic diversity of landscapes and providing the industry with new products. Many large nurseries also have their own research and development programs for new plant introductions. This strong interest in bringing new plants into the trade comes the chance that a potentially invasive taxon might be widely planted in a suitable host environment outside its native range.

Effects of some unfortunate past introductions by the nursery trade are infamous, despite their apparent positive landscape features at the time of introduction. While many of these taxa are exotics (non-natives or alien) to the USA, some American natives such as Southern Catalpa (Catalpa bignonioides Walter), Common Trumpetcreeper (Campsis radicans L.), and Black Locust (Robinia pseudoacacia L.) have become invasive weeds under favorable circumstances (Arnold, 1999; Buckstrup and Bassuk, 1998). The list of exotic introductions that have become noxious weeds in various regions of the USA include such notable plants as Kudzu (Pueraria lobata (Willd.) Ohwi, Fig. 1), Melaleuca (Melaleuca quinguinervia (Cav.) S.T. Blake), Japanese Honeysuckle (Lonicera japonica Thunb., Fig. 2), Chinese Privet (Ligustrum sinense Lour.), Amur Honeysuckle (Lonicera maackii (Rupr.) Maxim.), Chinaberry (Melia azedarach L., Fig. 3), and Tree-Of-Heaven (Ailanthus altissima (Mill.) Swingle) (Arnold, 1999; Dirr, 1998; Gilman, 1996; Reichard, 1997; Wade and Kafka, 1997).

The Presidential Executive Order of Feb. 3, 1999, attempts to prevent the introduction of invasive species, including plants, and to provide for their control. "Alien species" means, with respect to a particular ecosystem, any species that is not native to that ecosystem, while "invasive species" refers to alien species that cause economic or environmental harm. These definitions do not consider the impact of different dispersal rates on whether "economic or environmental harm" can be controlled.

As a land grant college Texas A&M University was one of the earliest locations for the introduction of many plants in Texas. With a campus of over 5,000 acres that included land along several streams, conditions were often ideal for plants to escape from cultivation at various abandoned nurseries and at some areas adjacent to landscape plantings.

The objectives of this study were to document the dispersal mechanism and local distribution of three introduced taxa which have escaped from cultivation on or near the TAMU campus.

Materials and Methods

Initial surveys of the cultivated and naturalized flora of Brazos County, Texas (contains College Station, Texas, at which TAMU is located), were begun in 1972. This study was conducted with the assistance of the late Drs. Omar Sperry (Botany), Homer Blackhurst (Horticultural Sciences, then a part of Soil and Crop Sciences), and Frank Gould (Rangeland Ecology) as advisors. Advisors provided information about the content and location of previous and current landscape plantings that have served as the source of persistent individuals in non-cultivated landscapes or seed for escaped plants. Additional ecological observations on invading species were continued through the present (1998) to document the extent of spread of species from cultivated landscapes.

Species were classified by location and distribution as occurring in cultivated landscapes only, or being persistent, adventive, or naturalized in non-cultivated landscapes. Persistent plants were those that once established in cultivated landscapes continued to survive when the area was no longer maintained. Adventive plants were those that had occasional seedlings or naturally occurring vegetative propagules that survived and became established in non-cultivated landscapes, but generally failed to establish permanent reproducing populations during the time frame of the study. Species that are currently adventive may, in time, become naturalized. Or alternatively, may only be experiencing a narrow window of favorable climatic conditions that allow them to briefly escape cultivation but never become permanently established given the longer term climate of the region. Naturalized species are those taxa which invaded non-cultivated landscapes and appeared to have established multiple generations of offspring that have become a permanent components of the flora of the region. In many cases these distinctions are dependent upon the time frame of reference (White, 1997). This invasion lag phase between the time of introduction of a species to its naturalization may be far beyond a single man's lifetime, or may be only a few years (White, 1997).

Results and Discussion

Ligustrum quihoui É. Carričre (Jones et al., 1997; Wyman, 1969), the Quihou Privet, has become a common invader of understories, woodland edges, and fence rows in Brazos county.The Quihou Privet is a native of China. Multiple generations and hundreds of thousands of individual plants have spread from existing or removed landscape plantings in the Bryan/College Station area. Ligustrum quihoui differs from the commonly planted Ligustrum sinense J. de Loureiro, or Chinese Privet, in inflorescence morphology having a more narrow elongated raceme of flowers (Fig. 4, Fig. 5) and differs in flowering date, occurring about two weeks later than that of L. sinenseLigustrum quihoui also has a leaf that is narrower in width compared to its length than on L. sinense. Populations of L. quihoui appear to be spread by birds, with new colonies starting beneath typical bird roost sites such as fence rows and beneath large trees (Fig. 6). Ligustrum quihoui was already naturalized and becoming a common weed species at the time of the initial 1972 survey.

Three sites are examples of the adventive and naturalized nature of Pistacia chinensis von Bunge, or Chinese Pistachio, from long established plantings on the TAMU campus. Chinese Pistachio is originally a native of Central China. Texas A&M University was one of the first sites of introduction of Chinese Pistachio and several mature reproducing specimens exist (McWilliams, 1991). Pistacia chinensis is a dioecious species and female trees do not produce large quantities of seeds until established in the landscape for fifteen or twenty years. Two distinctively different female flower structures and flower phenologies occur within the population on the TAMU campus Fig. 7, Fig. 8). One forms flowers earlier in the spring and has larger panicles than the other.

The Texas A&M University Nursery/Floral Field laboratory is adjacent to several 30 to 40 year old P. chinensis trees in cultivated or abandoned landscapes. The original trees have been persistent in both the cultivated and abandoned locations (Fig. 9). The surrounding woodland edges contain specimens of seedlings presumably from these trees Fig. 10, Fig. 11. Some of the adventive saplings have reached reproductive age and a diameter of at least 6 inches (15 cm) at breast height (Fig. 12). The P. chinensis seedlings are scattered beneath bird perch areas. The seedlings are surprisingly drought and shade tolerant. In many instances these seedlings are growing beneath and in some cases have grown through established plants to top mature Juniperus virginiana L. and Platycladus orientalis (L.) J. Franco plants. Similar spread from established landscape plants was evident on the TAMU golf course. The third location is at the TAMU airport where reproduction of at least a second generation post-landscape planted P. chinensis was found. At this site and the TAMU Nursery/Floral Field laboratory Pistacia chinensis have naturalized in the local ecosystem. Seedlings are most frequently found under avian roosting sites and several species of birds have been observed feeding on the fruit. Nearly all of the fall color in Fig. 9, 10, 11, and 12 are of P. chinensis seedlings.

A final example of a species at another stage, early stage of naturalization, is Ulmus parvifolia von Jacquin, the Lacebark Elm (Fig. 13). The Lacebark Elm is native to China and Southeast Asia. This tree has become, along with P. chinensis, one of the most widely planted shade trees in Texas (Arnold, 1999). In irrigated landscapes throughout the area and in specific environmental niches at the TAMU Nursery/Floral laboratory U. parvifolia has successfully established saplings (Fig. 14). At the field laboratory several volunteer seedlings have been found with a diameter of at least 4 (10 cm) inches or greater at breast height. Small seedlings of U. parvifolia are common in irrigated landscapes and in the surrounding landscape in the spring. These young seedlings appear to have a very high mortality rate during the summer droughts unless one happens to be located in an environmental niche where some soil moisture is available during summer droughts. Locations such as stream banks and drainage swales that retain additional moisture from sparse summer rains are prime locations for colonization. Ulmus parvifolia produces an abundance of seed (Fig. 15) that is primarily dispersed by wind and/or water. It is uncertain whether or not this species will naturalize in the drier portions of Texas, however first generation saplings have reached reproductive maturity at several sites near of College Station, Texas.


These observations suggest that there is a significant and presently unpredictable invasion lag phase between introduction of a plant into cultivation and the subsequent naturalization of the species. Climatic fluctuations influence the naturalization rate of each species in unique ways relating to the ecological life history and phenological stages of the plant (McDonnell and Roy, 1997; Reichard, 1997; White, 1997).

In the past, the nursery industry has enjoyed virtually unfettered introduction of exotic species into the trade. If the species naturalized or was planted and fell out of fashion, it was largely abandoned in the trade with little financial or regulatory consequences to the industry. In the future greater precautions and study of potential introductions to determine their propensity to become invasive will be necessary. Federal agencies are now working together and in cooperation with state governments and universities to address the issue of invasive species. Legislation and/or executive decrees may dictate the nursery/landscape industry's options with regard to introduction of new plants in the future. A proactive approach to invasive plant policy may allow the nursery/landscape industry to be public spirited and yet offer an opportunity for a voice in decisions that might adversely impact their livelihoods (Reichard et al., 1999). Several schemes that tolerate a variety of risk levels with regard to exotic plant introductions have been proposed (McDonnell and Roy, 1997; Reichard, 1997; White, 1997).

Examples presented in this paper illustrate that it may not be easy evaluate newly introduced exotic plants for invasiveness. The three species discussed here illustrate several ways in which plants are escape cultivation and are disbursed. The invasive lag phase appears to vary with species. Horticultural history, no doubt will continue to repeat itself as newly introduced plants escape in the future.

Literature Cited

Arnold, M.A. 1999. Landscape Plants for Texas and Environs. Stipes Publ. L.L.C., Champaign, IL. p. 596.

Buckstrup, M.J. and N.L. Bassuk. 1998. Native for native's sake? American Nurseryman 187(10):55-61.

Dirr, M.A. 1998. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses, Fifth Ed. Stipes Publ. L.L.C., Champaign, IL. p. 1187.

Gilman, E.F. 1996. Trees for Urban and Suburban Landscapes. Delmar Publ., Albany, NY. p. 662.

Jones, S.D., J.K. Wipff, and P.M. Montgomery. 1997. Vascular Plants of Texas: A Comprehensive Checklist Including Synonymy, Bibliography, and Index. Univ. Texas Press, Austin, TX. p. 404.

McDonnell, M.J. and E.A. Roy. 1997. Using ecological science to understand the spread of exotic species. Public Garden 12(2):28-29.

McWilliams, E. 1991. The impending naturalization of Pistacia chinensis (ANACARDIACEAE) in East Texas. Sida 14: 508‑510.

McWilliams, E. 1997. Invasive plant classifications. The Texas Nurseryman 27(8):34.

Reichard, S.H. 1997. Learning from the past. Public Garden 12(2):25-27.

Reichard, S.H. 1997. The prevention of invasive plant introductions on national and local levels. pp. 215-227. In: Assessment and Management of Plant Invasions (J. Luken and J. Theiret eds.), Springer-Verlag, New York, NY.

Reichard, S., B.R. Yinger, H. Gramling, D.J. Ellis, M. Yanny, R. Gass, J. Randall, K. Warren, C.J. Regelbrugge. 1999. Invasives round table. American Nurseryman 190(2):54-77.

Wade, G.L. and M. Kafka. 1997. Japanese invasion. Landscape and Nursery Digest 31(6):6-8.

White, P.S. 1997. A bill falls due: botanical gardens and the exotic species problem. Public Garden 12(2):22-25.

Wyman, D. 1969. Shrubs and Vines for American Gardens: Revised and Enlarged Edition. MacMillan Publ. Co., New York, NY. p. 613.

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