Improving Availability Of Under-Utilized Small Trees For Texas

Mitchell W. Goyne* and Michael A. Arnold

*Graduate Assistant and Assistant Professor of Landscape Horticulture, Texas A&M University, Dept. of Horticultural Sciences, College Station, Texas, 77843-2133

A paper from the Proceedings of the 9th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Columbus, Ohio, August 8-10, 1996.

Introduction

Researchers and extension personnel at Texas A&M University are working to identify small trees (20' to 25' tall) suitable for use in Texas urban and suburban landscapes that will be amenable to conventional container nursery production technology. Many of the popular small tree species sold in retail outlets were originally selected for use in the Eastern USA and are marginally adapted to Texas' challenging environments. Soils in Texas are frequently high in pH, alkaline, or salty and are often of high bulk density or underline by compacted layers a few inches below the surface. Many soils are frequently droughty or are poorly drained when water is available. High temperatures, both daily maximum and minimum night temperatures, are common for extended periods from spring to autumn. Precipitation ranges from over 60 inches/year in extreme southeast Texas to less than 10 inches/year in west Texas and is not evenly distributed throughout the year. Winter temperatures vary widely, from nearly tropical in the eastern Rio Grande Valley (USDA Hardiness Zone 10) in the panhandle's Zone 6. While numerous small trees are native to various regions of Texas, relatively few of these trees are widely available in the nursery trade.

Leaders in the nursery/landscape industry and academic horticultural community were surveyed to identify under-utilized small tree species with commercial potential, but were not widely available in the nursery trade. Participants were asked to consider ornamental attributes as well as potential adaptability to urban sites and conventional container production systems. Numerous species were suggested by survey respondents. From this group fourteen species were identified for further study, all but three species are Texas natives. Species not selected for initial studies will remain candidates for later propagation / production / establishment studies and clonal selection work as time and resources permit.

Acceptance and wide-spread use of new taxa requires that: 1. The product resemble or exceed in overall quality and appearance conventional products (small trees) currently on the market; 2. Plants can be readily established in the landscape; and 3. Plants be available when promoted for use at a comparable price to conventional alternatives. Before adding new taxa to their product mix, nursery professionals need to know how the new species will respond into their production systems and what associated costs and rotation times will be required. While new species adaptability to the various landscape regions in Texas can be estimated from natural distributions and/or previous landscape plantings, the production time and the easy of establishment are unknown. These questions will be answered by an integrated project using the newly renovated Nursery/Floral Crops Research and Education Laboratory in the Texas A&M University's Department of Horticultural Sciences and the Texas Agricultural Extension Service's Target 2000 Program (an integrated environmental plan for the Texas nursery and floral industries) and CEMAP (Coordinated Education and Marketing Assistance Program).

The first objective is to determine what modifications to conventional production practices will be necessary to produce acceptable wholesale and retail quality container nursery stock of the selected species. The second objective is to determine how these costs of production compare to those of conventional substitute plants already in the trade. A separate project directed by Dr. Michael Arnold (Horticultural Sciences) and Dr. Charles Hall (Agricultural Economics) and funded by the Griffin Corporation, Valdosta, GA, will provide economic data for conventional species production costs for comparison.

Initial work concentrated on determining the suitability of the fourteen selected species to conventional container production systems. The goal was to produce liners, shift the liners to "one" gallon containers, and the one gallon plants to three gallon containers to determine rotation times for salable stock. Under commercial conditions, salable one gallon plants are produced in less than one year and three gallon plants in 18 to 24 months from propagation. As finished stock becomes available, comparative field establishment with conventional small landscape trees will be investigated.

Narrowing the List of Potential Species

Propagation studies were initiated between Fall 1994 and Spring 1995. A popular, easily produced small tree species, Fraxinus velutina (Arizona Ash), was included as an industry standard for comparison. As limitations were encountered some species were eliminated from the study at the liner and 1 gallon stages. For instance, the three oak species, Quercus glaucoides, Q. mohriana, and Q. pungens var. vaseyana, were promising, but were eliminated at the liner stage due to limited seed availability. Few potential mother trees were in irrigated landscape settings; irrigations boosts seed production. Native stands have erratic seed production, and some species' distributions are limited to privately held lands. Chionanthus retusus was difficult to propagate; stem cuttings were difficult to root in commercially acceptable percentages and the seeds have a double dormancy. Cotinus obovatus, also rooted in low percentages, but those that rooted grew rapidly, thus further research will be conducted. Fraxinus cuspidata propagation was also complicated by an apparent extended seed dormancy, germinating only after being held for over a year at 40°F. While problems with these species may not be insurmountable, initial efforts were directed to those species with less impediments to production.

Several more species were eliminated from further study at the one gallon stage. Prosopis glandulosa exhibited acceptable growth responses to container production, but was eliminated from further study until a thornless selection can be identified and propagated. Sophora affinis has a slower growth rate than Acacia wrightii, Chitalpa linearis, X C. tashkentensis, and Rhus lanceolata, but plants remained attractive during production. Two to three foot tall S. affinis were produced in about 18 months, probably relegating this species to higher margin specialty markets. Ungnadia speciosa makes an adaptable landscape plant, but did not respond well to container production, suffering from repeated defoliation and susceptibility to root rots in four different media combinations. Pseudocydonia sinensis exhibited moderate growth rates, but developed what appeared to be salt burn on the leaves with drip irrigation and tended to defoliate with overhead applications of irrigation water. This would be a limitation to production in Texas as irrigation water with low salt content is often not available. While irrigation water at the test site was maintained at pH 6.3 to 6.7 via injection of sulfuric acid, the water contained 250 mg Na and 500 mg bicarbonates per liter.

Preliminary results suggest that Chilopsis linearis, X Chitalpa tashkentensis, and Rhus lanceolata height and trunk caliper growth were comparable to that of Fraxinus velutina; salable three gallon plants were produced in 12 to 18 months. Some refinement of media requirements, IPM practices, and pruning practices may be required. While A. wrightii had acceptable height growth, but had small caliper and thorns. The thorn might be an asset if a barrier plant is desired. A few thornless (or nearly so) selections of A. wrightii were identified in the study, but propagation methods which maintain the thornless trait are undeveloped. While C. linearis and X C. tashkentensis compared most favorably with F. velutina, they are not without limitations. Chilopsis linearis requires a well drained site and would not generally be suitable for sites with poor drainage or heavy clay soils. X Chitalpa tashkentensis is more tolerant of imperfect drainage, but is susceptible to Alterneria leaf spot (identified by Dr. Larry Barnes, TAEX Plant Pathologist) and powdery mildew in humid regions.

Plans are to continue investigations on production and market potential of the most promising species and expand the project to include other species, methods of production, and to access establishment potential. This project is one of several being developed at the newly renovated TAMU Nursery/Floral Crops Research and Education Laboratory in the TAMU Department of Horticultural Sciences in conjunction with the Texas Agricultural Extension Service's Target 2000 Program (an integrated environmental plan for the Texas nursery and floral industries) and CEMAP (Coordinated Education and Marketing Assistance Program).

We would like to thank our industry cooperators in the project and recognize the advice and assistance of Prof. Benny Simpson (Dallas), Dr. Yin-Tung Wang (Weslaco), Dr. Wayne Mackay (El Paso), Dr. William C. Welsh (College Station), Dr. Don C. Wilkerson (College Station) and Dr. Charles R. Hall (College Station). A special thanks to Mr. Todd Kinney and Mr. Gary Kinney of Kinney Bonded Warehouse (Texas) for the donation of supplies and materials need for initial stages of this project and to The Scotts Co. of Marysville (Ohio) for donation of coconut coir.

Bibliography

Miller, G.O. 1991. Landscaping With Native Plants of Texas and the Southwest. Voyageur Press, Inc., Stillwater, MN. p. 128.

Native Plant Project. 1995. Native Trees of the Lower Rio Grande Valley, Texas. Native Plant Project, Edinburg, TX. p. 36.

Native Plant Society of Texas. 199?. Texas Natives Ornamental Trees. Native Plant Society of Texas, Georgetown, TX. p. 40.

Nokes, J. 1986. How to Grow Native Plants of Texas and the Southwest. Texas Monthly Press, Austin, TX. p. 404.

Simpson, B.J. 1988. A Field Guide to Texas Trees. Texas Monthly Press, Austin, TX. p.372.

Simpson, B.J. 1994. The lacey oak. Neil Sperry's Gardens 8(8):50.

Wasowski, S. and A. Wasowski. 1988. Native Texas Plants Landscaping Region by Region. Texas Monthly Press. p. 406.


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