Harris County Precinct 4 Mercer Arboretum & Botanic Gardens
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The Large-Fruited Sand Verbena

  Large-Fruited Sand Verbena
 

Large-Fruited Sand Verbena

   

Mercer Arboretum & Botanic Gardens is one of 38 leading botanical gardens and arboreta in the United States that collectively maintain over 750 species for the Center for Plant Conservation’s (CPC), National Collection of Endangered Plants. The CPC preserves rare, native (American) plants in a centralized network of botanical institutions within 17 states and the Virgin Islands. In Texas, Mercer Arboretum & Botanic Gardens, San Antonio Botanic Gardens, and The Lady Bird Johnson Wildflower Center share the responsibility for the preservation of rare plants within seed banks or as live specimens. In addition, the CPC institutions, The Arboretum at Flagstaff and Desert Botanical Garden of Arizona, maintain the rare mountain and desert species for Texas. In order to preserve the nation’s rare botanical treasures, Mercer and the CPC’s other member institutions hold partnerships with concerned citizens, private and government conservation organizations, universities, and colleges.

The large-fruited sand verbena (Abronia macrocarpa), listed as endangered by the federal and Texas governments in 1988, is one of Mercer’s “charter” species. In 1990, the large-fruited sand verbena received full sponsorship for the CPC through generous donations from The Quaker Hill Foundation. The large-fruited sand verbena benefits from annual sponsorship stipends directed to Mercer for equipment and supplies for the plant’s recovery.

First discovered in 1968 by doctors Helen and Donovan Correll and described as a new species in 1972, Abronia macrocarpa occurs only in the sand dune habitats of post oak savannas in three east-central Texas counties: Freestone, Leon, and Robertson. The large-fruited sand verbena is an herbaceous perennial and is extraordinarily well-adapted to drought conditions by virtue of its persistent taproot and ability to remain dormant during the hot summer months. The magenta, golf-ball-sized flower heads of this wildflower typically appear following spring rains, and they produce an intense, sweet perfume at dusk. Fall rains occasionally stimulate a second bloom season.

Contrary to its common name, the large-fruited sand verbena is a member of the four-o’clock (Nyctaginaceae) family that includes the popular garden annual four-o’clock (Mirabilis) and the sub-tropical vine bougainvillea. In the United States, members of the Nyctaginaceae family mostly occur in the southern and Pacific regions.

Although hummingbirds typically pollinate bougainvillea, moths are important pollinators of many of the four-o’clock species and the sweet-scented Abronia macrocarpa. Mercer’s conservation partners, Dr. Paula Williamson and her students at the Texas State University-San Marcos, and Gena Jannsen of Janssen Biological, study the genetic diversity and biology of the endangered Abronia and monitor wild populations for the Texas Diversity Database Project. These partners also collect seed stock for the CPC’s National Collection of Endangered Plants. The Texas Natural Diversity Database Program is a member of NatureServe, an international network of natural heritage programs and conservation data centers found in all 50 U.S. states, Canada, Latin America and the Caribbean.

Dr. Williamson and her research group documented that hawk and noctuid moths are attracted to the perfume released from the open Abronia flowers at dusk. Pollination by these moths occurs until the Abronia’s flowers close in the morning. Cross-pollination between individual plants of this wildflower is required for the production of fertile seeds within Abronia macrocarpa’s characteristic large, five-winged fruits called “anthocarps.” The wings on the anthocarps serve to assist dispersal of the seed by wind. However, researchers have found that seed are rarely dispersed more than a foot from the mother plants. Ants are instrumental in gathering the seed and uneaten seed will germinate in the ants’ underground nests.

During April of 2010, Elizabeth Saunders, Ph.D., candidate of Southern Illinois University Carbondale, visited Mercer’s Endangered Species Garden. By means of a portable vacuum pump, she captured the scent from the open flowers of Abronia macrocarpa. Saunders is studying the relationship between pollinators and the chemical makeup of the scents of the 20, or so, species of sand verbenas found in North America. Her studies hope to reveal insights about the evolution of floral scents and its relationship with pollinators.

Abronia macrocarpa’s restricted occurrence within sites under threat from commercial development and invasive plant species has made wild populations of the plant especially vulnerable. Successful partnerships currently underway with private landowners where the large-fruited sand verbena exists in the wild will help preserve this endangered plant species.

The large-fruited sand verbena typically blooms during March and April and is displayed with other rare Texas plants in Mercer’s Endangered Species Garden. Awarded the Best of Backyard Habitats certificate by Texas Parks and Wildlife and the National Wildlife Federation in 2004, and certified as a butterfly garden by the North American Butterfly Association in 2009, the Endangered Species Garden serves as an educational tool for the importance of “wildscaping,” demonstrating the use of permanent water and food sources, composting, and organic management methods to benefit wildlife. Common native plants complement the rare species in this display garden and provide examples for home landscapes.

Anita A. Tiller, Botanist
Mercer Arboretum & Botanic Gardens
Adapted from Parkscape, Summer 2001
Photo by Greg Wieland

References

Center for Plant Conservation (CPC). 2010. National Collection of Endangered Plants. www.centerforplantconservation.org

Flora of North America North of Mexico. Volume 4. 2003. Edited by Flora of North America Editorial Committee. Oxford University Press, New York. 559 pp. Available online: www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=100040

Judd, W.S., et al. Plant Systematics: A Phylogenetic Approach. Second Edition. 2002. Sinauer Associates, Inc. Sunderland, MA. 576 pp.

Lawrence, G. H. M. Taxonomy of Vascular Plants. 1951. Macmillan. New York. 823 pp.

Poole, J. M.; Carr, W. R.; Price, D. M.; and Singhurst, J. R. Rare Plants of Texas. 2007. A&M Univ. Press College Station, TX.

Williamson, P.S.; Bazeer, S.K. 1997. Self-incompatibility in Abronia macrocarpa (Nyctaginaceae). Southwestern Naturalist. 42: 409-415.

Williamson, P.S.; Muliana, L.; Janssen, G.K. 1994. Pollination biology of Abronia macrocarpa (Nyctaginaceae), an endangered Texas species. Southwestern Naturalist. 39: 336-341.

Williamson, P.S.; Werth, C.R. 1999. Levels and patterns of genetic variation in the endangered species Abronia macrocarpa Galloway (Nyctaginaceae). American Journal of Botany. 86, 2: 293-301.