The large-fruited Sand Verbena

As mentioned in the Parkscape Spring 2001 article titled: "Plant Conservation a Priority at Mercer", Mercer Arboretum and Botanic Gardens is one of the 28 leading botanical gardens and arboreta in the United States that maintains the National Collection of Endangered Plants for the Center of Plant Conservation. 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 and is maintained in Mercer’s research conservation areas for the National Collection of Endangered Plants.

Abronia macrocarpa, first discovered in 1968 by the botanists, Doctors Donovan and Helen Correll, occurs only in the sand dune habitats of post oak savannas in three east-central Texas counties. The Large-fruited Sand Verbena is a 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 beautiful wildflower typically appear following Spring rains and produce an intense, sweet perfume at dusk.

Contrary to its common name, the Large-fruited Sand Verbena is a member of the Four-o’clock family or Nyctaginaceae. The Nyctaginaceae family includes the popular garden annual, Four-o’clock (Mirabilis) and the sub-tropical vine, Bougainvillea and consists primarily of tropical and subtropical herbs, shrubs and trees. 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 partner, Dr. Paula Williamson of Southwest Texas State University in San Marcos and her students study the genetic diversity and bio- logy of the endangered Abronia. 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".

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. Conservation and research efforts at Mercer Arboretum and Botanic Gardens, Southwest Texas State University, Stephen F. Austin Mast Arboretum and other private and government research facilities are crucial in insuring the preservation of the beautiful Abronia macrocarpa and all endangered Texas plants for generations to come.

Large-fruited Sand Verbena seedlings produced by the "charter" specimens are being transferred from Mercer’s conservation research area to the public Endangered Species Garden in preparation for their bloom in Spring 2002. The Endangered Species Garden established in 1994 with the generous support from Star Enterprises, offers the public a unique opportunity to view a number of rare plant species up-close at Mercer Arboretum & Botanic Gardens.

Anita A. Tiller, Botanist, Mercer Arboretum & Botanic Gardens, Summer 2001