Texas wild rice (Zizania texana Hitchc.), a rare and endangered species of aquatic grass, exists in the spring-fed headwaters of the San Marcos River within the city limits of San Marcos, Hays County, Texas. This unique plant is endemic to Texas and occurs, in the United States, naturally only in this location. Because of its endangered status and declining population, Zizania texana is protected by federal regulations. The plant is habitat specific. Its relatively small relict population is restricted to a 1½-mile length of the headwaters of the San Marcos River. In the artificially formed Spring Lake, which is part of that length, it has been eradicated. Historically and scientifically, little is known about Texas wild rice, and only limited interest is expressed in this rare species. It is closely related to annual wild rice (Zizania aquatica), an economically important plant that grows abundantly wild in the northern, central, and eastern wetland areas of North America but not in Texas. Texas wild rice is a coarse perennial forage plant that grows completely submerged in 0.3 meter to 2.0 meters of clear water of constant temperature, except for the flowering head, which rises 0.3 meter to 0.6 meter above the current. It grows out in the swift current away from banks and quiet waters. It has the general appearance of Johnsongrass, panicum, or sedge growing underwater. Its luxuriant growth of long, flat, narrow, highly pliable, bright green leaves gracefully ripples, as streamers below the surface reach lengths of up to 110 centimeters, and its stems grow to lengths of 3.7 meters or more.
This plant attaches itself firmly to the gravelly streambed by relatively short spongy roots serving as a sturdy anchoring system. A crown of tightly entangled roots develops into a plant clone or colony, which is exposed above the streambed soil. The basal area of a Zizania texana clone ranges from a few square centimeters to as much as a square meter. New plants other than seedlings are produced at the end of short coarse stolons (runners). Panicles (flowering heads) of the Zizania texana are normally developed during the fall and spring, usually in early spring. However, inflorescences appear sporadically throughout the year. This species has the capability of producing viable seeds. Unfortunately, it has little opportunity to do so because of scattered and thinning clones and man's activities in the natural habitat. It is highly unlikely that this rare plant is reproducing itself significantly by seeds. Modern man's impact upon the natural habitat of Texas wild rice began about 1831, when settlers arrived in the area. Before the influx of people the prime dangers to Zizania texana were habitat alterations by natural phenomena, which could alter significantly the spring flow, local geography, and water chemistry.
Scientific attention was first given to Texas wild rice in August 1892 by G. C. Nealley, a botanical collector commissioned by the United States Department of Agriculture to investigate the grasses and forage grasses in Southwest Texas. He reported his find of an aquatic grass growing in the spring-fed headwaters of the San Marcos River as a variety of Zizania aquatica (annual wild rice). In the late 1920s W. A. Silveus, an agrostologist of San Antonio, collected specimens of the aquatic grass at San Marcos. At this time he called it Zizania aquatica. Later Silveus noted several differences and thought that this plant might be a new species. He asked A. S. Hitchcock, an eminent agrostologist and widely traveled plant explorer, to verify his latest observations. The plant was named Zizania texana in 1932. In his writings in 1932 Silveus commented that cattle had been observed sticking their heads deep into the running water to graze on Zizania texana. In 1933 he noted that panicles of this plant were seen throughout the year and that its growth in the water was so luxuriant that the irrigation company had difficulty in keeping the artificial lake (Spring Lake) and ditches clear. In 1967 W. H. P. Emery, biology professor at Southwest Texas State Teachers College reported some drastic changes in the quantity and habitat of Zizania texana. In 1976 he measured 1,131 square meters of Texas wild rice in the San Marcos River. By 1986 coverage had fallen to 454 square meters but rose again in 1992 to 1,612.65 square meters. He concluded that the rapid decline of this rare species was probably caused by floating aquatic vegetation debris from Spring Lake, streambed plowing in the city park area, plant collecting, and pollution of the stream. The causes of population decline of Zizania texana suggested by Emery were supported in studies and surveys made by H. E. Beaty in 1972, W. H. P. Emery in 1977, J. E. Vaughn in 1986, and P. J. Power in 1992. In 1994 threats to Texas wild rice included dams, pollution, recreational use of the river, competition, and floating debris caused by the mowing of Spring Lake. Areal coverage continues to decrease in some areas of the river.
Texas wild rice is threatened with extinction as a direct result of human encroachment. There is concern that much of the genetic diversity once available in it may have already been irretrievably lost. Currently, its beneficial properties are under study. Research efforts have been made to combine certain highly desirable genetic properties of Zizania texana with a native North American cereal, Indian (northern) wild rice (Zizania palustris), a commercially successful species. If such efforts succeed, it is highly probable that Zizania texana's genes could contribute to the wild rice industry. Experiments conducted in 1986 showed that Texas wild rice grew best at water depths of less than two meters due to lack of light. In 1990 further experimentation showed that higher levels of germination occurred in lower oxygen concentrations. Southwest Texas State University and the Texas Parks and Wildlife Department are moving ahead with additional research in the hopes of discovering the habitat requirements of Texas wild rice, since it is extremely difficult to grow it outside the San Marcos River.