Walker's Manioc (Manihot walkerae)
- Other Names
- Walker's Manihot, Texas Tapioca
- Texas Status
- U.S. Status
- Endangered, Listed 10/02/1991
- Walker's manioc is a perennial, many-branched, reclining to erect herb up to 5 feet in height. The leaves are alternate, 5-lobed, and deeply incised. The narrow stems are smooth and grayish-brown. Separate male and female flowers occur on the same plant. Male flowers, which occur on elongated stems, are white with light purple streaks and almost tubular in shape. The tiny female flowers occur at the base of the male flowering stalks. The plants flower April to September following rains. The globe-like fruits, about 1/2 inch long, occur on slightly downward curved stalks and contain 3 seeds per capsule. The round or slightly flattened gray seeds have small irregular dark spots. The root of Walker's manioc, much smaller than that of cassava, measures about 4 inches long and is shaped like a carrot. All plant parts have a strong cyanide odor when fresh.
- Life History
- Walker's manioc is a member of the spurge family (Euphorbiaceae) and is related to the important crop plant, cassava, an important source of starch and a staple food for peoples of the tropics worldwide. Both cassava and probably Walker's manioc are poisonous if eaten raw. Little is known concerning the biology of Walker's manioc. The landowner protecting the one plant growing on private land in Hidalgo County reports that the white flowers open in clusters of three or four fragrant blossoms in late afternoon and last only one day. Specific pollinators have not yet been determined.
Walker's manioc is apparently self-fertile, since the one plant in Hidalgo county produced seed without cross pollination with other plants. Plants in the Euphorbiaceae family disperse their seed from capsules that split apart as they ripen, propelling the seeds 1-2 yards into the air. Small cloth bags may be fastened over the developing fruits to capture seeds as the capsules split apart.
Many of the plants recently found in the wild are growing within another shrub and therefore are very difficult to see. Walker's manioc plants may be defoliated and die back to tubers in response to prolonged drought, freezing weather, or excessive grazing. They grow in association with anacahuita, barreta, blue sage, Calderona krameria, cenizo, coyotillo, drago, elbowbush, guayacan, tasajillo, and wild oregano. Much of the native brush habitat in the historical range of Walker's manioc has been cleared for agriculture, urbanization, or improved pasture. It has been estimated that over 95% of the native brush in the Lower Rio Grande Valley has been converted to other land uses. As part of efforts to recover this species, transplanted specimens are under cultivation at the University of Texas, Austin and at the San Antonio Botanical Gardens.
The importance of Walker's manioc as a genetic resource highlights the need to protect this species. Because Walker's manioc may contain genes that provide resistance to drought, cold, or plant disease, and compounds that are useful for treating human disease, it is of special interest to botanists, plant breeders, and drug companies. If Walker's manioc and cassava can be interbred, it may be possible to reduce crop diseases or expand the range over which cassava can be grown, helping to feed more people.
- Walker's manioc grows in dense stands of native brush or in small openings. In Mexico, the species occurs in open brush and brush-invaded prairie. On the sites where it is found, Walker's manioc grows in areas that are somewhat shaded and relatively moist compared with the surrounding environment.
- Historically, Walker's manioc is known only from the lower Rio Grande valley of Texas (Hidalgo and Starr counties) and northern Tamaulipas, Mexico. Until recently, it was believed that this species was represented in the U. S. by a single plant in the wild, discovered in Hidalgo County in 1990. In 1995, Walker's manioc was located in three different areas on the Lower Rio Grande National Wildlife Refuge in Starr and Hidalgo Counties.
- Walker's manioc was listed as endangered by the U.S. Fish and Wildlife Service in October 1991. Landowners and land managers can help conservation efforts by learning to recognize this rare and potentially important plant, and by protecting the area where it grows from land use changes. Mechanical brush management and herbicide use should be carefully planned to avoid impacts to these sites.