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Apocynaceae
Amsonia tharpii occurs in only two populations in the United States. Plants are herbaceous perennials from a long woody taproot. They become dormant, dying back to the ground during November and emerging from dormancy during March. Leaves are sessile, linear-lanceolate with rounded tips, about 3 cm long and whorled on stems. Flowers are white, clustered at ends of stems, and are followed by two-lobed follicles. As follicles mature, they open completely, becoming flat and papery, dispersing all of the seeds inside except perhaps one remaining at the base of the follicle tube. Seeds are corky and cylindrical, about 1 cm long, and are irregularly truncated at the ends.
The plants in the New Mexico population are located on a northeast-facing side of a shallow canyon just beneath rocky limestone bluffs at edge of canyon. Approximately seventy-five plants were located east of the fenceline. The southwest aspect of the canyon was searched for plants, but none were found. Approximately twelve additional plants were found on the northeast side of a small unnamed side canyon.
The area surrounding the canyon where Amsonia tharpii grows in New Mexico appears to have been overgrazed for some time. Many trenches, some as deep as five feet and two to three feet wide, have been eroded into the southwest side of the canyon. Grazing has been discontinued for the last several years, and the canopy cover of grasses and forbs has regenerated; however, the presence of dense stands of Prosopis glandulosa saplings and cholla in the canyon bottom are reminders of past overgrazing. Because Amsonia tharpii is found on steep, relatively inaccessible areas along the sides of the canyon, grazing and trampling do not appear to be direct imminent threats. However, plants are subject to extirpation resulting from environmental changes brought about by overgrazing, such as severe erosion resulting in loss of habitat, and change in species composition, resulting in increased competion with non-native species.
The Texas population is situated on an entirely different soil substrate and has an entirely different topography. Plants are found only on one side of a paved road on a flat, open area that has been exclosed to keep out cattle. As plants are in the Apocynaceae family and have milky sap, they are unpalatable to cattle, but are vulnerable to trampling and soil compaction. Plants are also found growing along the highway easement, a narrow strip that is regularly mowed. During a drought year, Amsonia tharpii plants were the only green ones in the area, apparently extracting water from deeper in the soil through the long taproots.
Desert Botanical Garden has 600 seeds from the Texas population, and 959 seeds from the New Mexico population. Thirty-three plants grown from seed obtained in Texas have been maintained in pots beneath 30% shade for approximately 10 years. During this time, only seven seeds were produced in cultivation through hand-pollination efforts. Flowering is in April, but not always reliable on a yearly basis, even under controlled conditions in cultivation. Plans for A. tharpii include continuation of seed collection when conditions permit, and further attempts to produce seeds in cultivation.
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