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Plants are low herbaceous perennials, forming a rosette of branches that grow flat against the ground. The ends of the branches turn up a little, and the entire plant ranges from 10 cm in diameter in the wild, up to 30 cm wide in cultivation. Leaves are alternate, compound with 9-13 leaflets, and appear folded along the midrib. Inflorescences are clover-like, a globose head about 1 cm in diameter, with whitish petals with pale purplish tips. Fruits are small flat pods with 2-4 seeds, and are indehiscent (shed from the plant unopened). Fruits are partitioned into two separate halves by thin membranes, with the seeds enclosed in each half.
Astragalus hypoxylus is distinguished from other species of Astragalus by the rosette-forming growth habit and the clover-like inflorescences. This species is very distinctive and was first classified in the genus Hamosa.
Plants were first described in 1882 from the type location in southern Arizona near the Mexico-U.S. border. The location description was not detailed, and plants were not seen again until 1986 in another mountain range. Plants are found in clearings in oak woodland comprised of Quercus hypoleucoides, Q. emoryi, Juniperus deppeana, and Pinus edulis. Exposures are generally south to southwest at 5300-5500 feet. Plants were designated in 1992 as Category 2 or Species of Concern under the Endangered Species Act.
Threats are primarily attributed to trampling by humans and cattle. Monitoring plots were established in 1988 and 1989.
Growth and reproductivity trends were difficult to establish during the brief time plants were monitored, but seedling establishment and mortality were approximately equal. DBG staff noticed a number of plants growing in disturbed areas along roadsides; shaded plants were larger that plants growing in full sun.
Desert Botanical Garden collected seeds of Astragalus hypoxylus from both sites in 1991 and 1992. Two germination tests were conducted in 1992, yielding percentages of 66 and 76. These tests provided living plants to be used for research and production of additional seeds in cultivation.
In 1996, plants growing outdoors in a test area produced over 14,000 seeds which were hand-cleaned. This labor-intensive task took several months, involving separation of the two fruit halves, and removal of the two septae enclosing each half, and extracting seeds which were approximately 2 mm across. The seeds are currently in frozen storage at -20C at Desert Botanical Garden and at the National Seed Storage Lab in Ft. Collins, Colorado.
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