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This diminutive globular cactus has stems that reach only 3-6 cm in diameter, covered with tiny, grooved tubercules. Each tubercule has an areole filled with long white trichomes, and 11-17 radial spines. During dry seasons, plants lose enough water to 'shrink' down to ground level, virtually disappearing among the spaces between the rocky substrate. Seedlings are almost impossible to find. Speculation as to the ultimate lifespan of Coryphantha robbinsorum varies from 18 to 40 years. The difficulty in estimating ages of plants lies in the very slow growth (ca 1-2 mm/year in diameter). Flowers are light yellow-green, ca. 5 mm in diameter, and open only enough to expose the stigma and stamens. C. robbinsorum is an obligate outcrosser, pollinated by two species of tiny, native bees. Fruits are small and globular, red in color, eventually drying on the plants. Plants flower and fruit from late March into early May. The bright white radial spines and small size distinguish it from any other cactus in the area.
Only three populations of this species are known. All are located within walking distance from each other. A detailed census covering one square hectare was completed by A. Zimmerman in 1978-79, who found approximately 470 plants. The habitat is an unnamed range of hills comprised of limestone, broken into broad sheets, interspersed with patches of desert pavement. Elevation is about 3000-3500 feet. Surrounding species include Fouquieria splendens, Vaquelinia pauciflora, Larrea tridentata, Prosopis velutina, Rhus microphylla, Rhus coriophylla, Agave palmeri, and Dasyliron wheeleri.
Populations are 'bunched', occurring in groups, or sub-populations within the species' range. Plants were listed as Threatened under the Endangered Species Act in 1986. Understandably, the major threat to this species lies in its limited range and the scarcity of plants within that range. It would take only a small localized catastrohic event to decimate the species, or reduce its numbers to a point where it could no longer sustain itself. Overcollection, mineral exploration, or competition with introduced species associated with cattle grazing could potentially wipe Coryphantha robbinsorum from the face of the earth.
Desert Botanical Garden has only 2 field seed collections, both obtained from a population occurring on privately owned land. The first collection was made in 1987 when 812 seeds were collected. The seeds appeared immature at the time of collection, But because access to the ranch on which plants occur is restricted, seeds were collected anyway. Subsequent germination tests failed. The seeds had been immediately frozen following cleaning and counting, prior to establishing a baseline germination percentage. If the seeds were not fully mature, it is possible that freezing killed the developing embroyos. Another possibility arising from the failure to germinate seeds collected from any of the plants was that freezing may not be appropriate for some seeds of desert plants, notably members of Cactaceae. Experimentation with some other species of cactus has provided similar results. The protocol for the storage process for Cactaceae is being re-examined at DBG. Another collection was made from the same site during 1995. Five seeds were collected from each of 50 plants.
During 1999, DBG received 33 plants, each approximately 2 cm in diameter, from a private individual. These plants immediately flowered at the time they normally would have, and controlled cross-pollinations were conducted, producing 78 fruits and a total of 2827 seeds. The seeds were allowed to dry in the partially dry fruits, and then cleaned and counted by hand. They were stored in an airtight foil pouch after 6 weeks of drying at 63C. Germination tests have not yet been conducted on these seeds. Seedbanking for this species calls into question the protocol for establishing a conservation seedbank, as each plant may produce only 1200 seeds during its lifetime. Until germination tests provide more definite results, the Garden maintains a conservative approach to augmenting the seedbank it has established.
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