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Echinocereus viridiflorus v. davisii was discovered at the type locality in Texas, and described as E. davisii by Arthur Houghton in 1931. Similarities between E. davisii and E . viridiflorus compelled W. T. Marshall to make it a variety of E. viridiflorus in 1941. Examination of flavenoids in 1983 by J. Miller at Sul Ross University in Alpine, Texas determined that there were similarities betwood E. davisii and E. viridiflorus, supporting the varietal theory.
Plants are extremely diminuitive, up to 3 cm in diameter, and usually do not offset. Stems are attractively marked with regularly spaced purplish horizontal stripes, and plants have 6-9 ribs. The aereoles contain 8-13 spines, usually radial, and grayish. Flowers are yellow-green, up to 2 cm across, and fruits are greenish-purple. The fruits dehisce along a suture along the side, revealing up to 100 tiny black seeds. During drought (usually late fall, and early spring), plants utilize water stored in the stem for metabolic processes, causing a shrinking effect. During drought, or non-flowering times, plants are extremely difficult to find, as they are found growing with Hilaria sp, which has greyish, curling lower leaves very much resembling spines of E. viridiflorus v. davisii. Sometimes plants shrink well below ground level. Flowering and fruiting occur in April to May on 3 to 4 year-old plants.
The single known population occurs on a specific limestone substrate in west Texas, in a semi-desert grassland. A large percentage of the soil is covered by Selaginella sp., but other associated plants include Opuntia leptocaulis, Escobaria hesteri, Larrea tridentata and Dasyliron leiophyllum.
The majority of the limestone formation lies on privately owned land with no access. Historically, access was once granted to the limestone formation on which plants occur. Beginning approximately 25 years ago, the land has been inaccessible. It is believed that the present range is the same as the historic range, as plants are very substrate-specific. Only a small portion of plants growing along a 20 m strip next to a road can now be reached. These plants are being monitored by the Texas Parks and Wildlife staff in cooperation with USFWS. Populations appear stable, as collection is the main threat, and collectors are unable to get on the land where the majority of plants are growing. Neither grazing nor secondary habitat destruction due to grazing is especially a threat, due to the rocky substrate. Natural threats include interspecific competition, and restricted habitat. E. viridiflorus v. davisii was listed endangered in 1979.
Seeds and plants were collected by Desert Botanical garden as early as 1997. Immediately, attempts were made to germinate seeds. Germination is rapid and the percentages are high, but the survival rate of seedlings is low, even in cultivation, possibly due to the miniscule size of seedlings. Watering properly is challenging. Early germination experiments (1991) were conducted on seeds that had been cleaned, counted, dried, and frozen. One or more of the processing methods could have accounted for relative low germination percentages, or the timing of seed collections. In Cactaceae, seeds must be completely ripe (detached within fruits) in order to achieve high percentages of germination. Early germination techniques used were the Steve Brack screened box method, whereby a wooden box covered with window screening houses pots of seeds, and is misted twice daily. This method ensures stability of the round seedlings in the soil, by reducing the size of water droplets reaching plantlets.
Subsequent germinations using other methods have been successful as well. Seeds are collected annually by Desert Botanical Garden as stipulated by the Texas Office of the USFWS. Manipulations with seeds and plants are reported annually to Big Bend National Park (located nearby), Texas Parks and Wildlife, and the USFWS. These reports can serve as a means of assisting agencies to determine optimal management procedures in case the opportunity to use them becomes available. The collected seeds are stored both at DBG and the National Seed Storage Lab in Ft. Collins, Colorado.
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