Shaw’s agave is a large, slow-growing, perennial succulent with one or more basal rosettes. A rosette grows from a short stem; over time, additional rosettes may be cloned off (produced vegetatively) from the rootstock. Some plants exist as a single rosette but clones of 100 or more have been reported; such clones constitute a single individua.l A rosette is approximately spherical and three or more feet high (1 m). Leaves are thick and fleshy, oval to ovate in shape, usually concave upwards. They are medium to pale green and each is armed with strong, curved dark spines along the margins and a large, often reddish, terminal spine. The leaf surface may bear the imprint of the spined margins, formed when the leaf was furled. This varies from plant to plant and may depend upon the water content of the leaf.
A rosette grows slowly for years until enough energy has been stored to support a bloom. Each rosette may produce a single flower stalk, six to twelve feet high (2-4 m) which, in turn produces ten or more side branches, each of which supports a tight cluster of many (35-75+) large, upright flowers. Buds are red-tinged; open flowers are yellow often red-tinged with age. At the base of each cluster a large, succulent, reddish bract curves around the lateral stem.
A flower is bisexual with indistinguishable sepals and petals (together called tepals); these are united into a funnel that flares into six unequal lobes. There are six yellow stamens exserted from a flower throat. Anthers, 1-1½ inches long (2-3½ cm), are attached near their centers to thick yellow filaments, and pivot back and forth on the filaments under the weight of large insects. The single yellow pistil has an inferior ovary and a small three-lobed stigma atop a long style. The flower stalk is produced in summer; flowers usually open between January and March.
The fruit is an obovoid to oblong capsule, 2½ to three inches long (5½-7 cm), with an apical beak. There are three chambers with numerous flat, black seeds. Seeds are initially retained in the split capsule, and are gradually shaken out in the breeze. After fruiting, the parent rosette dies, leaving any clonal rosettes to bloom in future years.
The plant in front of the Nature Center at San Elijo Ecological Reserve, bloomed in 2020-21. In addition to the usual tall stalk of flowers from the largest rosette, several adjacent but smaller rosettes also produced much shorter stalks with a few clusters of flowers. I have not seen these abbreviated flowers described in the literature.