Shaw’s Agave

Shaw’s agave (Agave shawii var. shawii) is a statuesque plant, from the large, sharp spines on the rosette leaves, to the spectacular display of yellow blooms massed on tall spires. The species is indigenous to a small region of coastal Baja California with the northern boundary extending into San Diego County. The California population has been seriously reduced by urban development and the last remaining population north of the border was destroyed in 2008 by the construction of the border fence.  A single indigenous individual was left.

Today’s population has been increased with transplants into reserves, parks and other protected areas, but these populations are not reseeding at a rate sufficient to reestablish truly wild populations.

Description ^2,76,519

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.

Classification

Shaw’s agave (Agave shawii var shawii)  is a perennial monocot in the agave family (Agavaceae). Monocots are an early offshoot of flowering plants partly characterized by having a single cotyledon (instead of two), by having parallel veins in the leaves (instead of reticulated) and by having flower parts in multiples of three (instead of four or five).¹⁷⁶

Agaves have been cultivated for centuries, first by pre-Columbian Americans and later by horticulturists in the old world. The resulting plethora of cultivars includes a spectrum of hybrids and mutants that complicates modern taxonomy ⁵¹⁹. At present, the source we follow² retains the agave family as a separate family. Others place it as a subfamily of the asparagus family (Asparagaceae).^41,143 It has also been considered a subfamily of the lily family (Liliaceae). ^11,174

Members of the agave family are often found in dry habitats.⁷⁶ They are characterized by rosettes of stiff, fibrous leaves^11,76 and by a fruit with two or more chambers that becomes dry and splits open at maturity.⁴⁴ Many have large, erect, conspicuous flower clusters.⁴¹

Four indigenous species within the Agave family are found in the Reserve. In addition to Shaw’s agave, we have chaparral yucca, (Hesperoyucca whipplei), mojave yucca (Yucca schedigera) and soap plant (Chlorogalum parvifolium). Three are similar in general structure, being tree or shrub-like with rosettes of thick, fleshy leaves and tall upright stalks of flower clusters.  These genera are separated on the basis of flower structure. The fourth, soap plant,  is a diminutive plant that appears seasonally from a bulb.

There are two varieties of Agave shawii (sometimes called subspecies). The other,  A. shawii goldmaniana, replaces ours about 200 miles (350 km) south of the US-Mexico border.⁵¹⁹

Ecology ^519,520

Among the important management concerns for an endangered species is the production of seeds and seedlings. These often result from cross-pollination, a mechanism that facilitates population variability, change and ultimately population survival.  Shaw’s agave readily reproduces asexually by cloning, but both the production of viable seeds and the production of seedlings are depressed below the rates seen in natural populations further south.

As of 2014,⁵¹⁹ we did not know the extent to which Shaw’s agave was self-compatible (able to fertilize itself), or whether successful fertilization was restricted by the low numbers of individuals, and the low fraction of plants blooming at any one time. Nor did  we know what organisms pollinate Shaw’s agave. After successful pollination, seedling germination and growth may be limited by lack of open ground, by predation of the seeds or by grazing on the tender young plants.

In the intervening years, scientists from both U.S. and Mexico have begun to work on the reproduction problem. Biologists at National Park Service studied germination success of viable seeds by growing  half of them with in wire cages to exclude herbivores. Numerous seedlings were found inside the cages; none were seen outside. Thus, some of the recruitment mystery may be explained by hungry herbivores.

To my knowledge, the degree of cross-pollination is still unknown. Comparison with a closely related species suggests that both are possible but cross-pollination may cause self-pollinated seeds to abort.

The search for missing pollinators continues. Several characteristics of the agave flower suggest chiropterophily (pollination by bats): the flowers are tubular and yellow (or white) in color, and held upright in separate clusters with lots of air space around them, and abundant pollen and nectar. At the southern end of the species range, bat pollination seems to be common, but the pollinating bat species no longer (if ever) extend into San Diego County. Some botanists believe that Shaw’s agave is currently evolving away from bat pollination to pollination by daylight organisms.  North of the border, several diurnal visitors are common, including bees, hawkmoths and birds, but no active transfer of pollen has yet been observed. The questions of cross-pollination and principle pollinators remain open.

Human Uses ^75,272,519

Many agave species were used interchangeably by indigenous Americans and may not always have been distinguished by the users.²⁷²  As a group, agaves concentrate liquid carbohydrates in their stems and leaves and they were an important food staple, the ‘daily bread” ²⁷² of the coastal tribes, somewhat analogous to acorns and pinon nuts for the more inland tribes.

An agave rosette was harvested with a stout hardwood digging stick, usually sharpened at one end into a point or chisel and fire hardened. This was also used to remove the leaves and stalk from the base. Parts were all baked in a large, stone-lined roasting pit usually for 1-2 days. (Campbell⁷⁵ pp 75-77, has photographs of the entire operation.) The result has been described as “bland”⁷⁶ and “deliciously sweet,”⁷⁵ possibly depending upon the agave species.

Shaw’s agave also produces a strong fiber, ⁵¹⁹ used for string, nets and cordage.²⁷²

Interesting Facts ^21,521

Shaw’s agave is named for Henry Shaw, the founder of the Missouri Botanical Gardens. Shaw came from England at the age of 18, and settled near the growing river town of St. Louis, buying property on the edge of the prairie –  “covered with tall, luxuriant grass, undulated by the gentle breeze of spring.” He opened a business in hardware and cutlery and, at the age of 39, retired with a substantial fortune. Inspired by Kew Gardens, he developed his property into a magnificent garden, herbarium, museum and library and gave it to the people of St. Louis. Today, The Missouri Botanical Garden is considered among the top botanical gardens in the world.