fbpx

Shaw’s Agave

Agave shawii shawii

cluster of yellow flowers from Shaw's Agave
Nature Center | Jan. 2021

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 native to a small region of coastal Baja California with the northern boundary extending into San Diego County. The Calfornia population has been seriously reduced by urban development and last remaining population north of the border was destroyed in 2008 by the construction of the border fence.  A single native 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.

Other Common Names:

Coastal agave, mescal, maguay

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.

clump of Shaw's agave

One clone: two blooming stalks, three stalks with last years fruit (two fallen) and several old rosettes | Torrey Pines State Park | Feb. 2021

rosette of fleshy, spiny leaves

A single rosette; note pattern impressed on lower leaf surfaces | Nature Center board walk | Jan. 2021

cluster of yellow flowers with bee

Flower cluster showing (c.w. from lower right) tepal, filament, anther, style, stigma

Distribution 7,89,519  

Shaw’s agave is native to northern coastal Baja California, with its northern boundary extending into southwestern San Diego County. Historically the population north of the border occurred in coastal sage scrub vegetation on coastal bluffs and nearby mesas and foothills below 300 ft (100 m). Most of these native populations have been eliminated by urban development. In 2008, during the construction of the border wall on Border Field, the last native population was reduced to one clump of about 50 rosettes (a single individual). A second population on Point Loma is thought to contain some native plants, supplemented by recent transplants. As of 2014, four other significant populations had been established from transplants on public lands: three on Point Loma and one at Torrey Pines State Park. Shaw’s agave is considered seriously endangered in California, but is not classified by the federal government or by Mexico.

In San Elijo Lagoon Ecological Reserve there are no native individuals. The largest plant is a dense clone of approximately 50 rosettes in front of the Nature Center. Another individual grows on the bank behind the Nature Center where the original rosette bloomed in 2013. The original is gone but a clone of six or more rosettes remains. At least two rosettes were planted in the hills above the Rios trail where the fierce armor was intended to stop off-trail foot traffic. More recently a few transplants were placed along the boardwalk at the Nature Center.

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 leaves11,76 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 native 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

Alternate Scientific Names:

Agave orcuttiana, A. pachyacantha

Jepson eFlora Taxon Page
rosettes of spiny leaves in native plant garden

Whitaker Garden, Torrey Pines State Park | Feb. 2021

Cluster of agave with current and old flower stalks

Torrey Pines State Park | Feb. 2021

close up of sharp leaf spines

Nature Center | Jan 2021

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.

bee, crawling among yellow anthers

Nature Center | Jan 2021

spike of yellow flowers next to a parking lot

Nature Center | Feb. 2021

dried cluster of opened seed pods

Cluster of opened seed capsules; oval, white seeds are infertile |Torrey Pines State Park | Feb. 2021

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

Agave clusters blocking a closed trail

Rosettes planted to block a closed trail | Central Basin, south side | June 2019.

rosette of fleshy, spiny leaves

Single rosette; note pattern impressed on lower surfaces | Nature Center board walk | Jan. 2021

clusters of yellow flowers

Note large bracts at base of each cluster (lines) | Nature Center | Feb 2021

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.

Shaws agave with young flower stalk without flowers

Flower stalk produced in summer | Nature Center | July 2020

Shaw's agave starting to bloom

Same flower stalk first blooming in following winter | Nature Center | Feb. 2021

salk with clusters of yellow flowers

Nature Center | Jan. 2013

Photo Gallery