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   Yucca angustissima and Yucca harrimaniae are so similar that there is disagreement among experts about exactly what distinguishes one species from the other.

After I compared many descriptions of the two species, I found what appears to be several agreed upon characteristics that separate the two species:  

The flower stalk and flower cluster (the peduncle and inflorescence) of Y. angustissima can be 4+ feet tall.  Y. harrimaniae peduncle and inflorescence are typically no more than 2 1/2 feet tall and the lowest flowers are often a bit below the leaf tips.  Y. angustissima usually begins flowering above the leaf tips.  Leaves of Y. harrimaniae are concave (not flattened), usually wider, stiffer, and longer than those of Y. angustissima, but I find that leaf length varies enormously in both species. Some plants have leaves only 6 inches long, whereas other plants have leaves over 20 inches long.   

Flowers of Y. harrimaniae are broadly bell-shaped; those of Y. angustissima generally taper at both ends, somewhat ovoid shaped.  The flowers of Y. harrimaniae more often have a purple tinge; those of Y. angustissima are white to pale green.  Stanley Welsh, Utah plant expert, indicates that the style of Y. harrimaniae is green and the style of Y. angustissima is "white to pale green".  

One would think that with this many distinguishing characteristics it would be easy to identify the two plants.  It often is not, because these characteristics are highly variable and Welsh points out that the two plants form "apparent intermediates".

Pollination is the same for both Yuccas.  Their flowers are thick, waxy, and attractive.  But these characteristics do not bring about normal pollination.  Because of the unusual structure of Yucca flowers, they can only be pollinated by having pollen forced onto the stigma.  Yucca harrimaniaeThis task is accomplished only by the Tegeticula genus of moth, primarily Tegeticula yuccasella, the Pronuba Moth.  The usually night flying Pronuba drills a hole in the ovary of the flower, lays eggs, and in the process pollinates the Yucca. The Yucca then becomes food for the larva.  The hole can be seen in every Yucca fruit.

Linnaeus named the Yucca genus in 1753.  We do not know why he used the Carib word "Yucca" that had been previously applied as a common name for Manihot esculenta, the Manihot Plant or Cassava.  Although the Manihot and Yucca do have large tuberous roots they otherwise do not resemble each other and in fact are in two different families: Manihot is Euphorbiaceae and Yucca is Asparagaceae. 

Yucca angustissima

Yucca angustissima

Yucca angustissima (Narrow Leaf Yucca)
Asparagaceae (Asparagus Family)
Synonym: Agavaceae (Agave Family)

Semi-desert, foothills. Woodlands, canyons, openings. Spring.
Canyons of the Ancients National Monument, April 27, 2007 and May 11, 2016.

Narrow Leaf Yucca is very common in Utah Canyon Country, northern Arizona, western New Mexico, and southwestern Colorado, often replacing the Broad Leaf Yucca as the dominant YuccaYucca angustissima leaves are a delicate light green, seven to twenty-four inches long, and, usually, quite narrow.  From the center of the leaves a flower stalk emerges in early spring and grows rapidly to about three or four feet (although the stalk can be as much as six feet tall).  A long cluster of very handsome somewhat egg-shaped flowers follow.

The plant was first collected by John Bigelow in 1854 and famed St. Louis Doctor and botanist, George Engelmann, named it, probably soon after it was collected, but for some reason did not publish the name.  Trelease published the description of this species in 1902 and retained the name that Engelmann had given this species.

"Angust" is Latin for "narrow" and "issima" is a Latin suffix indicating the superlative, hence "very narrow" leaves.

Click for more Yucca angustissima and Yucca harrimaniae photographs.

Yucca harrimaniae
Yucca harrimaniae (Harriman's Narrow Leaf Yucca) 
Asparagaceae (Asparagus Family)
Synonym: Agavaceae (Agave Family)

Semi-desert, foothills. Woodlands, canyons, openings. Spring.
Arches National Park, Utah, May 4, 2005.

Yucca harrimaniae occurs in southwestern Colorado and westward through the Four Corners across northern Arizona and southern Utah into eastern Nevada.  It is, as discussed at the top of this page, very similar to Yucca angustissima in appearance and range.  It is quite distinct from Yucca baccata, having much thinner and narrower leaves and a flower stalk which is narrower and much taller in proportion to the plant.

The species name honors the Harrimans, railroad owners and sponsors of the Harriman Alaska Expedition.  The plant was specifically named for Mrs. Harriman by the botanist and Missouri Botanical Garden Director, William Trelease (1857-1945) who was on the Harriman Expedition to Alaska.  After the Expedition, the train carrying trip participants stalled in Helper, Utah. Trelease passed the time botanizing and discovered Yucca harrimaniae.
(
More biographical information about the Harrimans.)

Click for more Yucca angustissima and Yucca harrimaniae photographs.

Range map © John Kartesz,
Floristic Synthesis of North America

State Color Key

Species present in state and native
Species present in state and exotic
Species not present in state

County Color Key

Species present and not rare
Species present and rare
Species extirpated (historic)
Species extinct
Species noxious
Species exotic and present
Native species, but adventive in state
Eradicated
Questionable presence

Range map for Yucca angustissima

Range map for Yucca harrimaniae