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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.

There are several characteristics that seem to help in separating the two species, the most visually apparent being the purple tinge to the flowers of Y. harrimaniae; the tendency for its flower stalk to begin blooming a bit below the leaf tips; and its concave, usually wider, stiffer, longer leaves.  

Flowers of Y. harrimaniae are broadly bell-shaped, those of Y. angustissima generally taper at both ends.  The styles of Y. harrimaniae are bright green, those of Y. angustissima are 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.

Yucca flowers of both species are thick, waxy, and attractive.  But these characteristics do not assure pollination.  Because of the unusual structure of Yucca flowers, they can only be pollinated by having pollen forced onto the stigma.  This task is accomplished only by the Tegeticula genus of moth, primarily Tegeticula yuccasella, the Pronuba Moth.  The 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 this genus in 1753.  Why he used the word "Yucca" we do not know, but it is one of the many common names for the Manihot Plant, Cassava, Manihot esculenta, a Euphorbiaceae (Spurge Family) of the Caribbean area.  The two plants do have large tuberous roots but otherwise do not resemble each other at all and Linnaeus must have known this as he also named Manihot esculenta.

 

 Yucca angustissima (Narrow Leaf Yucca)
Agavaceae (Agave Family)

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

Narrow Leaf Yucca is very common in Utah Canyon Country, northern Arizona, western New Mexico, and into Colorado, often replacing the Broad Leaf Yucca as the dominant Yucca.  Its leaves are a delicate light green, seven to twenty-four inches long and quite narrow.  From the center of the leaves a flower stalk emerges in early spring and grows rapidly to about three or four feet.  A long cluster of very handsome sphere-shaped flowers follow.  (See the very similar Yucca harrimaniae below.

The plant was first collected by John Bigelow in 1854 and famed St. Louis Doctor and botanist, George Engelmann, named it, probably soon after, 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.
Yucca angustissima (Narrow Leaf Yucca)
Agavaceae (Agave Family)

Semi-desert, foothills. Woodlands, canyons, openings. Spring.
Canyons of the Ancients National Monument, April 27, 2005.

Flower stalks grow rapidly to three or four feet tall.
Yucca angustissima (Narrow Leaf Yucca)
Agavaceae (Agave Family)

Semi-desert, foothills. Woodlands, canyons, openings. Spring.
Canyons of the Ancients National Monument, November 4, 2004.

It is common for Yucca angustissima to be found in colonies of a dozen or more plants sprouted from roots of the parent plant.

 
Yucca angustissima (Narrow Leaf Yucca)
Agavaceae (Agave Family)

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

Flower clusters (often three feet long) are delicate and very eye-catching in the spring Pinyon-Juniper forest.  Flower petals are thick, waxy, and long-lasting.  The flower of Yucca angustissima is very similar to that of Yucca harrimaniae, but usually Y. angustissima flowers are somewhat egg-shaped, i.e., the top and bottom of the flower taper from a cylindrical center.  The base of Y. harrimaniae flowers is broader than the top.
Yucca angustissima (Narrow Leaf Yucca)
Agavaceae (Agave Family)

Semi-desert, foothills. Woodlands, canyons, openings. Spring.
Corona Trail, Utah, June 7, 2007.

These young developing green seed capsules still have the flower petals hanging on.  The capsules will dry to a buff brown, split, and spill their seeds.

 
Yucca angustissima (Narrow Leaf Yucca)
Agavaceae (Agave Family)

Semi-desert, foothills. Woodlands, canyons, openings. Spring.
Canyons of the Ancients National Monument, March 13, 2007.

Pack Rats, Deer, and other critters nibble Yucca leaves.  This plant was nibbled over the 2006-2007 winter and you can see from the first buff colored row of nibbled leaves below the green that the critters have been returning to nibble for several years.  At the very bottom of the plant there are, however, many long leaves indicating that until about five years ago no critters were eating the leaves.
Yucca harrimaniae (Narrow Leaf Yucca) 
Agavaceae (Agave Family)

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

Yucca harrimaniae is common in the southern part of Colorado and westward through the Four Corners into northern Arizona and eastern Nevada.  It is very similar to Yucca angustissima in appearance and range.  It is quite distinct from Yucca baccata, having much thinner and narrower leaves than Yucca baccata and a flower stalk which is narrower and much taller in proportion to the plant.

The species name honors the Harrimans, sponsors of the Harriman Alaska Expedition.  The plant was 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 home stalled in Helper, Utah and Trelease passed the time botanizing; he discovered Yucca harrimaniae. (More biographical information.)
Yucca harrimaniae (Narrow Leaf Yucca) 
Agavaceae (Agave Family)

Semi-desert, foothills. Woodlands, canyons, openings. Spring.
Arches National Park, Utah, May 4, 2005.
Yucca harrimaniae (Narrow Leaf Yucca) 
Agavaceae (Agave Family)

Semi-desert, foothills. Woodlands, canyons, openings. Spring.
Corona and Bow Tie Arches Trail, Utah, April 14, 2008.

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