Rocky Mountain National Park staff, on very rare occasions, find ourselves viewing our spectacular winter landscape and thinking of slightly warmer parks (perhaps Haleakala National Park, Hawaii). In so musing, one might find one's self pondering the similarities and differences between Rocky Mountain National Park's soapweed yuccas (also known as Spanish bayonets) and Haleakala's silverswords. These two unrelated plants live in similar enough habitats to have developed many similarities in lifestyle and appearance. Scientists call such species ecomorphs.
Yuccas and silverswords have long, narrow leaves that tend to reflect intense sunlight (yuccas because of their light blue-green color, and silverswords because they are covered with white hairs.) They are both succulent and grow from a taproot that helps with water storage and to keep the plant anchored in strong winds. Both plants grow in the form of globe-shaped rosettes and bear several flowers on long spikes. Both occur on dry, open, high altitude, high light intensity, often windy slopes.
There are many differences between the two species, despite their superficial similarities in appearance. Soapweed yuccas (Yucca glauca), are members of the century-plant family (Agavaceae), and are long-lived perennials. The Agavaceae are closely related to the lily family (Liliaceae), and some authors still classify it in that family. Yucca flowers are greenish-white, produced annually, have three petals, three sepals that look like petals, and about ten to fifteen occur on a spike. The leaves contain tough, fibrous, parallel veins and are up to two feet long. Yuccas are widely distributed across the west central part of North America, west of the Missouri River, from southern Alberta to Texas. They grow up to about 8,500 ft. altitude.
The Haleakala silversword (Argyroxiphium sandwicense ssp. macrocephalum) is an endemic plant, known only from an area of about 2,500 acres on the upper slopes (6,890 to 9,843 ft) of Haleakala Volcano. It is a member of the Composite family (Asteraceae), and flowers only once in its 15 to 50 year life. When a plant flowers at the end of its life, it produces a one and a half to six and a half foot tall flowering stalk, usually with hundreds of maroon sunflower-like flower heads.
For more information about the Haleakala silversword check out the article by Lloyd Loope, U.S. Geological Survey researcher, for a more scientific description of the plant, or the outstanding web page by U.S. Geological Survey Research Associates Forest and Kim Starr, featuring more than 8,000 high resolution, public domain photos of native and non-native Hawaiian plants.