Alien Invasion, Part 11:

Giant Reed (Arundo Donax)

While not an invasive species that is present in the Elfin Forest currently, giant reed (Arundo donax) is present nearby and could spread into the park. It is found in both the Los Osos Creek and Chorro Creek watersheds, as close as a half mile from the Elfin Forest. Arundo donax has spread along creeks and rivers and wherever water is available throughout California. The California Exotic Pest Plant Council lists arundo as one of the most invasive and widespread of the wildland pest plants.

Giant Reed (Arundo donax)

Photo courtesy of John M. Randall/ The Nature Conservancy

Giant reed is a tall, erect, perennial cane that is a reed-like grass. It reaches heights in excess of 20 feet, and it is one of the largest of the herbaceous grasses. Arundo donax forms dense bunches of stalks that each have opposite, sword-like leaves. The base of each leaf curves cup-like around the stalk. Stalks have nodes like bamboo, 5-10 inches in length. The plants have well established root systems and are very resistant to drought. New shoots are susceptible to frost damage.

Flowers form in late summer or early fall, and are arranged in plumes at the top of the stalks. They are dense and silvery to purplish in color. In California, the viability of the seeds is in question. Usually, giant reed reproduces by rhizome formation. To spread, sections of the root mass are broken off, such as during floods. The root sections are carried down stream and then take root, giving rise to new stands of giant reed.

Arundo donax originated in the Mediterranean region. The Egyptians used it as early as 5000 BC to line underground grain storage vaults. Egyptians also wrapped their mummies in arundo leaves in the fourth century BC. Besides using giant reed for construction applications, arundo has been used in weapons, erosion control, ornamental planting and medicinal purposes. It is also has been used for musical instruments since ancient times. It is the reed material used in the earliest pipe organs and panpipes. To this day, arundo is cultivated for use in woodwind instrument reeds. It is not difficult to see why human activity has spread arundo into most subtropical and warm temperate areas of the world.

In California, arundo has been planted widely as an ornamental. It was popular in the Southwestern US for erosion control along ditches. As early as 1820 people gathered it from along the Los Angeles River for use as roofing material. It is now found in virtually all the river and creek beds in southern California.

In this region of the state, it can be viewed along Highway 101 in Monterey County in the Salinas River watershed. Closer to home in San Luis Obispo, it is seen along Highway 101 in San Luis Creek as well. Much to the consternation of this author, there is a stand continuing to grow along the street behind the clubhouse at Sea Pines golf course in Los Osos.

Arundo donax can be controlled in several ways. Mechanical pulling and cutting are effective by themselves only if done before seeds fully form, and are only effective if roots are removed. Otherwise, the roots can re-sprout new shoots. Repeated cutting can work, especially if done at the beginning of seed formation. This will deplete the stored nutrients in the root system most severely, and eventually will cause the roots to die. Similarly, grazing young plants (with goats or sheep) repeatedly can control and perhaps even eliminate arundo, if done for a prolonged period.

Chemical control methods have been used successfully on giant reed. Dowpon-C-grass killer, glyphosate (Roundup) and paraquat have been used with success. California weed abatement efforts have reported the use of cutting at the beginning of seed formation, usually to a height of one-to-two feet, followed by application of glyphosate to the cut stalks. This effectively killed the roots and eliminated stands of arundo.

Because arundo continues to be planted commercially, potential biological controls (insects and such) probably are not going to be used in the future.

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