Nonnative plants are exotic plants introduced from other parts of the world that often need human help to survive. Many nonnatives have been introduced as ornamental plants, and to provide shade, erosion control, and windbreaks. Invasive exotics are nonnatives that are able to spread and invade into natural areas, especially after disturbances such as fire or grazing. Exotics can outcompete native species, disrupt food chains, and change nutrient cycles. Eradicating invasive exotics is difficult, but can help native species reestablish themselves. Removal methods include bulldozing, manual removal, controlled burns, and chemical control.
There are 83 exotic plant species in Glen Canyon National Recreation Area (NRA). Of the exotic species present, eight are controlled because of the threats they pose to native habitats: Russian knapweed (Acroptilon repens), African mustard (Brassica tournefortii), Russian olive (Elaeagnus angustifolia L.), Camelthorn (Alhagi macrorum), Fivestamen tamarisk (salt cedar, Tamarix chinensis), giant reed (Arundo donax), Uruguayan pampas grass (Cortaderia selloana), and ravennagrass (Saccharum ravennae). The rest are not prone to being invasive and are not a threat, or are too abundant and too difficult to control, like Russian thistle (Salsola pestifer) or cheatgrass (Bromus tectorum).
Tamarisk (Tamarix chinensis)
Tamarisk has invaded Glen Canyon NRA and much of the western United States. This invasive, low growing shrub or tree is common in riparian communities throughout the Southwest. It can easily be identified by its wispy foliage composed of small, fleshy leaves, and its feathery pink flowers which produce large amounts of seeds. The trunk and branches of mature plants are very dark, while the shoots of seedlings have a reddish color. Tamarisk consumes an excessive amount of scarce desert water and can evaporate or transpire up to 300 gallons per day, significantly more than a similarly sized cottonwood or willow. The dropped leaves of a tamarisk make the soil too salty for most native species to thrive.
In the late 1800s, tamarisk was brought to North America from its native habitat which ranges from Spain through northern Africa and Asia. It was introduced in the western United States as an ornamental plant, windbreaker, and stabilizer for stream banks prone to erosion. Tamarisk quickly adapted to the Southwest and spread along low elevation riverbanks and wash bottoms throughout the region, crowding out native riparian plant and animal communities as it eventually extended its range from Canada to Mexico. By the 1940s, tamarisk was well established along the Colorado River. Without any natural checks on population growth, it replaced riparian vegetation communities and significantly reduced the biodiversity and the health of ecosystems along many of the region’s riparian areas.
Tamarisk is resistant to drought, saline soils, cold temperatures, and fire. In addition, its rapid seed production and ability to outcompete native riparian plants like narrowleaf willow (Salix exigua) and the Fremont cottonwood (Populus fremontii S. Wats.) make this invasive exotic difficult to control. Eradication techniques include mechanical removal including bulldozing, controlled burns, and chemical control. The most effective method is to cut tamarisk plants close to the ground, and then carefully treat each stump with herbicide to prevent the root system from producing new shoots. Tamarisk removal efforts have occurred in many parks in the National Park Service system in an attempt to restore native ecosystems.
Russian olive (Elaeagnus angustifolia)
Russian olive is a shrub or small tree with silvery leaves, small thorns, and fragrant yellow flowers which turn to fruit. Native to southern Europe and central Asia, Russian olive was introduced to the United States as an ornamental plant in the 1800s. In the early 1900s it was recommended as a windbreaker, control for erosion, and wildlife cover. Though birds will eat the fruit of Russian olive, bird species diversity is higher in riparian areas which are dominated by native vegetation.
Russian olive can outcompete and dominate native vegetation, particularly in riparian areas, and is present in Glen Canyon NRA, especially along the Escalante River. It can form a dense shrub layer, displace native species, and close open areas. Russian olive can survive in a variety of habitats including bare mineral substrates. It is resistant to infrequent fire, temporary flooding, browsing, and mechanical cutting. Herbicides are an effective control of this invasive exotic if applied correctly and over several years. Removal of this invasive exotic can restore native species to riparian ecosystems. The National Park Service has ongoing efforts to remove Russian olive from the Escalante and San Juan rivers.
Ravennagrass (Saccharum ravennae)
Ravennagrass grows in clumps reaching up to 10 feet in height and is related to sugarcane. Its tall stems grow purple plumes in the fall. Native to northern Africa and the Mediterranean, this grass was introduced to the United States as an ornamental plant. Ravennagrass grows in a variety of soil and canopy conditions and is highly competitive, but provides little habitat for birds and other wildlife. Ravennagrass threatens riparian habitats and hanging gardens and is becoming one of the most invasive and dangerous exotics in the West.
Ravennagrass was introduced to the Glen Canyon area as an ornamental grass planted around buildings associated with Glen Canyon NRA. Populations are essentially under control in parts of the San Juan River and Colorado River below the dam, but remain in some side canyons. Ravennagrass can be manually removed, but is a costly process. Recent work has tested various herbicides to see if they are effective at controlling the species.
Non-chemical control methods include bulldozing, manual removal, and controlled burns. The use of chemical herbicides to remove invasive exotics is often necessary, although controversial. When necessary, Glen Canyon NRA uses Garlon 4 with vegetable mixing oil in a 1:1 ratio. This mixture is carefully applied for the treatment of stumps only, breaks down quickly, and is not dangerous to humans or wildlife. Garlon 4 is used to control tamarisk and Russian olive.
A checklist of exotic plant species documented in Glen Canyon NRA is available here.