The Rio Grande River forms the border between the USA and Mexico, and it is the source of irrigation for the billion-dollar agricultural industry in the Rio Grande Valley. The river has many domestic, municipal, and industrial uses. Its unique ecological community is being threatened by various changes to the river.
Already strained by human population growth and agricultural demands on both banks, the Rio Grande River and its reservoirs are invaded by several non-native plants. The banks of the Upper Rio Grande, from El Paso to the Big Bend, are being sucked dry by the alien plant species salt cedar/tamarisk. The Middle Rio Grande, from Del Rio to Zapata, suffers from the effects of Giant cane. The Lower Rio Grande, from Lake Amistad to the Gulf of Mexico, has been clogged with Hydrilla and Water hyacinth. These invasive species are threatening the ecology of the Rio Grande. In 1978, a 196-mile section of the Rio Grande was designated by Congress as a Wild and Scenic River. Rivers that bear this designation are to be preserved in their free-flowing condition, their ecosystems actively protected in their natural state. Of the USA’s 3.5 million lineal river miles, 10,763 miles bear this honor. The Act states that designated rivers “...be preserved in free-flowing condition, and that they and their immediate environments be protected for the benefit and enjoyment of the present and future generations.”
Description of Giant Cane
Giant cane (sientific name: Arundo donax L.) is a tall, rapidly growing perennial cane. Other common names include Carrizo, Spanish cane, wild cane, giant reed and arundo. Native to eastern Asia, this species has been widely planted and naturalised in the temperate and subtropical regions of both hemispheres. It forms dense stands on disturbed sites, sand dunes, in wetlands and riparian habitats. Resembling bamboo and native cane, it has a hollow diameter, and alternating, broad, grey-green leaves, and commonly grows to 6m. It is among the fastest growing terrestrial plants in the world (nearly 10 centimeters/ day). Giant cane stems and leaves contain a variety of harmful chemicals, including silica and various alkaloids, which protects it from most insect herbivores and deters wildlife browsing. Stems of the Giant cane have been used to provide reeds for musical instruments.
Giant cane is highly flammable throughout the year, and during the drier months of the year, it can increase the probability, intensity, and spread of wildfires through the riparian environment, changing the communities from flood-defined to fire-defined. After fires, rhizomes can re-sprout quickly, outgrowing native plants, which can result in large stands of A.donax along riparian corridors. Fire events thus push the system further toward mono-specific stands.
The Giant cane does not provide any food sources or nesting habitats for native wildlife. Thus, resources provided by the crowded-out native plants are not being replaced by Giant cane stands.
Giant cane reproduction
Giant cane flowers in late summer, bearing upright, feathery plumes 40-60 cm long, but the seeds are rarely fertile. Instead, it chiefly reproduces vegetatively, by underground rhizomes. The rhizomes are tough and fibrous and form knotty, spreading mats that penetrate deep into the soil up to one meter deep. Stem and rhizome pieces less than five cm long and containing a single node readily sprouted under a variety of conditions. This vegetative growth appears to be well adapted to floods, which may break up individual clumps, spreading the pieces, which may sprout and colonise further downstream.
Threats to the river
The spread of invasive aquatic plants is creating significant problems for Texas water bodies and the people who depend on them. These plants significantly reduce available water supplies, choke waterways, interfere with power generation and agricultural irrigation, degrade water quality, and have contributed to a number of drowning deaths. They make boat travel and recreation dangerous, while threatening the health of native plants and animals that make Texas unique. As the river passes through stands of cane and salt cedar, the water velocity slows, resulting of increased deposition of sediment and nutrients. These deposits aggregate on the river banks, raising the floodplain and narrowing the channel. In turn, the built-up banks provide more places for salt cedar and cane to flourish.
A waterside plant community dominated by Giant cane may also have reduced canopy shading of the in-stream habitat, which may result in increased water temperatures. This may lead to decreased oxygen concentrations and lower diversity of aquatic animals.
Giant reed can completely suppress and remove native vegetation very easily, which reduces wildlife habitat, increases fire risks and interferes with flood control It uses large amounts of water from its wet habitat to supply the rapid rate of growth.
By regularly scouring away mud, silt and vegetation, floods kept the river channel shallow and wide, and maintained its gently sloping banks and low, open floodplain. But dense stands of cane and salt cedar lock mud and silt in place, so even when there are floods now, the inundation’s cleansing power is diminished. Resource managers hope that removing cane and salt cedar will destabilize the banks and allow smaller floods and the occasional big flood to do their job, moving mud and silt downstream and helping the river resume a more natural form.
Other potential problems
Some local residents and the U.S. Border Patrol are concerned that dense stands of Giant cane along the Rio Grande provide cover for illegal immigrants crossing the Rio Grande from Mexico into the United States. These crossings can increase crime and drug trafficking across the border.
There are several different ways to control invasive aquatic species including biological, mechanical, and chemical methods. Giant cane must be controlled before the quality of the Rio Grande can be improved and native species can regenerate. Recent projects in Big Bend National Park have utilized herbicides, salt cedar-eating beetles, and old-fashioned tree felling to clear a few river miles of invasive species. “We’re definitely not trying to take the river back to the way it was before the dams were in, but we’d like it in better condition than it is today,” says Mark Briggs, Big Bend coordinator for the World Wildlife Fund.
Insect herbivores from the Giant cane's native range can be used to stop the spread of the plant. These specialized insects are natural predators and are thus adapted to feed on the cane. Their feeding activity can cause considerable damage to the plant. Three of these specialist herbivore insects have been imported from Mediterranean Europe and are being evaluated as biological control agents. The three insects are the Arundo wasp, Tetramesa romana; the Arundo scale, Rhizaspidiotus donacis; and the Arundo fly, Cryptonevra spp. Before these are released into the wild, they must be carefully studied so more damage is not done and native species are not threatened.
Small stands of cane can be removed manually, as long as the entire root mass and all rhizome parts are removed. Dense growth and thick root masses make manual or mechanical removal a slow and difficult process. Rhizomes buried under one to three meters of soil may re-sprout, and the disturbance caused by physical removal to the soil and surrounding communities may be severe.
Seedlings can be pulled or dug out, when the soil is moist after a heavy rain. Large individual plants can be cut with a chainsaw; moreover, roots must be dug up with a shovel, pickax, or brush ax to prevent re-sprouting. Alternatively heavy equipment can be used to remove the stands.
Systemic herbicides may be applied after flowering as a cut-stump treatment or foliar spray to kill the root mass (Bell 1997). Glyphosate is used in EPA-approved formulations for use in wetlands. For detailed information on the use and effects of glyphosate, see Tu et al., 2001. If herbicides are applied, it is vital to follow label instructions when using herbicides, in order to avoid harm to other organisms.
Both treated and non-treated stems can be left on-site to decompose, although they break down very slowly. If left to compost, debris must be kept well away from river waters. For stems that have not been chemically treated and in areas where it is feasible, the debris can be burned. Otherwise, the canes can be chipped into very small pieces for mulching. Chipped material can be disposed of either in green waste containers, or spread out to dry and possibly sprayed with herbicide if any regrowth occurs from chipped debris.
Governmental support is needed to address problems caused by invasive plant species through (1) financial assistance with operational control of existing problems and (2) technical assistance in developing and implementing scientifically sound, ecosystem-based management strategies for restoring native plant communities in weed-infested water bodies. The U.S. Army Corps of Engineers, through its Aquatic Plant Control (APC) Program and Aquatic Plant Control Research Program (APCRP), has the mission and the mandate to satisfy both needs.
The Corps’ APC program has not been funded in recent years; in the past it has provided both fundingand technical support for aquatic plant management operations in cost-sharing states, including Texas. The Corps’ ARCRP, the only federally authorized research program for aquatic plant management, provides an ecosystem-based approach that seeks to integrate the use of biological, chemical, mechanical, and cultural controls to eliminate or greatly reduce the problematic non-native plants while promoting the establishment of beneficial native plants.
Executive Order 13112, adopted in 1999, created the National Invasive Species Council to coordinate the invasive species programs that are spread through 40 different federal agencies and to work with state, local, and private organizations on this critical area. (TWCA)
References and further reading
Arundo donax L. U.S. Department of Agriculture
Giant Reed Plant Conservation Alliance Alien Plant Working Group
Arundo donax United States Forest Service
Alden, P., F. Heath, A. Leventer, R. Keen, W. B. Zomfler, eds. 1998. National Audubon Society Field Guide to California. Knopf, New York.
Bell, G. P. 1997. Ecology and Management of Arundo donax, and approaches to riparian habitat restoration in southern California. In Plant Invasions: Studies from North America and Europe, eds. J. H. Brock, M. Wade, P. Pysêk, and D. Green. Pp. 103-113. Backhuys, Leiden, the Netherlands.
Boose, A. B., and J. S. Holt. 1999. Environmental effects on asexual reproduction in Arundo donax. Weeds Research 39: 117-127.
Dudley, T. L. 2000. Noxious wildland weeds of California: Arundo donax. In: Invasive plants of California's wildlands. C. Bossard, J. Randall, & M. Hoshovsky (eds.).
Herrera, A., and T. L. Dudley. 2003. Invertebrate community reduction in response to Arundo donax invasion at Sonoma Creek. Biol.Invas 5:167-177.
Mackenzie, A. 2004. Giant Reed. In: The Weed Workers' Handbook. C. Harrington and A. Hayes (eds.) www.cal-ipc.org/file_library/19646.pdf
Miles, D. H., K. Tunsuwan, V. Chittawong, U. Kokpol, M. I. Choudhary, and J. Clardy. 1993. Boll weevil antifeedants from Arundo donax. Phytochemistry 34: 1277-1279.
Perdue, R. E. 1958. Arundo donax – source of musical reeds and industrial cellulose. Economic Botany 12: 368-404.
Scott, G. 1994. Fire threat from Arundo donax. pp. 17-18 in: November 1993 Arundo donax workshop proceedings, Jackson, N.E. P. Frandsen, S. Douthit (eds.). Ontario, CA
Tu, M., C. Hurd, and J. M. Randall. 2001. Weed Control Methods Handbook: Tools and Techniques for Use in Natural Areas. The Nature Conservancy.