Arundo donax
(giant reed)

A. donax is an aggressive species with an ability to reproduce quickly, allowing it to out-compete native plant species, and has established itself as one of the primary threats to native riparian habitats in its introduced range, dramatically altering ecological and successional processes and altering habitats towards dense, monotypic stands up to 8 m tall. It is listed as one of the 100 world’s worst invasive alien species (ISSG, 2011). This species represent a serious concern in arid and semiarid habitats because it outcompete native vegetation in the access to soil-water. It uses more water than native plants, lowering groundwater tables. A. donax is highly flammable and can change fire regimes in invaded areas (USDA-ARS, 2014).

Control is an expensive process involving cutting plants to the ground and manual application of herbicides to avoid harming native species. Biological control efforts are being developed as one of the primary tools for the long-term management of this pest. However, A. donax is a valuable and very fast-growing crop that is being promoted for the production of fuel, fibres and pulp, and is also widely used as an ornamental. This means that further introductions are likely and the balance between exploitation and threat as an invasive plant requires careful consideration (Raghu et al., 2006; Low and Booth, 2007).

A. donax is native to tropical and temperate regions of the Old World. Although it is often considered indigenous to the Mediterranean region (Hickman, 1993) or to warmer regions of the Old World (Munz, 1959), it may be an ancient introduction to Europe from South Asia, and the native range is here taken as Asiatic (i.e. temperate and tropical Asia; USDA-ARS, 2007). It is now widely dispersed into all similar climates of the world, and has also become naturalized and invasive in many regions, including southern Africa, sub-tropical USA (below 300 m altitude), Mexico, the Caribbean, South America and Pacific islands (Hafliger and Scholtz, 1981).

Further spread is highly probable given its ease of propagation from water-carried rhizome fragments and the commercial availability/agricultural plantings of A. donax and value as an alternative to tree products and as a soil-stabilizing ornamental species. Regulatory processes have been initiated in California to add it to the CDFA (California Department of Food and Agriculture) Noxious Weed List which may result in state-wide ordinances preventing sale or transfer of A. donax. The California Exotic Pest Plant Council puts A. donax on its 'List A: Most Invasive Wildland Pests'. However, A. donax growth and productivity is also being studied to assess its suitability for the production of biomass for energy, paper pulp, and the construction of building materials which would ultimately lead to the establishment of more A. donax plantations and an economic industry based on it.

A. donax is a hydrophyte but can grow in a wide range of conditions, from moist well-drained soils to those with a water table at or near the surface. Soil preferences are also broad, occurring on coarse sands, gravelly soils, heavy clays and river sediments from freshwater to semi-saline soils on brackish estuaries or in ditches, and along the banks of streams, rivers and lakes. There are abundant naturalized populations in California, along the Rio Grande River, Texas, USA, and in Mexico. It is almost certain that invasive populations are the result of escapes and displacement of plants from commercial plantations and horticultural propagation. It is spread throughout the southern USA in private gardens and is spreading into riparian habitats from Arkansas and Texas to California, and may be established in coastal areas from Maryland south to Florida. It is particularly prominent in the coastal river basins in southern California where it sometimes occupies entire river channels from bank to bank (Jackson et al., 1994) and is thought to have invaded following large storms in the late 1960s. In Hawaii, it has naturalized in coastal areas, often in thickets (Wagner et al., 1999). In Fiji, it is widespread on hillsides, in open forest, and along roadsides, up to about 200 m (Smith, 1979).

A. donax is not usually a weed of crops, rather tending to out-compete and displace native vegetation in riparian habitats. However, it has been reported as invasive in pasture/cropland in South Africa, Tanzania, Egypt, Argentina, Uruguay, Chile, Puerto Rico, and the Dominican Republic (ISSG, 2007; Randall, 2012).

Genetics

The base chromosome is x=12, but with numerous ploidy levels reported, with 2n=60, 2n=72, 2n=110 and 2n=112. A chromosome number of 2n=40 has been recorded for A. donax var. macrophylla. There are generally low levels of genetic variation in introduced populations as they tend to reproduce only vegetatively.

Physiology and Phenology

New shoots arise from rhizomes in nearly any season, but most commonly in the spring. Growth also occurs in any season, but is highly sensitive to temperature and moisture. Under favourable conditions (warm and wet months), A. donax grows very rapidly. Growth rates of up to 0.7 m/week have been recorded, putting it among the fastest growing terrestrial plants. Young culms develop at the full diameter of older canes and further growth involves thickening of the walls. The new growth is soft, very high in moisture and has little wind resistance (Perdue, 1958). Age of individual culms is certainly more than one year and branching seems to represent stem growth in later years, whereas rhizomes show indeterminate growth. Branches can also form when a stem is cut or laid over. Die-back is infrequently observed but culms fade or become partially brown in winter, becoming dormant under cold conditions.

Reproductive Biology

Very little information is available in the literature regarding the biology of A. donax. Although plants have been grown in scattered locations from seed collected in Asia, it is reported that A. donax does not produce viable seeds in most areas where it is apparently well-adapted. The importance of sexual reproduction to the species, as well as seed viability, dormancy, germination and seedling establishment have yet to be well studied. Population expansion thus occurs almost exclusively through vegetative reproduction in most reported cases, either from underground rhizome extension of a colony or from plant fragments carried downstream, to become rooted and form new clones (Else, 1996). Root formation can also occur where an attached culm has fallen over and is in contact with the substrate, and such layering has now been proposed as a more common means of reproduction in invasive stands in the USA than either via rhizome extension or plant fragments (Boland, 2006). Much of the cultivation of A. donax is therefore initiated by planting rhizomes that root and sprout readily. Wild stands in the USA have reportedly yielded over 20 t of oven-dry biomass per ha (Perdue, 1958).

Environmental Requirements

A. donax is extremely tolerant to different climates and can survive and grow at almost any time under a wide variety of environmental conditions. However, it does not appear to tolerate high elevations over much of its native and introduced ranges, nor does it like continental environments where regular freezing occurs. In Egypt, it was found to tolerate both higher and lower water table levels than Phragmites australis. Often found on sand dunes near seashores. It tends to favour low gradients <2% grade) over steeper and smaller channels, but scattered colonies can be found in moist sites or springs on steeper slopes. It is reported to tolerate annual precipitation of 300-4000 mm, annual temperatures of 9-29°C and a soil pH of 5.0-8.7 (Duke, 1975). It has been found at altitudes approaching 4000 m in Ecuador (Missouri Botanical Garden, 2007).

Armillaria mellea, Leptostroma donacis, Papularia sphaerosperma, Puccinia coronata and Selenophoma donacis have been identified as affecting A. donax (Bell, 1997). The effects and specificity of Phoma glomerata and Alveophoma arundinis attacking the leaves and stems have yet to be ascertained. The leaves of A. donax contain a number of toxic and unpalatable natural minerals and chemicals, such as silica, cardiac glycosides, hydromaxic acids, and alkaloids that protect the plant from native insects (Bell, 1993). Due to the presence of these toxic chemicals and minerals that protect the plant from native insects that might attempt to feed or reproduce upon it, such species become scarce with the invasion of A. donax. Unfortunately, little is known about predators in its invasive range that can damage or kill A. donax. A number of invertebrates are known to feed on the grass in Eurasia and Africa (El-Enany, 1985; Hoshovsky, 1987). Caterpillars of Phothedes dulcis have been reported feeding on A. donax in France (Dufay, 1979); Zyginidia guyumi in Pakistan (Ahmed et al., 1977) and the moth borer, Diatraea saccharalis in Barbados (Tucker, 1940) have also been reported.

Environmental

• Agroforestry
• Amenity
• Boundary, barrier or support
• Erosion control or dune stabilization
• Landscape improvement
• Shade and shelter
• Soil conservation
• Windbreak

Fuels

• Biofuels
• Fuelwood
• Miscellaneous fuels

General

• Ornamental

Materials

• Fibre
• Miscellaneous materials

Ornamental

• Propagation material

Roundwood

• Roundwood structures

Sawn or hewn building timbers

• Exterior fittings
• Fences
• Flooring
• For light construction

Woodware

• Musical instruments

A. donax can be confused with the closely related common reed (Phragmites australis or Phragmites communis) and pampas grass (Cortaderia spp.), also with cultivated bamboos, and in its earlier growth stages with some large-stature grasses such as Elymus spp. Common reed (Phragmites spp.) is, however, less than 4 m tall and has panicles with long hairs between the florets.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Control

Cultural control

Prescribed burning has been used to control A. donax, with a flame thrower being used as a cheap, alternative spot treatment to heat-girdle the stems at the base of the plant. Larger, mature, infestations can be burnt by broadcast burning with or without a prior pre-spray of herbicides to kill and desiccate the plants. This is generally not recommended as it does not kill the underground rhizomes and probably encourages A. donax germination over native riparian species. Burning presents containment risks and the possibility of damage to beneficial species, resulting from soil disturbances which may result from firebreak construction as well as from difficulties of promoting fire through patchily distributed stands. Cut material is often burnt on site because of the difficulties associated with collection and removal of all the chipping material.

Prescribed grazing is a managerial control method sometimes employed to control A. donax. Although A. donax is not very palatable to cattle, during the drier seasons they do browse young shoots, followed by upper parts of more mature plants (Wynd et al., 1948). In parts of California, USA, goats have been used quite effectively to control A. donax (Daar, 1983) although they tend to be less selective than sheep and the latter have been shown in feeding experiments to survive for extended periods on a diet of A. donax alone (Frattegglani-Bianchi, 1963). Although sheep may prove a more practical alternative to mowing in some cases, it is important to manage this so as to avoid soil compaction problems in overly damp areas. It has also been suggested that wild geese breeds might contribute to A. donax control efforts given their capacity to consume weed grasses and sedges.

Since A. donax in its invasive range appears to be unable to regenerate much, if at all, from wind or water-carried seeds or small propagules, its invasiveness could be controlled by not planting within the 50- or 100-year floodplain, and placing barrier screen systems along irrigation canals.

Mechanical control

Smaller infestations can be eradicated by manual methods, especially where there is a risk of damage to sensitive native plants and wildlife by other methods. This is successful with young plants less than 2 m in height, but care must be taken to remove all the rhizome material, and as such may be more effective in loose soils and after periods of rain when the substrate is more workable. Plants can also be removed using hand tools such as pick-axes and shovels, particularly in combination with the cutting of stems near the base with pruning shears or a chainsaw. Stems and roots should be removed or burned on site to avoid re-rooting and a chipper can be used to reduce the volume of cut material. For larger infestations on accessible terrain, heavier tools such as rotary brush-cutters, chainsaws and tractor mounted mowers may facilitate biomass reduction and should be followed either by rhizome removal or chemical treatment. These methods may be of limited use on inaccessible or sloping terrain, and may interfere with the re-establishment of native plants and animals (Hoshovsky, 1987). Mechanical control tends to be very difficult as even rhizomes buried 1-3 m deep readily resprout (Else, 1996) and removal of all such material is not practical, especially in sensitive sites where soil disturbance is disruptive or where soils are susceptible to compaction or erosion or when they are saturated.

Chemical control

In many situations, this may be necessary and is usually carried out in combination with mechanical control. Glyphosate is most commonly used against A. donax, which is approved for use in wetlands. As a broad-spectrum herbicide, care should be taken to avoid application or drift onto desirable vegetation. This can be achieved by treating the culms directly, also reducing herbicide costs, with fair results being achieved year round with best kill in autumn (Else, 1996). Concentrated glyphosate solution is applied to the stems, cut at a height of 5-10 cm, by painting with a sponge or spraying with a hand mister. Adding a dye to the solution can help identify the treated material. The solution must be applied immediately after cutting, as translocation ceases within minutes of cutting. As new growth is sensitive to herbicides, so a common alternative is to cut or mow a patch and allow regeneration, returning 3 weeks to 3 months later to treat the new growth. With all chemical methods, follow-up treatment and assessment are necessary. The optimal application period is post-flowering and pre-dormancy, usually in late July to early October when plants are translocating nutrients into roots and rhizomes. Foliar uptake and kill are best achieved by spraying during the active growing periods from late Spring to early Autumn and small patches can be treated from the ground using backpack or towed sprayers, and major infestations have been aerially sprayed using helicopters in the USA (Zembal and Hoffman, 2000).

Biological control

Pest surveys and assessments have been initiated for the biological control of A. donax, but no biological control agents have yet been released. A number of invertebrates is known to feed on the grass in Eurasia and Africa (El-Enany, 1985; Hoshovsky, 1987). Caterpillars of Phothedes dulcis have been reported feeding on A. donax in France (Dufay, 1979); Zyginidia guyumi in Pakistan (Ahmed et al., 1977) and the moth borer, Diatraea saccharalis in Barbados (Tucker, 1940) have also been reported. A review of the CABI Bioscience herbarium suggests that there are obligate biotrophic fungi associated with A. donax in China, but not India or Pakistan, lending weight to the theory that China may in fact be a truer centre of origin for the species. As with invertebrates, a number of pathogens are also associated with A. donax in its naturalized range. Given the commercial value of A. donax, the use of insects and/or pathogens would undoubtedly engender conflicts of interest. Biological control is thought to offer one of the best options for long-term, affordable and environmentally friendly management of A. donax. The ARS European Biological Control Laboratory (Montpellier, France) in cooperation with a USDA-ARS laboratory initiated a foreign exploration programme in 1999 which has already located potential agents in the Mediterranean Basin, India and Sri Lanka (Kirk et al., 2003). A. donax stems and leaves contain a wide array of noxious chemicals, including silica (Jackson and Nunez, 1964), tri-terpines and sterols (Chandhuri and Ghosal, 1970), cardiac glycosides, curare-mimicking indoles (Ghosal et al., 1972), hydroxamic acid (Zuñiga et al., 1983), and numerous other alkaloids which, however, probably protect it from most native insects and other grazers (Zuñiga et al., 1983; Miles et al., 1993).

Integrated Control

A suite of methods is needed to control A. donax depending upon the presence or absence of native plants, the size of the stand, the amount of biomass which must be dealt with, the terrain and the season. Several technical approaches can be successful, with the best one for a particular site dependent on available labour resources, the size of the infestion, degree of intermixing with desirable native riparian vegetation, site accessibility and other factors. Follow-up treatments are usually necessary for one to five years after the initial control. Often the lower, frequently flooded stream banks will re-vegetate spontaneously, and upper, drier banks may need re-planting with native species. Where A. donax is the only vegetation on stream banks, post-eradication revegetation efforts may be critical for bank stabilization. Team Arundo del Norte (2002), comprising a number of organizations and agencies dedicated to tackling this problem weed, is producing a brochure for the general public, a handbook for landowners who are becoming interested in A. donax removal, a guide for organizations who want to start an A. donax removal programme, and a public-education video.

22/04/14 Updated by:

Julissa Rojas-Sandoval, Department of Botany-Smithsonian NMNH, Washington DC, USA

Pedro Acevedo-Rodríguez, Department of Botany-Smithsonian NMNH, Washington DC, USA

26/10/2007 Updated by:

Nick Pasiecznik, Consultant, France