Invasive alien plants in southern
Africa
Part 5
The Grasses (
In southern Africa the Poaceae is one of the largest plant families with 194 genera, 967 species and infraspecific taxa, 847 indigenous species, and 115 naturalised species (Gibbs Russell et al.1990).
Eight of the 198 species listed as declared plants in South Africa belong to the Poaceae; seven species are prohibited (Category 1), while one species is permitted under conditions of controlled cultivation (Category 2). A further six species have been proposed as Category 3 Plants, but more information is needed before they can be listed. Legislation concerning their control, cultivation, and trade are contained in the Conservation of Agricultural Resources Act (CARA), Act 43 of 1983, and amended in 2001. (See appendix for species list.)
Giant
Reed
The Giant Reed, Arundo donax, is the most widespread and
abundant invasive grass species in southern Africa. It was introduced by early
colonists to the Cape, probably in the 1700s or even earlier. One of the earliest
botanical explorers in southern Africa, William J. Burchell, refers to it
in 1811. He describes the ceiling of a house in Genadendal near Caledon, which
was constructed with this reed and says that it was known as the Spanish Reed
and was widely cultivated by farmers (Burchell 1822). Although Giant Reed
is often reported as a native of the Mediterranean, it was probably introduced
to this region many years ago, and originally came from eastern Asia (Fornell
1990).
It is a large and robust reed 2–6 m high, spreading from horizontal rootstocks or rhizomes. It can be distinguished from indigenous reeds (Phragmites species) by the following combination of features:
Leaf
tips are soft or firm but not rigid and penetrating (as in P. mauritianus).
Leaf
sheaths are persistent on stem.
Inflorescence
is compact and spear-shaped.
Often
grows on roadsides or other sites away from water, unlike indigenous reeds.
In
South Africa it rarely flowers above an altitude of 1,000 m and then usually
next to busy roads. Propagation appears to be entirely vegetative from rhizome
and stem fragments. It has been reported that it does not produce viable seed
in most areas where it has been introduced (Perdue 1958).
Giant Reed competes with and replaces indigenous species. It forms very dense stands on riverbanks and in riverbeds, which results in the narrowing of water channels, increased siltation, and the exclusion of smaller and less vigorous riverbank species. During floods, stem and rhizome fragments are carried downstream and dumped against bridges, drainage pipes and other flood control structures, causing blockages, damage, and even structural failure. Dense stands obstruct access to watercourses and impede vision for motorists along road verges; they are also a fire hazard.
Pampas
Grasses
Pampas grasses, Cortaderia selloana and C. jubata,
are South American tussock grasses that have been planted for decoration and
mine dump stabilisation. They have become invasive not only in South Africa,
but also in California, Hawaii, New Zealand, and Australia. They are now listed
as Category 1 Plants in South Africa and may no longer be cultivated (with
the exception of sterile cultivars of C. selloana), and must
be controlled or eradicated where possible. They are invading grassland, forest
edges, riverbanks and other seasonally wet areas, roadsides, and wasteland.
Purple Pampas or Jubata Grass (C. jubata) has longer flowering stems and a darker purplish, loosely branching inflorescence. Silver Pampas (C. selloana) has a paler, silvery-white to mauve, stiffly branching inflorescence; the flowering stems are only slightly taller than the foliage. Silver Pampas has male and female plants and reproduces sexually. Both species can be propagated vegetatively from rhizomes. Studies in California (DiTomaso et al. 2002) have shown that Jubata Grass produces only female flowers and is able to reproduce apomictically—that is, seeds are produced without pollen. Each inflorescence can produce over 100,000 seeds and all seeds are genetically identical to the parent plant.
Once established, mature plants of both species are very competitive and large infestations crowd out indigenous species. The accumulation of dry material creates a fire hazard. Leaves have sharp, cutting edges and the flowers can cause severe asthma.
Tussock
Grasses
Nassella trichotoma, Nassella Tussock, and N. tenuissima,
White Tussock, are South American grasses that are suspected of being accidentally
introduced into South Africa during the Anglo-Boer War (1899–1902) with hay
imported from Argentina (Henderson et al. 1987, Wells & De Beer
1987). Both species have invaded natural grasslands in the Eastern Cape, while
Nassella invasion extends into the Western Cape. Both Nassella species
are listed as Category 1 Plants.
In the 1980s, Nassella was predicted to have the potential to invade two million hectares of grassland in South Africa (Wells no date). Its success as an invader can be attributed to its vigorous growth, high seed production, long seed viability, efficient seed dispersal by wind, water and animals, its unpalatability, and its ability to thrive under a wide range of climatic and soil conditions. It can completely replace the natural veld once this has been disturbed, and because of its inedibility, reduces the carrying capacity of the land. A vigorous campaign using herbicides has greatly helped to control this species.
Pennisetum
Pennisetum setaceum, Fountain Grass, and P. villosum,
Feathertop, are North African tufted grasses that have been cultivated for
ornamentation and mine dump stabilisation. Fountain Grass is tussock-forming
and widespread, preferring hot, dry sites, whereas Feathertop has a more open,
creeping habit and prefers cooler, wetter sites. Both species can form stands
along roadsides and in other disturbed sites, competing with indigenous ruderal
species. They have the potential to spread into adjacent natural veld. Fountain
Grass has become a formidable weed in dry habitats in several states of the
USA, including Hawaii (Benton 2002). It is a fire-adapted colonizer and readily
out-competes indigenous species. It flowers and seeds prolifically, and wind,
water, animals, and humans disperse the seed. Both Fountain Grass and Feathertop
are Category 1 Plants in South Africa.
Sorghum
Sorghum halepense, Johnson or Columbus Grass, is both a
troublesome weed and a valuable fodder plant for livestock. Wilted foliage
and young sprouts, however, cause prussic acid poisoning in livestock (Kellerman
et al. 1988). It has been cultivated in many parts of the world and
is widely naturalised. Its region of origin is now obscure, although it may
be native to the Mediterranean. Johnson Grass can form spreading stands that
are difficult to eradicate because of long, deeply-buried rhizomes. It invades
agricultural lands, disturbed sites, roadsides, grassland, and particularly
riverbanks and riverbeds. It has been listed as a Category 2 Plant, which
means that it may still be cultivated under controlled conditions, but must
be prevented from spreading.
Proposed
Species
A further six invasive grass species have been proposed as Category
3 Plants, but because they are also useful cultivated plants, conflicts of
interest will have to be resolved before they can be listed.
Ammophila
arenaria (Marram Grass)
Cynodon
dactylon (Couch or Kweek)
Lolium
multiflorum (Italian Ryegrass)
Lolium
perenne (Perennial Ryegrass)
Pennisetum
clandestinum (Kikuyu Grass)
Pennisetum
purpureum (Napier Grass)
Marram Grass has been used for stabilising coastal sand dunes, Couch and Kikuyu are both widely used for lawns, fodder, and for erosion control. The ryegrasses are used as fodder crops and erosion control. Napier Grass is cultivated for fodder and as a screen.
—by Lesley Henderson
SABONET
News 7.3: 178
List
of declared plants in the Poaceae in South Africa
BURCHELL, W.J. 1822. Travels in the interior of southern Africa. Longman, London.
DITOMASO, J.M., HEALY, E., BELL, C.E., DREWITZ, J. & TSCHOHL, A. 2002-08-27. Pampasgrass and Jubatagrass threaten California coastal habitats. University of California Weed Research & Information Center, http://wric.ucdavis.edu/information/pampasgrass.html.
GIBBS RUSSELL, G.E., WATSON, L., KOEKEMOER, M., SMOOK, L., BARKER, N.P., ANDERSON, H.M. & DALLWITZ, M.J. 1990. Grasses of southern Africa. Memoirs of the Botanical Survey of South Africa No. 58.
FORNELL, T.C. 1990. Widespread adventive plants in Catalunia. In: F. Di Castri, A.J. Hansen & M. Debussche (eds), Biological invasions in Europe and the Mediterranean basin, pp. 85–104. Kluwer Academic Publishers, Boston, USA.
HENDERSON, M., FOURIE, D.M.C., WELLS, M.J. & HENDERSON, L. 1987. Declared weeds and alien invader plants in South Africa. Department of Agriculture and Water Supply, Pretoria.
HENDERSON, L. 2001. Alien weeds and invasive plants. Plant Protection Research Institute Handbook No. 12. Agricultural Research Council, Pretoria.
KELLERMAN, T.S., COETZER, J.A.W. & NAUDÉ, T.W. 1988. Plant poisonings and mycotoxicoses of livestock in southern Africa. Oxford University Press, Cape Town.
PERDUE, R.E. 1958. Arundo donax—source of musical reeds and industrial cellulose. Economic Botany 12: 368–404.
WELLS,
M.J. & DE BEER, H. 1987. Nassella tussock. Weeds A.21/1987. Farming
in South Africa.
WELLS, M.J. no date. Nassella tussock threatens you. Department of Agricultural Technical Services, Pretoria.
