Invasive alien plants in southern Africa
Part 3
The Daisies (Asteraceae)
Some 14 of the 198 (7%) species listed as declared plants in South Africa belong to the Asteraceae. These plants are all invasive and regarded as harmful to the natural resources of South Africa. Legislation concerning their control, cultivation, and trade are contained in the Conservation of Agricultural Resources Act, Act 43 of 1983, and amended in 2001.
The Big Six
Six species belonging to the tribe Eupatoriae are amongst the most invasive
species in southern Africa and threaten the biodiversity of the region. They
are:
Ageratina
adenophora
Ageratina riparia
Ageratum conyzoides
Ageratum houstonianum
Campuloclinium macrocephalum
Chromolaena odorata
These species originate from tropical and subtropical America and the West Indies. They are invading savanna, grassland, forest margins, and commercial plantations in South Africa and neighbouring Swaziland, Mozambique, and Zimbabwe. One species, Chromolaena odorata, extends further northwards to central, eastern, and western Africa.
Chromolaena
Chromolaena odorata (=Eupatorium odoratum) is commonly known as chromolaena,
triffid weed, or paraffin bush in South Africa, and as Siam weed in other
parts of the world. It is one of the most invasive plants not only in southern
Africa, but also the whole of tropical Africa and beyond to India, southeast
Asia, and Indonesia. It is native to Central and South America and the West
Indies. It is a scrambling shrub up to 4 m or higher and forms thickets that
smother other vegetation. It grows rapidly, flowers within one season, and
produces vast quantities of viable, windblown seeds that can also be dispersed
long-distance by attaching to fur, feathers, and clothing. It can also reproduce
vegetatively by producing adventitious roots where the stems touch the ground.
Oils in the leaves make the plants inflammable even when green. Plants are
relatively unpalatable and young shoots may contain toxic quantities of nitrate.
Chromolaena is thought to have been unintentionally introduced to Durban,
South Africa, during the 1940s and has spread rapidly over the past 60 years
(see map). Climatic factors limiting its spread are low rainfall and frost.
Although chemical control of triffid weed is effective, the growth rate and
spread of the plant make it impossible to contain in the long term over the
large, often low-value and inaccessible, areas that it invades. Biological
control, using natural enemies, is being investigated in southern Africa and
is the only hope for its successful control.
Ageratina
Ageratina adenophora (=Eupatorium adenophorum), crofton weed, is similar
in appearance to chromolaena but lower-growing, usually 1–2 m tall, and either
herbaceous or softly woody. It is of Mexican origin and has become a serious
weed in Hawaii, Australia, New Zealand, China, and India. In South Africa
it has invaded streambanks, forest margins, and plantations in the Magaliesberg,
Magoebaskloof, Pietermaritzburg, Plettenberg Bay, Stellenbosch, and Cape Town
areas. Natural enemies have been realeased as part of a biological control
programme but their effect is negligible. Ageratina riparia (=Eupatorium
riparium), creeping crofton weed, is a weak-stemmed herb seldom more than
1 m high and creeping in its lower parts. So far it is only known from the
Pietermaritzburg area but, has the potential to spread more widely.
Ageratum
Ageratum conyzoides and A. houstonianum are annual herbs with
mauve, blue, or pinkish inflorescences and seldom grow taller than 1 m. A.
houstonianum has larger, fluffier inflorescences and has been cultivated
as an ornamental. Both species invade riverbeds, riverbanks, forest margins,
plantations, grassland, wetlands, and croplands. They often form dense stands
that exclude other species. There is evidence to suggest that they have allelopathic
properties that inhibit the growth and germination of other plants.
Campuloclinium
Campuloclinium macrocephalum (=Eupatorium macrocephalum), pompom weed,
is an erect perennial herb up to 1.3 m tall, with bristly hairs covering the
leaves and stems. The flowerheads are large, pink, and showy. The stem dies
back annually to a root crown. It invades moist grassland and survives winter
fires. This is a very ornamental species that was first recorded in the Fountains
Valley, Pretoria, in the mid-1970s. In the past five years it has spread dramatically
and now forms extensive stands in the Pretoria and Centurion areas. It is
also invasive in the Tzaneen, White River, Barberton, Pietermaritzburg, Durban,
and Port St Johns areas. Pom-pom weed is a serious threat to biodiversity
as it has the ability to invade undisturbed grassland. It is also unpalatable
and spreads at the expense of palatable grassland species.
Xanthium
Xanthium spinosum, spiny cocklebur, was the first species to be declared
a noxious weed in South Africa in 1860. The Afrikaans name ‘boetebossie’ (‘bush
for which one is fined’) is a direct result of this proclamation and concomitant
fines. Both X. spinosum and X. strumarium, large cocklebur,
can cause severe losses to the wool industry because of their large spiny
burs that adhere to wool. The burs become entwined in tails, manes, and coats
of domestic livestock, causing the animals much discomfort. Plants invade
crop lands, overgrazed pastures, and riverbanks. The seedlings are particularly
toxic to domestic livestock. Both species are probably native to tropical
America but have become cosmopolitan weeds.
Parthenium
Parthenium hysterophorus, commonly known as parthenium weed in Australia,
congress grass in India, and feverfew in the Caribbean, where it is native,
poses a major health problem and a threat to agriculture in South Africa.
Not only does it inhibit the growth and seed germination of other plants through
allelopathy (the chemical inhibition of one organism by another), but it also
causes asthma and serious dermatitis in humans. The sesquiterpene lactone
parthenin is a major allergen produced by the species. Parthenium is an annual
chrysanthemum-like herb, 0.3–1.5 m high. It has an erect, longitudinally grooved
stem, which branches above into a large, open panicle of small, white flowerheads,
each measuring about 3 mm across. Although it was first recorded in Natal,
South Africa, in 1880, it appears to have become common and troublesome only
since the 1980s. It is an aggressive colonizer of wasteland, roadsides, railsides,
watercourses, cultivated fields, and overgrazed pastures. In the subtropical
lowlands of South Africa it has become a major nuisance in sugarcane and banana
plantations. It spreads from seeds which are adapted to dispersal mainly by
water and to a lesser extent by wind. Most long-distance dispersal is by animals,
vehicles, farm machinery, and in mud.
Tithonia
Tithonia diversifolia, Mexican sunflower (yellow-flowered), and T.
rotundifolia, red sunflower, are native to Central America, including
Mexico. They are ornamental shrubs up to 3 m high with large showy flowers.
T. diversifolia is annual or perennial, usually occurring at lower
altitudes (0–600 m), and very common along the KwaZulu-Natal coast. T.
rotundifolia is an annual, and usually occurs at moderate altitudes (600–1,400
m). They form dense stands along roadsides, riverbanks, and in wasteland and
can invade savanna and grassland.
Montanoa
Montanoa hibiscifolia, tree daisy, native to Central America, is a
perennial shrub or small tree up to 6 m tall. It has large palmately lobed
leaves and white flowers in terminal, branching inflorescences. It invades
forest margins, woodland, bush clumps, roadsides, and riverbanks, mainly along
the KwaZulu-Natal coast.
Thelechitonia
Thelechitonia trilobata (=Wedelia trilobata), Singapore daisy, is
an ornamental herb with creeping and rooting stems. A native of tropical America,
it has been used as a groundcover, especially in sandy coastal areas where
it can invade dunes. It also invades riverbanks, wetland margins, roadsides,
and urban open space. It has been declared a noxious weed in Australia.
Next Instalment
The next article in this series deals with the Myrtaceae (eucalypt–myrtle
family). This group includes important commercial species, such as Eucalyptus
grandis, which is grown for timber, and Psidium guajava, the guava,
which is grown for its edible fruit. It also includes some ornamental species,
such as Metrosideros excelsa, the New Zealand bottlebrush, and species
of Eugenia and Syzygium. Species belonging to the genera Callistemon
and Melaleuca are also invasive but have not yet been listed as
declared plants.
—by Lesley Henderson
SABONET News 7.1: 32
HENDERSON, L. 2001. Alien weeds and invasive plants. Plant Protection Research Institute Handbook No. 12. Agricultural Research Council, Pretoria.
LIGGIT, B. 1983. The invasive alien plant Chromolaena odorata, with regard to its status and control in Natal. Institute of Natural Resources, University of Natal, Pietermaritzburg. Rural Studies Series, Monograph 2.
OLCKERS, T & HILL, M.P. 1999. Biological control of weeds in South Africa (1990–1998). African Entomology Memoir No. 1
STAFF OF THE PLANT PROTECTION RESEARCH INSTITUTE. Weeds/Onkruid. Farming in South Africa. Series of pamphlets produced from 1981 to 1992.
