Invasive alien plants in southern Africa
Part 4
Eucalypts and Myrtles
Sixteen
of the 198 species (8%) listed as declared plants in South Africa belong to
the Myrtaceae. This group of plants is unusual in that almost half of the
listed species are regarded as both a threat to the natural resources, and
as valuable commercial and utility trees. 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.
The
Myrtaceae is predominantly a woody family of the subtropics and tropics, particularly
Asia, America, and Australia. Leaves are usually opposite, simple, and glandular-dotted.
The Eucalyptus species, however,
are unusual in having adult leaves that are apparently alternate. Flowers
have many showy stamens and the ovary is usually inferior. The fruit is usually a berry or capsule and
is tipped with the remains of the calyx.
Eucalyptus
The
genus Eucalyptus has the highest number of invasive species in the myrtle
family in South Africa. The eucalypts are best known for their commercial
use as timber trees and windbreaks. They are also cultivated for shade, firewood,
ornamental purposes, and honey production. The South African public is often
surprised to hear that some species are invasive and a threat to the natural
resources of the country. The listed species are
They
originate from Australia and are invading watercourses, forest margins, forest
gaps, and fynbos. With the exception of spider gum, all are recognised as
being valuable commercial or utility trees and may be cultivated in demarcated
areas under controlled conditions. They are prohibited outside demarcated
areas. Spider Gum has been listed as a category 1
plant (prohibited) in the Western Cape, as it is a serious threat to coastal
fynbos and its use as a sand-binder and windbreak could be performed by other,
non-invasive species. The common name probably alludes to the spider-like
inflorescence with its strange elongated, curved bud caps covering the stamens.
Eucalypts
are well known for their ability to use large volumes of water, which increases
with growing availability of water, for example, along watercourses. All the
declared eucalypts invade watercourses and according to CARA, they should
be removed from these habitats. Red River Gum, in particular, can form extensive
stands along watercourses and is one of the primary target species for removal
by the Working for Water programme, South Africa’s largest and most costly
alien plant control programme. Red River Gum is the most widespread eucalypt
in Australia, growing along and near watercourses, and potentially has a very
wide distribution in South Africa. Sugar Gum and Karri have been cultivated
mainly in the Western Cape and it is here that they are invasive. Saligna
Gum is better suited to the more tropical regions of KwaZulu-Natal and the
northern provinces and is invasive in these regions. Grey Ironbark is suited
to the warm and humid summer rainfall zone, but little data is available of
its naturalised distribution. Black Ironbark is adapted to the dry and cold
interior; its naturalised distribution is also likely to be wider than shown
on the map.
Myrtle
Leptospermum
laevigatum (Australian Myrtle)
has a similar distribution to Spider Gum, invading sandy coastal areas of
the Western and Eastern Cape. It is a large, densely branching shrub or small
tree up to 8 m high and is used as a windbreak and hedge. It is an aggressive
invader, forming dense stands that exclude indigenous fynbos and forest species.
In places, this invader is replacing another invasive species, Acacia saligna (Port Jackson), which has been brought under bio control
by a gall-forming rust fungus (Gordon 1999). Fortunately a bio control programme
against Australian Myrtle is progressing well and should help to curb its
further spread.
Metrosideros
Metrosideros
excelsa (New Zealand Bottlebrush
or Christmas Tree) is a popular ornamental tree and hedge plant in coastal
areas of the Western Cape. In its native New Zealand, it is known as “Pohutukawa”
meaning “spray-sprinkled”, as it rarely grows far from the sea or an inland
lake. It is so well adapted to sea conditions that oysters may even be found
on branches that dip into the sea (Laing & Blackwell 1907)! In South Africa
this species is invading hygrophilous fynbos (fynbos on moist peaty soils)
at Betty’s Bay and Kleinmond. Similar habitat types are threatened between
Hermanus and Gordon’s Bay and on the Cape Peninsula. Large plants are difficult
to eradicate because they coppice when cut and the wood is very tough; roots
can penetrate rocky crevices and cracks. It produces large quantities of fine,
wind-blown seed. Moist conditions are necessary for seed germination and establishment.
Guavas
Psidium
guajava (Guava) is a shrub or small
tree of tropical American origin. It is grown commercially in the warm, frost-free
parts of South Africa for its edible fruit. It is also one of the most aggressive
invasive species in these regions, rivalling even Lantana camara in the coastal belt of KwaZulu-Natal
and the lowveld of Mpumalanga. Its fruit is eaten by
birds and mammals, who spread the seed far and wide. P. guineense (Brazilian Guava) is an ornamental species whose fruits
are bitter and resinous, but still attractive to birds and other animals,
who assist in dispersing its seed. Psidium
X durbanensis is a hybrid between
P. guajava and P. guineense and is found around Durban and on the southern KwaZulu-Natal
coast. It is apparently sterile and spreads by suckering. P. cattleianum (= P. littorale var. longipes;
Strawberry or Cherry Guava), native to Brazil, is grown for both its ornamental
value and its edible fruit. This species is invasive on the KwaZulu-Natal
coast and probably also in the lowveld of Mpumalanga.
Eugenia
Eugenia
uniflora (Pitanga or Surinam Cherry)
is another Brazilian tree that has been cultivated for
hedging,
for use in producing ornaments, and for its edible fruit. It is an
aggressive invader of riverbanks, coastal bush, forest edges, and forest understories
in KwaZulu-Natal. It is likely to invade similar habitats in Mpumalanga and
Limpopo Province. It has been declared a category 1 plant (prohibited) in
all these regions. Pitanga can be easily distinguished from other Eugenia
and Syzygium species by its very distinctive
eight-ribbed fruits, which are yellow, turning deep crimson when ripe.
Syzygium
Syzygium
cumini (Jambolan) and S. jambos (Rose Apple) are evergreen trees
of Asian origin that have been cultivated as ornamentals and for their edible
fruit. They invade coastal bush and savanna in frost-free areas. Jambolan
can easily be confused with the indigenous S. guineense (Water Pear); however, Jambolan can be distinguished
by its longer leaves (up to 150 mm) with many closely spaced lateral veins,
abruptly tapering leaf apex, oval to pear-shaped fruits, and much-branched
sub-terminal inflorescence, usually arising from old leaf scars. Jambolan
fruits are purplish-black when ripe; those of Rose Apple are creamy yellow,
tinged with pink.
Unlisted
Species
Three additional myrtaceous species
have been proposed as declared plants, but more information is required before
they can be listed. They are Callistemon
rigidus (Stiff-leaved Bottlebrush), a popular ornamental shrub, which
has been seen to invade fynbos and forest edges, Leptospermum scoparium (Manuka Myrtle or New Zealand Tea Ttree), and
Syzygium paniculatum (=Eugenia myrtifolia;
Australian Brush-cherry). Australian Brush-cherry is a popular ornamental
tree and hedge plant with edible fruits. Birds in many parts of the country
favour it; judging by the prolific seedling establishment in urban gardens,
it is likely to become invasive beyond the urban environment.
Other
myrtaceous species that have been recorded as naturalised in the Western Cape,
but have not yet been proposed as declared plants, are Melaleuca
hypericifolia (Red-flowering Tea Tree), M.
wilsonii (Violet or Wilson’s Honeymyrtle), and Callistemon citrinus (Lemon Bottlebrush).
SABONET
News 7.2: 106
List
of declared plants belonging to the Myrtaceae in South Africa
GORDON,
A.J. 1999. Biological control of Australian myrtle, Leptospermum
laevigatum (J. Gaertn.) F. Muell. African
Entomology Memoir 1: 139–143.
HENDERSON,
L. 2001. Alien weeds and invasive plants. Plant
Protection Research Institute Handbook No. 12. Agricultural Research Council,
Pretoria.
LAING,
R.M. & BLACKWELL, E.W. 1907. Plants of New Zealand, 2nd
edition. Whitcombe & Tombs Ltd, Wellington.
