JOHANNESBURG`S TOP WEEDS/ALIEN INVASIVE PLANTS

JOHANNESBURG`S TOP WEEDS/ALIEN INVASIVE PLANTS
JOHANNESBURG’S TOP WEEDS/ALIEN INVASIVE PLANTS
BUGWEED
Bugweed
Family: Solanaceae
Scientific Name: Solanum mauritianum
Common name: Luisboom, Bugweed
Origin: South America (Argentina, Brazil, Paraguay and Uruguay)
Reason for Introduction: Ornamental Value
Legal Status (The Conservation of Agricultural Resources Act 1983 (Act No 43 of 1983)
(CARA): Declared Category 1 (The weed which requires that it should be controlled
immediately)
It is an unarmed (thornless) shrub or small tree 2-4 m in height and all parts, except older
stems, are covered with white velvety hairs. The leaves are large, dull green and velvety
above, white- felty below and emit a strong smell when bruised or crushed. The hairy leaves
and stems can cause a skin allergy and asthma.
The fruits, which look like berries, are initially green turning yellow when mature. The unripe
fruits are poisonous. The flowers are purple in compact clusters at the end of branches.
It is a highly invasive plant and inhibits the growth of natural vegetation. It is thus also
classified as a transformer invader plant. Countrywide, it grows along waterways, in urban
open space as well as along forest margins.
In Johannesburg, bugweed can be seen flourishing on sidewalks, road islands, flower beds,
along most streams and even between paving blocks. It is growing in many residential
gardens, some metres high and very well tended. The bugweed is often found germinating
under trees as the seeds are spread by birds.
Bugweed can be controlled by pulling out seedlings when they are first noticed; this is the
easiest form of control and the least expensive. Even young leaves are also strongly
aromatic. Plants from 0,5 to 1m in height can be sprayed with a registered herbicide. Large
plants need to be cut at ground level and any re-growth treated with a suitable herbicide.
BLACK WATTLE
Black Wattle
Family: Fabaceae
Scientific Name: Acacia mearnsii
Common name: Black wattle, Swartwattel
Origin: Southeast Australia and Tasmania
Reason for Introduction: Shelter, tanning bark, wood chips, shade, fire wood and
construction
Legal Status (The Conservation of Agricultural Resources Act 1983 (Act No 43 of 1983)
(CARA) : Declared Invader category 2 (Plants that can only be grown if a demarcation permit
is acquired {excluding forestation permit})
It is an unarmed (thornless) tree 5-10m in height and all parts finely hairy; growth tips golden
hairy. The fine leaves are dark olive-green.
The fruit is a dark brown pod, containing the seeds. The flowers, in flower heads, are pale
yellow or cream with a pleasant fragrance (August to September).
It is a highly invasive plant and inhibits the growth of natural vegetation. It is thus also
classified as a transformer invader plant. Countrywide, it grows along waterways, in urban
open space as well as along forest margins, grassland and roadsides. Found mostly in the
eastern interior and coastal regions of South Africa. In Johannesburg, black wattle trees
grow prolifically along many streams, in disturbed grassland, on ridges and in many gardens;
all areas a source of seed.
Black wattle can be controlled by pulling out seedlings when they are first noticed; this is the
easiest form of control and the least expensive. Seedlings and young plants from 0,5 to 1m
in height can be sprayed with a registered herbicide. Large plants need to be cut at ground
level and the cut stump chemically treated. Any re-growth must be treated with a suitably
registered herbicide. Seeds may continue germinating at a removal site for many years.
Seed-feeding biocontrol agents are available in South Africa.
LANTANA
Lantana
Family: Verbenaceae
Scientific Name: Lantana camara
Common name: Lantana
Origin: Central and South America
Reason for Introduction: Ornamental Value and Hedging
Legal Status (The Conservation of Agricultural Resources Act 1983 (Act No 43 of 1983)
(CARA) : Declared Category 1 (The weed which requires that it should be controlled
immediately)
It’s a compact untidy scrambling plant, 2 metres or higher. The stem is four angled and
usually covered with hooked prickles (thorns). The leaves are dark green, rough and hairy.
They have a strong smell when crushed and can cause skin irritation.
Flowers are pink, red, orange, yellow, creamson or white. Has incompact flat-topped
flowering heads (September to April).
The fruit is small glossy green berry turning purplish black when ripe and contains one seed.
It is a highly invasive plant and inhibits the growth of natural vegetation. It is thus also
classified as a transformer invader weed. Countrywide, it grows along waterways, in urban
open space as well as along forest margins, savannah, degraded land and roadsides. Found
mostly in the eastern interior and coastal regions of South Africa
Lantana can be controlled by pulling out seedlings when they are first noticed; this is the
easiest form of control and the least expensive. Seedlings and young plants from 0,5 to 1m
in height can be sprayed with a registered herbicide. Large shrubs need to be cut at ground
level and the cut stump chemically treated. Any re-growth must be treated with a suitably
registered herbicide.
Biocontrol agents are available in South Africa.
GREY POPLARS
Grey Poplar trees
Family: Salicaceae
Scientific Name: Populus X canescenes
Common name: Grey Poplars;
Origin: Europe and Asia
Reason for Introduction: Timber, shelter, ornaments, donga reclamation, commercial (match
stick, wagons)
Legal Status (The Conservation of Agricultural Resources Act 1983 (Act No 43 of 1983)
(CARA) : Declared Invader : Category 2 (Plants that can only be grown if a demarcation
permit is acquired).
It’s deciduous or semi- evergreen tree 10 to 20 metres high, the bark is white or grey. Young
plants sprout and grow from the roots.
Leaves are dark green shiny above and grey or white woolly beneath
Flowers reddish, male flowers only.
Bears no fruit.
It is a highly invasive plant and inhibits the growth of natural vegetation. It is thus also
classified as a transformer invader weed. Countrywide, it grows along waterways, river
banks, dongas and wetlands. Found mostly in the eastern and southern regions of South
Africa
Poplars can be controlled by foliar treatment. Seedlings should not be pulled (because they
are part of the root system). Seedlings and young plants from 0,5 to 1m in height can be
sprayed with a registered herbicide. Large trees need to be cut at ground level and the cut
stump chemically treated. Any re-growth must be treated with a suitably registered herbicide.
No biocontrol agents are available in South Africa.
BLUE GUMS
Blue Gum tree
Family: Myrtaceae
Scientific Name: Eucalyptus camaldulensis (this is one of many gums species)
Common name: Blue Gum tree
Origin: Australia (All states expect Tasmania)
Reason for Introduction: Shelter, timber, fire wood, ornamental, bee keeping, fencing and
telephone poles, mining industry
Legal Status (The Conservation of Agricultural Resources Act 1983 (Act No 43 of 1983)
(CARA): Declared Invader: Category 2 (Plants that can only be grown if a demarcation
permit is acquired)
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It is an evergreen tree 18 to 40 metres high with the spreading crown with smooth with or
grey bark.
Leaves are pale dark green, flowers are cream coloured
The fruit is a red to brown capsule
It invades perennial, seasonal and intermittent water courses
Blue gums can be controlled by pulling out seedlings when they are first noticed; this is the
easiest form of control and the least expensive. Seedlings and young plants from 0,5 to 1m
in height can be sprayed with a registered herbicide. Large trees need to be cut at ground
level and the cut stump chemically treated. Any re-growth must be treated with a suitably
registered herbicide.
No biocontrol agents are available in South Africa.
CONCLUSION:
Johannesburg City Parks (JCP), which is responsible for applying the relevant alien plant
legislation, has an annual alien plant programme. However, bugweed and other invader
plants cannot be successfully brought under control in Johannesburg without the help of
residents, residents’ associations, business, nurseries and organizations. We need to all join
hands and together make an attempt in controlling these plants. Residents and business can
make a big difference as the sources of infestations are often gardens.
JCP encourages all residents to become aware of these plants and to remove or treat them
where ever possible.
References:
Henderson, Lesley. 2001. Alien Weeds and Invasive Plants – a complete guide to declared
weeds and invaders in South Africa. Plant Protection Research Institute, Handbook No 12,
Agricultural Research Council, South Africa
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