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GRAANGIDS
2016
GRAIN GUIDE
Ten main invasive plants in South Africa
Continued from p. 79
Opuntia robusta Wendl.
The common prickly pear or wheel cactus originally comes
from Mexico. It is a large, erect, succulent shrub-like cactus
that can grow up to 1 m to 2 m tall, but also as tall as 4 m.
The plant does not really have leaves, but blue-grey cladodes
containing several thorns. The root system of the common
cactus is shallow and fibrous. It grows relatively easily in any
type of soil and is drought resistant. The common prickly pear
can propagate with seed or vegetatively. The negative aspects
of the common prickly pear are the following: It competes for
light, space, water and nutrients; displaces grazing plants if it
occurs in dense stands; changes the landscape and habitat;
reduces biodiversity and grazing capacity if in dense stands;
obstruction (movement); thorns cause injuries in people and
animals; thorny types cannot be eaten by animals; causes
skin irritations. A cactus moth serves as biological control
agent for this plant and is relatively successful. However, the cactus is relatively resistant to cactus
cochineal. Mechanical control of the plant is often difficult because of the thorny cladodes
– any part of the plant that is left behind will also grow again. There are currently no chemicals
registered to control this plant – the Department of Environmental Affairs can be contacted to
determine whether a substance has been registered for minor control.
Chromolaena odorata (L.) R.M. King
Triffid weed originally came from North, Central and South America and might have entered the
country as an ornamental plant. It is regarded as one of the 100 most dangerous weeds in the
world. It is an erect or creeper shrub that can grow 1,5 m to 3 m tall – as a creeper it can grow up
to 10 m tall. Triffid weed occurs in higher rainfall areas, where it grows preferably in well-drained
soils. However, it can also occur in a variety of soils and among various types of plants – from
water courses to forested gorges. It has an extensive, shallow root system. Some of the roots de-
velop into a bulbous storage organ from which new plants can form if the parts above ground are
damaged. The plant forms a multitude of seeds that are easily disseminated by the wind, water,
vehicles and people. The seeds can survive for a relatively long period in the soil.
The negative aspects of triffid weed are the following: Plants
contain oils that make them highly flammable when it is dry;
they displace indigenous vegetation quickly; change the land-
scape and habitat; compete with other plants for light, water,
space and nutrients; penetrate river beds and banks and
destroy the habitat of aquatic animals; they are allelopathic
– secrete chemicals that suppress the growth of other plants;
obstruction (movement and vision); contaminant (seed); toxic
to animals; cause skin irritations and asthma problems in
humans. Various biological control agents are being tested
for this plant and some of them have already been released,
but the success of the bio-agents is not yet known. The
plants can be controlled with fire or mechanically, but new
plants can develop from the bulbous storage organ. Various
chemicals are registered to control the plant and can be used
in combination with fire and mechanical control. According to
the CARA Act this is a category 1 plant – the landowner must
therefore control it.
Opuntia robusta
Chromolaena odorata
Continued on p. 85