ALIEN PLANTS 

Alien Plant Report 2007  (PDF file)

The aggressive competition of weeds costs South Africa about one billion rand each year in crop losses, and causes immeasurable harm to water supplies and to indigenous plants and animals.  Without effective control measures it will be impossible to feed our growing population, alien plants will alter landscapes and habitats and rare indigenous species will be lost. Ideally, control measures should include combinations of chemical, biological and mechanical methods to achieve environmentally safe and cost effective results.

Alien plants have many characteristics which may contribute to their ability to spread.  Some of the important reasons are:

• Lack of consumers/herbivores: plants generally have a large number of insects and diseases which feed on or affect them.  If a plant is transferred to an environment that does not have these consumers/ herbivores/diseases, it may then grow unhindered.
• Occupation of a niche - if indigenous vegetation is removed, a niche is made available and is often occupied by alien invasive plants.
• Disturbance by man or natural occurrences stimulates plant succession.  Alien plants compete with indigenous plants and often win.

Triffid Weed (Chromolaena odorata)

Origin 
This plant originates from a large area in south and central America and the Caribbean and is believed to have been introduced to our country in packing material for goods landed in Durban in the 1940s.

Identification
An individual shrub can grow up to 3 m but will scramble higher when growing amongst trees.  Its leaves are triangular, simple and have three conspicuous veins arising from the base.  Leaves are strong-smelling when crushed.  Flowering is in July and August, the flowers are whitish tufts grouped together at the end of branches.  The seeds and fruits look like black-jack seeds except the hairs do not stick to clothing.  Up to 1,3 million wind-dispersed seeds are produced per plant.

Distribution and Habitat
Triffid weed occurs up to an altitude of 1 000 m but is extremely invasive below 600 m.  KwaZulu-Natal is the worst affected area in South Africa, with large areas of the coastal region covered.  It invades indigenous vegetation and can thrive in weakened forests and open veld.

Problems Caused
It replaces and modifies indigenous plant communities.  It can form solid stands in gardens and conservation areas.  This plant is the biggest threat to the long term survival of nature conservation areas in the province.