Contributor: Leonie Twentyman Jones
If there was any doubt in your mind regarding the problems caused by alien invasive plants – a visit to Umngazi River mouth in Pondoland would dispel those doubts very rapidly. The coastal hills and valleys are dominated by large scrambling colourful thickets of Lantana camara (Cherry pie) and the golden yellow flowers of the Cassia/Senna didymobotrya (Peanut butter bush).
Lantana camara, a shrub originally from Central and South America, flourishes in hot dry conditions. The pretty flat-topped heads of pink, red, orange, yellow and white flowers appear from September to April and are followed by glossy purplish-black berries which are spread by fruit-eating birds. It is poisonous to humans and livestock and competes with and replaces indigenous species. The dense stands obstruct access and reduce the grazing potential of the land. The more the shrubs are cut back the more enthusiastically they grow. Understandably Lantana camara has been declared a Category 1b invader which means it must be controlled and where possible removed and destroyed. It is hard to imagine how this will be achieved given the extent to which the shrub has taken over the landscape. Various methods of removal are recommended e.g. clearing and digging out the roots, applying herbicides by spraying or painting the chemical onto the stumps of the shrub after having cut them down. The introduction of biocontrol agents has been unable to stop the plant from spreading but in some areas has reduced its rate of growth and reproduction.
Similarly the Cassia or Senna didymobotrya, which comes from tropical Africa, is another invasive alien in the area. It has attractive bright yellow flowers all year round, which when rubbed give off a strong scent of peanut butter. The leaves are poisonous, but the seeds, contained in pods, are ground up by local people to use as a coffee substitute. It grows rapidly into a shrub or small tree forming dense impenetrable thickets which impede the growth and regeneration of indigenous plants. It invades grasslands, forests and coastal scrub. This is also a Category 1b invader and manual control is the only option. Again one cannot imagine how this will be achieved.
The invasion of these aliens is clearly a major problem and should make us all acutely aware how important it is not to allow it to happen here. There are some of these shrubs growing in Knysna and surroundings and every effort should be made to remove them before it is too late.