Invasive Alien Plants


In this feature, The Water Page highlights a problem which faces many countries - that of invasive alien plant species. This has particularly devastating impacts in developing countries which lack the resources to tackle the problem. One of the leading examples of the problem being taken seriously is in South Africa where, through the establishment of a "Working for Water" Programme, alien plants are being eradicated on a large scale whilst at the same time providing employment to a large number of people.

Invasive Alien Plants are plants which are imported into an eco-system which is not their natural habitat. They proceed to grow prolifically, threatening the indigenous plant and animal secrecies. They tend to utilise more resources to grow, in particular water. This can seriously deplete soil moisture and reduce groundwater recharge, the base-flow of springs and ultimately the flow in rivers and streams. In catchment areas which are infested with such plants, the eradication of invasive alien plants can often lead to significant increases in water available for other uses and may offer an alternative to traditional engineering solutions such as the construction of reservoirs and dams.

Invasive aliens include a variety of plants from aquatic weeds such as Water Hyacinth to large trees such as Eucalyptus and Wattle. Such plants in their own native eco-systems are constrained and of no harm but, when introduced into foreign habitats can cause enormous damage.

Billions of Dollars are lost every year in many countries due to damage done by invading alien plants. They are the single biggest threat to plant and animal biodiversity. As an example, there are 161 introduced tree and other plant species that have become invasive in a single country - South Africa. If uncontrolled, the problem are likely to double within 15 years.

Alien invasive plants are a problem because they:

  • can waste a large proportion of national water resources (7% estimated in South Africa);

  • lead to the loss of potentially productive land, and the loss of grazing potential and livestock production;

  • reduce the ability to harvest indigenous natural resources;

  • increase the problems associated with flooding and fires;

  • cause erosion, destruction of rivers, siltation of dams and estuaries, and poor water quality;

  • can cause a mass extinction of indigenous plants and animals,

Programs to eradicate invasive alien plant species can have a number of benefits:

  • Increased water security with enhanced streamflow and improved water quality,

  • more productive wetlands, estuaries and water tables;

  • Rehabilitation of degraded land with a strong emphasis on Land Care to secure the sustainable productivity of land;

  • Conservation of biodiversity and catchment integrity and the reduction in the frequency and intensity of fires and floods;

  • Development of secondary industries based on the cleared wood, and

  • Empowerment of people through the labour-intensive approach to the work, (The Working For Water programme in South Africa has employed over 42 000 people in a five year period.)


Control Options

Pre-Requisites for Effective National Programs to Eradicate Invasive Vegetation

In order to get any effective national scale program under way there are a number of pre-requisites:

Acknowledgement Of The Problem

  • The first and most important requirement is recognition that there is a problem. Because of the implications of extensive invasions, politicians and officials are often apt to hope that the problem will somehow solve itself until a point is reached where action has to be taken. [Picture - Going nowhere - Water Hyacinth gets out of control on Lake Victoria.]

Political Will

  • The cost and effort required to make a significant impact on invasive alien plants is not seen as a politically attractive allocation of resources - it is remedial effort, the results of which are generally not obvious to the public eye. However it is not possible to establish a sustainable eradication program without political will.

Technical Expertise

  • In order to be able to implement effective eradication programs with the correct mixture of mechanical, chemical and biological control, expert input is required, often including detailed research.

Adequate Legal and Institutional Framework

  • In order to be effective in both eradicating and preventing the spread of alien plants, it is necessary to establish an adequate legal framework in order to be able to address the problem where ever it may arise. Regulations should provide for the classification of plants into suitable categories, whereby landowners are required to ensure that plants belonging to certain species are removed from their property. Government efforts to address the problem often involve several departments including water, forestry, agriculture, environment etc. - interaction and co-operation between these departments is important for eradication programs to be effective.


  • Eradication programs cost money. In poor countries resources used for such programs may be regarded as diverting resources from other more pressing issues, however not attending to such problems often leads to larger costs in the long run. In some instances it is possible to derive benefit from eradication programs such as the "Working for Water" program in South Africa which provides employment to a large number of workers clearing invasive plants.

Control Options

  • The information provided below provides a basic indication of the methods which need to be employed to address the growing problem of alien vegetation.

Effective management of invading alien plants in natural and semi-natural systems is imperative if we are to prevent enormous impacts. An integrated approach involving the combined use of range of methods is usually necessary to control invasive alien plants effectively. The various methods that are available are usually classified as follows:

  • Mechanical methods (felling, removal of invading alien plants, often in conjunction with burning);

  • Chemical methods (using environmentally safe herbicides and pesticides);

  • Biological control (using species-specific insects and diseases from the alien plant, insect of animal's country of origin).

Approaches available for integrated control depend on the species under consideration (features of individual species and the number and identity of species that occur together), features of the invaded systems, the availability of resources and other factors. Mechanical and chemical control are short-term activities, whereas rigorous and disciplined follow-up and rehabilitation are necessary in the medium term. Biological control can provide effective control in the short and medium term in some cases, and it is often the only really sustainable solution in the longer term.

The biological attributes of plants represent a stable set of attributes, which enable managers to devise control approaches, but such approaches are likely to be upset by stochastic events such as fires, floods or budget cuts. While an approach of adaptive management, based on trial, error and continual improvement is a logical way in which to progress, the advent of powerful computer simulation modelling technologies will allow managers to do hundreds of "trial and error" runs in order to explore the consequences of certain courses of action. This should represent an improvement on the current state of affairs, and should allow for better decision-making.

Any control programme for alien vegetation must include the following 3 phases:

  • Initial control: drastic reduction of existing population

  • Follow-up control: control of seedlings, root suckers and coppice growth

  • Maintenance control: sustain low alien plant numbers with annual control

NB: it is important to rehabilitate land that has been infested with alien plants.

Where trees cannot be utilised (for example on steep slopes), do not fell trees, control them in situ.

Select from the following options:

  • Basal bark: apply recommended herbicide mixed in diesel carrier to the base of the stem of trees and saplings. Do not cut the bark. Apply herbicide mix with paintbrushes of knapsack sprayers at low pressure.

  • Strip bark: Using a bush knife, strip bark away from tree from waist height down to soil. Cambium is stripped with the bark. No herbicide used.

  • Hand pull: Grip the young plant low down and pull out by hand (using gloves).

  • Ring barking: Strip all bark and cambium from a height of 400-500mm down to just below soil level. Cut a ring at the top and pull strips.

  • Frill: Using an axe or bush knife. Make angled cuts downward into the cambium layer through the bark in a ring. Ensure to effect the cuts around the entire stem and apply herbicide into the cuts.

Where trees can be felled and removed use chainsaws, bowsaws, brushcutters or machetes.

  • Cut stump treatment: Apply recommended herbicide mixture to the cut surface with hand sprayers, paintbrush or knapsack sprayer at low pressure. Apply only to the cambium or outer layer of large stumps and the entire cut surface of small stumps. Ensure the stumps are cut as low to the ground as possible about 100 – 150 mm.

For invasive cactus species

  • Stem injection: Punch downward slanting holes into the main stem using a sharpened metal spike. Space holes around entire circumference of lower stems. Inject the herbicide directly into the plant – ensuring to inject around the stem. Follow label recommendations.