By Roelof Bezuidenhout
Most farmers will encounter at least one declared invasive alien plant species on their land. These plants are typically fast-growing and spread so quickly that, without early intervention, they can become unmanageable in no time. But tackling them without proper preparation can be a costly mistake – both financially and in terms of time and energy.
Although swift action is important, it must be guided by careful planning and budgeting. Invasive alien plants are not just a local nuisance but also pose a serious national threat. Dense stands of exotic trees such as black wattle, Port Jackson and various eucalyptus species consume enormous volumes of groundwater, choke streams, and collectively drain enough water each year to fill large reservoirs. Encouragingly, where these species have been successfully cleared, downstream water supply often improves immediately. The Working for Water programme – a public employment initiative launched in 1995 to combat invasive plants while creating jobs – has already restored thousands of hectares through targeted removal of these water-hungry invaders.
Invasive alien plants may appear harmless at first, but once they reach a certain density, they can quickly become overwhelming. Depending on the climate and local conditions, this tipping point might come within a few years – or take several decades to emerge.
Some of these invaders have been with us for over a century. Trees like black wattle and Port Jackson were imported from Australia for timber. Mesquite (or prosopis) came from Mexico and the southern United States as a source of shade and fodder. It has since spread aggressively through watercourses in the Northern Cape. Prickly pear, initially brought in for livestock feed, was nearly eradicated 80 years ago through the introduction of the cochineal insect. But other cacti – none of which are native to South Africa – remain difficult to control. Many were planted in gardens for ornamental use and have since spread into the wild. While not all cacti are invasive, it’s important to note that South Africa has no indigenous cactus species.
Act early, but start smart
Control methods include uprooting, felling, cutting or burning, applying a registered herbicide, and using biological control. In some cases, felled invasive trees make excellent firewood that can be sold – just be sure you’re removing declared invaders and not valuable indigenous species.
It’s also worth noting that some mesquite and eucalyptus species are not as aggressively invasive as others.
The World Wide Fund for Nature (WWF) has published a practical guide to help landowners in the Cape Floral Region manage invasive alien plants – it’s a good place to start if you’re not sure how to proceed. The guide identifies nine priorities:
1. Prevent new invasions: Identify and target emerging or newly arrived species before they have a chance to set seed and spread.
2. Follow up first: Prioritise areas that require follow-up treatment over areas that still require initial clearing. Follow-up treatment is essential to curb further growth and spread, and reinforces efforts in which you have already invested time and money. This also applies to an area that has recently burnt – the fire acts as an initial clearing mechanism, so if you prioritise such an area, it will cost less to clear while the growth is young.
3. Limit wildfire risk: Invasive aliens that pose a wildfire risk to houses or infrastructure should be a priority. Create effective firebreaks where woody or fire-prone plants are located in dense stands near settlements and power lines.
4. Start with less dense stands: Prioritise low-density, young invasions to halt the invasion and prevent the build-up of seedbanks. This is especially important in fast-maturing, wind-dispersed species such as hakea and pine. Less dense areas will also require fewer resources and easier follow-up treatment. Dense mature stands should be left for last, as they probably will not increase in density or pose a greater threat than they do at present. Clearing very dense areas requires a commitment to expensive, long-term follow-up treatments.
5. Start upslope: Consider the natural gradient of the area that needs clearing. All operations should ideally follow the slope or drainage lines. Clearing should start from the highest point and move downstream and downslope. This ensures that potential sources of the aliens – seeds and other regenerative material – are eliminated upstream of the working area to avoid reinfestation.
6. Work from the outside inwards: On gentle gradients, start clearing from the outside of a work block and move inwards towards the centre to contain invaders within a confined area.
7. Follow contours: To avoid soil erosion when clearing dense infestations of invasive plants on steep slopes, progress horizontally along the contours. The plants should be cut in bands of 3 m wide along the slope contour, and the cut material should then be rolled back so that it forms a “frill” along the band. Openings between contoured stacks should be staggered to further reduce water run-off.
8. Focus on riparian areas: Rivers, streams and wetlands are a priority when planning the clearing of invasive aliens. They use a lot of water and can reduce water flow and quality. And waterways provide a way for them to spread rapidly downstream.
9. Collaborate with neighbours: Collective management and planning with neighbours allow for more cost-effective clearing and maintenance.
Develop a management plan
The WWF guide suggests these four steps for developing an effective management plan to deal with invasive alien plants on your land:
1. Survey the area. A suitably experienced person should survey the areas to be cleared and identify the invasive plants that grow there. For very large areas, mapping of alien coverage is essential. It is best to walk the area to be mapped, and not just to rely on viewing it from a distance or via an aerial photo. Take photographs of the site to assist the process of monitoring the impact of the management programme.
2. Identify units. Break the property down into sensible management units, such as a block of land with uniform species and densities, or similar soil and slope. Give each management unit an identification number, and compile an inventory for each one, including the density and age of the invasive plants.
3. Plan actions. Identify what actions are needed in each management unit. Determine the sequence and which methods or combination of methods is best for the site and target species. Consider what field equipment and herbicides are required. Plan the order in which management actions should be implemented, taking the season into account. Check with your local fire protection association to ensure that your operation has received approval from the relevant authorities.
4. Calculate costs. Using the information gathered on the size of the management units, the density of the plants and the selected control methods, calculate the labour required and the likely costs. Keep a record of all expenses incurred, as these costs can be tax deductible under certain circumstances.
Legal obligations
Invasive alien plants can have a serious negative impact on agriculture, which is why South Africa’s agricultural laws include regulations to prevent their spread. These legal obligations may apply either to the landowner or to any person with the right to farm the land.
The Conservation of Agricultural Resources Act (Act No. 43 of 1983), administered by the Department of Agriculture, Land Reform and Rural Development, aims to protect South Africa’s agricultural resources. It prohibits activities that could contribute to the spread of declared weeds – such as selling, advertising, keeping, possessing or distributing them. It may also require land users to apply specific control methods, depending on the species and the surrounding ecosystem.
The WWF’s guide to managing invasive alien plants can be downloaded here.
 | Roelof Bezuidenhout is a fourth-generation wool, mohair, mutton and game farmer and freelance journalist. Attended Free State University, majoring in animal husbandry and pasture science. Other interests include agricultural extension and rural development. |
