CONSERVATIon

​National Weedbuster Week: An Introduction to
Alien Invasive Plants

OCT 12, 2020 | Written by Zoë Chapman Poulsen. Photos by Rupert Koopman, IsAbel Johnson, ‘MartinMartin’ (www.inaturalist.org) and Zoë Chapman Poulsen. 

Above: Hakea and pine dominate Overberg Sandstone Fynbos south of Botrivier. Photo: Rupert Koopman.

National Weedbuster Week

National Weedbuster Week is an initiative spearheaded by the Department of Environmental Affairs, implemented each year by their Environmental Programmes branch. It aims to raise awareness about alien invasive plants, their impact on South Africa’s highly biodiverse ecosystems as well as the country’s precious water resources.

What is an invasive alien plant?

An invasive alien plant is a plant that is not indigenous to South Africa or the ecosystem in which it is found growing within the country. When an invasive alien plant species takes hold within the ecosystem and starts to spread, they can displace indigenous plant species through competition, as well as bringing about changes in ecosystem structure and function.

This may include impacts on ecosystem services such as water resources and hydrology as well as biodiversity. Impacts of alien invasive plants can be worse in areas where human-induced disturbance has already increased vulnerability of the ecosystem to invasion.

Above: Port Jackson (Acacia saligna) in Atlantis Sand Fynbos near Chatsworth. Photo: Rupert Koopman. 

Invasive alien plant categories 

The South African government has divided the country’s alien plant invaders up into categories, thus making it easier to understand which species are the most ecologically damaging and where resources should be directed in order of priority for control. On this government gazette a total of 559 alien species are listed, with a further 560 species listed as prohibited from being introduced into the country. These categories are as follows:

Category 1a: These species are recognised as the most invasive and environmentally damaging. Any category 1a species need to be, by law, removed from the environment. No permits will be issued for their cultivation or distribution.

Category 1b: Any invasive species placed in this category qualify for removal under government funded alien invasive clearance programmes. No permits will be issued for invasive species within this category.

Category 2: Category 2 invasive species are regulated by area of the country. Demarcation permits are required to grow, move, sell, buy or accept any of these species as a gift. No category 2 invasives are allowed to be introduced into riparian zones.

Category 3: A permit for category 3 invasives is required to grow, move, buy or sell any species that fall into this group. Permits will not be issued for introduction of these species to any riparian areas.

Above: Port Jackson (Acacia saligna) in Lourensford Alluvium Fynbos at Broadlands near Strand. Photo: Zoë Chapman Poulsen.

Which are some of the worst alien invasive plants in South Africa?

Port Jackson (Acacia saligna): This medium sized Australian tree was introduced to South Africa in 1848 with the aim of using it to stabilise sand dunes in coastal areas and along roadsides. It was planted extensively both on the Cape Flats and in the Port Elizabeth area. This rapidly growing species is now found in huge thickets throughout the Cape Floristic Region, with advanced invasions excluding all other indigenous species. Control is a challenge due to its resprouting habit and large seed set.

Cluster Pine (Pinus pinaster): This highly invasive tree was brought over to South Africa at the request of the first colonial governor of the Cape, Jan Van Riebeek. Initially grown for timber, this species has escaped from plantations and invaded many mountain areas across South Africa, increasing fire danger, impacting on biodiversity and water resources in mountain catchments.

Water Hyacinth (Pontederia crassipes): This aquatic species originates from the Amazon River in South America, being introduced to South Africa for horticulture around 1910. It quickly escaped and is now common in waterways across the country. This species is considered the world’s worst water weed. Entire waterways can become blocked, preventing navigation and blocking irrigation canals. Growth is extremely rapid, with the number of plants doubling every 5-15 days.

What impacts do alien invasive plant species have?

Alien invasive plants have widespread rampant impacts on ecosystem services, biodiversity, water resources and levels of fire danger. Aquatic invasives such as can choke watercourses, blocking fishing, water access and navigation. Trees such as pines, Acacia and Eucalyptus are highly flammable, and when fire moves through the landscape in fire prone and fire dependent vegetation such as fynbos and grasslands, the fire burns with greater intensity at higher temperatures. Many of these invasives also increase evapo-transpiration of water, removing water from economically and ecologically important catchment areas.

Above: Common Water Hyacinth (Pontederia crassipes) is one of the world’s worst water weeds, invading entire waterways with its rapid growth. Photo by ‘martinmartin’ sourced from www.inaturalist.org.

What can you do to help?  

Invasion of South Africa’s ecosystems by alien plants is one of the most significant threats to the country’s biodiversity and provision of ecosystem services. It is up to everyone to contribute what they can to help control the problem.

Clearance of alien invasive plants is an important source of job creation through the Working for Water and Working on Fire programmes. Support is also available to landowners for this through the Department of Public Works. There are also many community hacking groups that clear alien invasive plants in their local area, so consider volunteering your time with an existing group, or even starting your own group to contribute to biodiversity conservation in your area. There are exciting plans for BotSoc Branches to get more strategically involved in addressing this major threat to our indigenous plants so watch this space!  

Above: Commercial afforestation in KwaZulu-Natal mistbelt grasslands. Plantations such as these have been a significant source of propagules for invasive pine trees to spread into surrounding natural vegetation. Photo: Isabel Johnson.

Further reading

Bromilow, C. (2010) Problem Plants and Alien Weed of South Africa, Briza Publications, Pretoria, South Africa.