Pompom weed (Campuloclinium macrocephalum (Less.) DC.) is a South American herb belonging to the daisy family, Asteraceae. It is rapidly becoming the most serious threat to the conservation of grasslands in South Africa. Infestations become conspicuous when the plants are in flower between December and March, transforming the veld from green to pink. The plant initially establishes itself in disturbed sites such as roadsides, but then invades natural grasslands, open savanna and wetlands. This weed displaces native species, reducing both the biological diversity and carrying capacity of wetlands and veld. During winter the plant is not visible above-ground.
Pompom weed is a perennial, erect herb up to 1.5 m high. The stems and leaves are covered with rough, bristly hairs. Leaves are scattered along the length of the stem but clustered at the base to form a rosette. The plant has a short woody rootstock that ends in thick tuber-like perennial roots. In spring, shoots arise from, and in autumn die back to, the rootstock. The showy pink flowerheads (inflorescences) are produced in dense clusters at the ends of the aerial stems, and look like bright-pink powderpuffs.
Each flowerhead, measuring 15 mm long × 25 mm wide, consists of hundreds of tiny, star-shaped florets (individual "flowers") surrounded by purple bracts (brightly coloured, leaf-like scales). A long, protruding style from each floret gives the flowerhead a fluffy appearance. Mature florets each produce a single-seeded dry fruit (achene) with a tuft of brown hairs (pappus) that promote wind dispersal.
How or when the plant was introduced into South Africa remains unknown. Currently it is most prominent in Gauteng, but also occurs in Mpumalanga, Limpopo, North West, KwaZulu-Natal, Eastern Cape and Free State Provinces. A small patch of plants in the Outeniqua mountains near George has been eradicated but the site will be monitored for regrowth.
Pompom weed is a listed invasive species (category 1b plant) in terms of the IAS Regulations (August 2014) of the National Environmental Management: Biodiviersity Act, (NEM:BA) (2004). It is illegal to harbour, plant, propagate or sell pompom weed. Landowners are therefore compelled to control pompom weed by whatever means are deemed appropriate.
Management should aim to maintain the natural vegetation in a healthy, productive state as this will help to limit pompom invasion.
Chemical control of pompom weed has been severely compromised by the rust biocontrol agent, Puccinia eupatorii. The rust becomes very damaging to mature pompom shoots from February, and herbicide absorption and translocation is thus adversely affected. It is not advisable to spray pompom weed after February; this leaves a narrow window of opportunity in which to conduct spraying operations: 2-3 months depending on climatic conditions (December to February). In years with early spring rainfall it may be possible to spray in late November. Herbicides should be applied onto actively growing plants at least 0.5 m tall. The selective, broadleaf herbicides picloram and metsulfuron methyl are both registered on pompom weed and will not affect veld grasses. As there are numerous registered products on the market, please consult Mr. Thilivhali Nepfumbada, Technical Advisor: Agricultural Remedies, or Crop Life South Africa, before you go out to spray. It is imperative that only registered, selective, broadleaf herbicides are used in grasslands. Non-selective herbicides should never be used to control pompom weed in the veld or along grassy road reserves. Non-selective herbicides should only be used where pompom weed occurs in croplands and industrial situations, e.g. concrete drains, pavements etc. Selective broadleaf herbicides will damage all broad-leaved plants exposed to the spray including native forbs and shrubs, so spot spraying is advised rather than indiscriminate broadcast applications. Each area sprayed must be revisited for the next seven years to neutralise the soil seedbank.
In general, physical methods of control, such as uprooting or hoeing, are ineffective and make the problem worse through disturbance. It is not advisable to plough lands with pompom weed, as this will damage the rootstock, stimulating further vegetative growth and denser stands. Spread of the plant can be limited by preventing seed production. Aerial stems can be cut right back before the flowers produce seed. However, be warned that the plants will be stimulated to produce more stems. In order for this method to work, the plants will have to be cut back several times until the end of the growing season. Repeated cutting back of aerial growth to deplete nutrients stored in the roots and thus to weaken the plant, is not recommended as it will require many years of meticulous dedication to each infestation and incur high costs for labour and transportation. This method is, however, only practical on a small scale and primarily to prevent flowering and seed set. It is advisable to remove all flower heads from the site, being careful not to spread the weed further, and dispose by burning.
In the case of single or very few pompom plants in an area, each plant can be dug up, taking care to remove at least the rootstock crown (the area where the stem is attached to the swollen, finger-like roots) from the soil. To be safe the rootstock should be removed with a portion of the roots. Once the rootstock crown has been removed, the roots remaining in the soil will die.
It is important to cause as little soil disturbance as possible, in order to prevent the mass-germination of pompom seeds. Regular follow-up visits to the site for the next seven years are essential to ensure that all seedlings have been removed.
Burning can be a double-edged sword, as it has been found to kill seeds on and just below the soil surface but also causes spontaneous regrowth form rootstock. If areas are burnt and left, the weed will become denser. We believe that burning, integrated with chemical control, will be an effective tool to target both the weed seedbank and mature rooted plants, but this assumption needs to be researched further. Burning in late winter will produce a flush of regrowth in spring before the veld grasses and native forbs emerge, and thus help reduce non-target herbicide damage to desirable species, It will also lengthen the window of opportunity for spraying before the rust sets in.
A stem-galling thrips (Liothrips tractabilis) and the rust fungus (Puccinia eupatorii) are currently the two main biological control agents in use against pompom weed and it is expected that the pompom thrips, in conjunction with the pompom rust, as well as chemical control efforts, will go a long way in reducing the spread and impact of this invasive plant.
Adult and nymphal stages of the thrips cause significant feeding damage to pompom stems and leaves which, in turn, drastically deforms plant growth, thus reducing flowering. Since the initial first release, over 100 000 adults and nymphs have been released in over 45 localities throughout Gauteng, Limpopo, Mpumalanga and North West. Follow-up surveys and feedback from landowners have confirmed the persistence of the insects, and methods are currently being investigated to increase their dispersal within and between pompom infestations.
Liothrips tractabilis is currently being mass-reared at the ARC-PPRI facility in Cedara, KwaZulu-Natal and couriered to ARC-PPRI in Pretoria, from where releases are coordinated according to guidelines provided by the National Pompom Steering Committee. Once releases have been completed in all areas prioritized by the Steering Committee, and mass-rearing expands to Gauteng, insects will be more readily available to meet the ever-increasing requests of residents' associations and private land owners.
Anyone wanting to include biological control in their management strategy against pompom weed can have their names, contact details and localities added to an existing database by contacting: Liamé van der Westhuizen (VdwesthuizenL@arc.agric.za).
The Southern African Plant Invaders Atlas project (SAPIA), coordinated by Lesley Henderson from ARC-PPRI, is aimed at keeping track of all infestations of pompom weed. Persons who are aware of any new sightings of pompom weed are requested to send these records to Lesley Henderson (stationed at SANBI), Private Bag X101, Pretoria, 0001; Tel 012 843 5035; Fax 012 804 3211; e-mail L.Henderson@sanbi.org.za. Please include date, GPS or approximate locality, habitat and abundance.
The following fact sheets can be downloaded:
Liamé van der Westhuizen. E-mail: VdwesthuizenL@arc.agric.za