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Crofton Weed

The Mexican Devil Weed (Crofton Weed) in the Magaliesberg
Please be on the lookout!

Ageratina adenophora - crofton weed (Family Asteraceae)

The Mexican Devil weed (crofton weed; Ageratina adenophora) has been making its appearance in the past five years in many Magaliesberg kloofs near and in water streams with a major infestation at Easter Kloof. Before being noted as a weed in South Africa, the plant assumed serious weed status in Hawaii, Australia, New Zealand, China and in India. Recently it was noted as having formed large thickets in the foothills of the Himalayas, in Nepal. It is believed to be primarily a garden escape.

Please make sure that you will recognise this weed, pull it out and report any findings!

Some biocontrol measures are currently in place in various countries. In the Northern Province (Tzaneen area), KwaZulu-Natal and the Western Cape, host-specific gall flies and a leaf-spot fungus from Mexico are being released, but these are not expected to be able to control the weed in our area. Because of low priority on a national scale (but not so for the Magaliesberg and for other inland mountain ranges!), research funding for more destructive, effective natural enemies (insects or diseases from the areas of origin) has not been forthcoming.

Recommended measures: manual removal, with chemical control (especially spot-spraying) where there are large infestations. When there are sparse plants and dense infestations, control efforts should be concentrated on the sparse plants first, as these could rapidly develop into more dense stands. Dense stands will hardly get denser, and could be left for last while biocontrol agents may be multiplying in them, even though they will be a source of seeds. If dense infestations are too much too handle properly at once (removing each shoot from under soil level as it will resprout if broken off and not entirely removed), cutting the flower stalks before the seeds are shed in early summer and putting them in closed plastic bags until decomposed) will at least reduce the copious seed production. Seeds are readily dispersed by wind over long distances, and also by water. Because of the dense regeneration from seeds in the soil, follow-up work over several years is essential in any cleared areas. Oversowing with seeds of pioneer grasses of the area will probably be an essential part of rehabilitating any areas cleared of dense infestations (manually or with the judicious use of chemicals) unless rigorous follow-up clearing is practised annually. It is imperative to check all kloofs from top to bottom annually in order to be able to prevent the weed from forming seeds there. Plants may grow high up on damp, vertical cliffs.
Stefan Neser, Plant Protection Research Institute, Agricultural Research Council.

The following adapted from: 'Plant invaders of South Africa - a pocket field guide to the identification of 161 of the most important and potentially important alien species', Plant Protection Research Institute Handbook No 5, ARC, 1995, Lesley Henderson, p. 5.

Line sketch

Crofton eradication at Easter Kloof - pictures