Weeds in our Area (Part Forty Seven)
By Bob and Ena McIntyre – Garden Route Branch.
Ageratum conyzoides
It is always astounding how even an insignificant little plant can become a serious headache in a
receptive and predator-free environment. Asteracea is a huge and very popular plant family,
therefore it is not surprising to find that Ageratum conyziodes (Invading ageratum) and
A.houstonianum (garden ageratum, floss flower, Todd’s curse), both Asteracea, were introduced
as ornamentals from Mexico, Southern and Central America and the West Indies. A.houstonianum
was used successfully in horticulture and some stunning
cultivars were developed. The floss flower became a problem
as far back as 1883 when it was first recorded as a weed. Sadly
we still see evidence of these plants being cultivated in home
gardens and many examples are found along roadsides and in
degraded urban areas. A.conyziodes is common in the warmer
eastern climes and has also established itself locally, where it
is frequently found in gardens. A.houstonianum seems to
prefer the KZN midlands and can be distinguished by its large
flower heads. A.conyziodes is used medicinally and in folk
remedies in various parts of the world. The leaves are used as
wound dressings and a multipurpose tea.
Identification: The plants are softly hairy and slightly aromatic.
The flowers are pale blue, mauve, pinkish or white fluffy
terminal flower heads and the foliage is bright green with the
leaves in opposite pairs with shallow toothed margins.
Invasive status: Both Ageratum conyziodes and A.houstonianum are special effects weeds. They
are common weeds in annual crops and can be particularly troublesome in cotton cultivation
where the seeds contaminate the lint and the wind-dispersed seeds can cause eye-irritation for
workers. The plants are competitive and poisonous and are declared category one weeds
(Excluding cultivars of A.houstonianum). They invade forest, woodland, grassland, cultivated land,
riverbanks, wetlands and coastal dunes.
Control: The plants are quite easy to control by regular cultivation and physical removal. The
application of pre-emergent herbicides is also effective. Once established though, they are difficult
to control with post-emergent herbicides.
Indigenous substitutes: Tetraselago natalensis (Natal Blue Haze), Vernonia capensis (Narrowleaved Vernonia), Vernonia natalensis (Silver Vernonia)
References: Problem plants of SA by Clive Bromilow, Alien Weeds and invasive plants by
Lesley Henderson
Photo: From Alien Weeds and invasive plants by Lesley Henderson. Copyright © 2001
Agricultural Research Council