Download Article 92 Robinia pseudoacacia (False Acacia)

Weeds in our Area (Part Ninety Two)
By Bob and Ena McIntyre – Garden Route
Robinia pseudoacacia
(False acacia/Black locust/Witakasia)
An important note with regard to CONTROL - clearly, the most obvious solution to prevent the
proliferation of invasive species by wind or bird dispersal is, where practical, to prevent them
from flowering. In the case of Agaves (Part Ninety One), simply cut off the pole before the plants
set seed or form bulbils.
In researching our topic for this edition we came across Robinia pseudoacacia originating from the
eastern and central parts of the USA. This particular subject illustrates beautifully just how easily
keen gardeners can inadvertently plant a problem. Robinia pseudoacacia have been around in
South African gardens for many decades and were cultivated for a variety of other uses. Tolerant
of all sorts of neglect and harsh conditions with the added bonus that it shed its foliage in winter –
letting in the sunlight - and then came alive in spring with a profusion of attractive, fragrant
flowers, it was highly recommended as an ornamental in all our reference books pre-1980. Not
that it disappears after that date, on the contrary, one then reads about it being used specifically
as rootstock for propagation by grafting, of Robinia hispida (Rose acacia), a much smaller plant but
very popular as a standard. Of course today with an abundance of commercial nurseries, home
gardeners are less likely to do their own grafting and we buy already grafted plants. Even the
earliest books acknowledged the awful invasive habits of the Robinia pseudoacacia rootstock
warning that all root-suckers of Robinia hispida are to be removed the moment they appear. To
add to its list of undesirable qualities; the seeds, leaves and inner bark of Robinia pseudoacacia are
poisonous.
Identification: The False acacia is a fairly tall (up to 25m)
deciduous tree with an attractive shape. The light green
leaves are compound (pinnate) and the trunk is dark
brown and irregular. Young stems and branches are
armed with short paired spines. The flowers are white
and fragrant in drooping sprays of 100-200mm and
appear from September to November. Fruits are reddish
brown pods that remain on the tree in the winter
months.
Invasive status: Declared invader Category 2. (Can only
be grown with a permit in demarcated areas and
measures in place to prevent their spread) The plants
sucker prolifically and invade riverbanks, dongas and
roadsides where they form dense stands of plants
interconnected by one root system.
Control: Two herbicides are registered for basal stem application (up to 100mm diameter); Ranger
240EC and Garlon 480 EC. For foliar application there are Confront 360 SL and Plenum 160 ME as
full cover spray on actively growing plants. Continue monitoring for possible re-growth. Even when
physically removing this tenacious plant, follow-up is necessary. Cutting down suckers stimulate
growth.
Indigenous substitutes: Baphia racemosa (Violet pea, Natal Camwood), Peltophorum africanum
(Weeping Wattle) Podylaria calyptrata (Sweetpea Bush),Indigofera frutescens (River Indigo),
podylaria myrtifolia (September bush),
References: Alien Weeds and Invasive Plants by Lesley Henderson, Copyright © 2001 Agricultural
Research Council, Problem plants of SA by Clive Bromilow, A-Z of Gardening in SA, W. Sheat.
Ornamental Shrubs and Trees, Una v/d Spuy, Flowering Shrubs and Trees, Sima Eliovson