Weeds in our Area (Part Ninety Two) By Bob and Ena McIntyre – Garden Route Robinia pseudoacacia

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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

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