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Download Article 129 Senna didymobotrya 2 - Botanical Society of South Africa
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Weeds in our Area (Part One Hundred and Twenty Nine) By Bob and Ena McIntyre – Garden Route Region. Senna didymobotrya (=Cassia didymobotrya) (Peanut-butter Cassia) In Sima Eliovson’s 1952 gardening book Flowering Shrubs and Trees for South African Gardens, Cassia didymobotrya is described enthusiastically as a: “striking large shrub growing to about 12 feet in Durban”. A note at the end regrets that it is unfortunately a short-lived plant. That was very long ago and the Peanut-butter cassia has since proven itself to be extraordinarily enduring and robust in making a home for itself - progressing to invader status today. We first examined Peanut Butter Cassia in 2006 and have noted an increasing footprint in Wilderness (see photo of a spreading patch in Waterside Rd, Wilderness) justifying an update. Another collection of goodlooking specimens can be seen upstream as you cross the little bridge on your way into the Milkwoods village complex. Although being indigenous to Africa (from the tropical regions) the plant has become invasive in the eastern sub-tropical summer rainfall regions of the country and is showing invasive tendencies in the western areas of the Eastern Cape and in some areas of the Western Cape, gradually migrating westwards. Introduced as an ornamental and hedging plant, its robust growth habit, and rapid easy seed germination give it a distinct advantage over indigenous species that are readily overwhelmed. Simple to identify, the local specimens are very obvious, many in full flower thanks to the ideal local conditions. The plants flower throughout the year and of interest is that the leaves of the Peanut Butter Cassia are poisonous. Identification: Senna didymobotrya is an impressive looking evergreen (in frost free areas), rounded shrub or small tree normally between 1 to 3 meters but can reach up to 6 meters in ideal conditions. The flowers and cut stems smell of peanut butter explaining the common name. The dark green leaves are compound (pinnate) with 8 to 21 pairs of leaflets with tapering points. Flowers are buttercup shaped and bright yellow with dark brown buds in narrow erect racemes up to 450 mm long. Flowers open from the bottom of the flowerspike. The seed pods contain numerous seeds and are flattened, ± 100mm long and 20mm wide, initially green, turning dark brown. Invasive status: Special effect weed – competitive, the leaves are poisonous. It invades coastal scrub, woodland, riverbanks and roadsides. Declared invader (category three: existing plants at 30 March 2001 may remain - no new plantings i.e. hand-pull emerging seedlings) Control: No herbicide is registered for the control of this species making manual control the only option. Large plants should be cut as close to the root ball as possible to reduce the chances of coppicing. Senna didymobotrya reproduce from seed only. This makes recognising seedlings very important in order to weed them while still small. It is essential to follow up on re-growth from cut stumps and to check for emerging seedlings. Indigenous Substitutes: Cortalaria capensis (Cape Rattle-pod) and Calpurnia aurea (Common Calpurnia). References: “ALIEN WEEDS AND INVASIVE PLANTS”: Lesley Henderson. Copyright © 2001 Agricultural Research Council. Problem Plants of SA – Clive Bromilow. Flowering Shrubs and Trees for South African Gardens – Sima Eliovson.