Download Article 129 Senna didymobotrya 2 - Botanical Society of South Africa

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.