Allamanda hendersonii is a synonym although some nurseries show them as different plants.
The widely cultivated Yellow Allamanda is naturalised, and a weed, in some areas of Queensland.
All parts have a poisonous, milky sap.
A vigorous shrub to 2 m high or a clambering vine with stems to 6 m long.
The young branches are green and the older ones greyish and woody.
They may be smooth or hairy.
The simple leaves are opposite or in whorls of 4 (3 to 5).
On stalks around 5 mm long the blades are up to around 15 cm long and 4 cm wide.
They can be ovate, obovate to elliptic with a wedge-shaped base and a variable, pointed tip.
They are leathery, dark glossy green with a wavy or rolled under edge.
The yellow-green lower surface may have hairs on the veins and midrib.
The pale midvein is prominent on both surfaces.
There are small glands in the leaf axils.
Hairy, branched inflorescences on a stalk to 2 cm long, are terminal or axillary.
There are bracts at the base of each branch but they fall off early.
The 10 to 12 flowers are on stalks around 6 mm long.
The bell or funnel-shaped flowers, around 10 cm long, have parts in 5’s.
Flowers may appear all year.
The unequal green sepals, 1.5 to 2 cm long, have the bases fused into a short tube.
The corolla tube, 8 to 9 cm long, has a narrow cylindrical base then widens above.
The spreading, slightly overlapping, round to ovate lobes are 3 to 4 cm long.
Flowers are a bright yellow with a pale to dark reddish throat that has darker lines.
The stamens are inserted near the top of the narrow part of the corolla tube.
They are on very short filaments and there are tufts of hairs above and below.
The 3.5 mm long anthers are stuck to the style head but not fused to it.
The ovary, of 2 fused carpels, has 2 locules with numerous ovules.
The single style, around 2.5 cm long, holds a large style head.
The style head has 2 apical lobes, a central receptive area and an inferior skirt.
Mature fruit are rarely seen and plants reproduce mainly by cuttings or layering.
The roundish capsules, 3 to 4 cm in size, are covered in long spines.
Each contains numerous flattened brown seeds with a narrow, paler, marginal wing.
There are a number of cultivars with small to large flowers.
A. cathartica ‘Williamsii’ and A. cathartica ‘Stansill’s Double’ are two cultivars with double flowers.
They are commonly seen just as Allamanda williamsii and Allamanda Stansill’s double.