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pronounced: HYPE-tiss cap-it-AH-tuh
(Lamiaceae – the lavender family)
common name: Knobweed
This native of Central America is a weed that grows in disturbed areas such as roadsides, creek banks, cultivated areas, run-down pastures, preferring heavy soils retaining above-average moisture, and full sunlight. The plants photographed were growing near the Horseshoe Bay lagoon. It is not yet widely naturalized in Australia, and is mostly confined to the coastal areas of North Queensland. It has also been recorded in Central Queensland, the Northern Territory and on Christmas Island.
Knobweed is a large upright perennial herbaceous plant with several branching stems, growing 60 – 250 cm tall. The pubescent stems are four-angled, and the widely spaced, bright green leaves, up to 12 by 5 cm in size, are opposite. The leaves have irregularly serrate margins and acute apices, and there are small oil glands on the underside.
The small white flowers are borne in stalked globular clusters, about 1.5 cm in diameter, that resemble flower-heads. They are borne on peduncles 2 – 9 cm long that are produced in the upper leaf axils. The individual flowers are sessile and their white petals (about 5 mm long) are partly fused into a corolla tube. They separate into 2 lobes near the top. The lower of the lobes is sac-shaped and bent sharply downwards, while the upper lobe usually has some faint purplish spots. Each flower is surrounded by a calyx tube with 5 lobes, which is made up of the fused sepals (3 – 4 mm long). Flowering usually occurs during late autumn and early winter.
After the flowers die, the calyces become enlarged (up to 1 cm long), and eventually turn brown. Inside each of the calyx tubes is produced a small 4-lobed schizocarp, which divides into 4 small mericarps. These are dark brown to black, 1 – 2 mm across, with 2 distinct white markings at one end. They are relatively smooth in texture, and subglobular in shape. The brown seed-heads persist on the plant.
The seed-heads readily adhere to animals, clothing and vehicles, thus spreading the seeds, which may also be dispersed by water or agricultural machinery, or in contaminated agricultural produce. The long-lived rootstock can also be spread by cultivation practices.
This is regarded as an environmental weed in Queensland, and as a potential one in the Northern Territory. It can form dense thickets and dominate or replace the natural shrub or herb layer of more open plant communities, although it is regarded as invading relatively slowly.
The larvae of the Perilla Leaf Moth Pyrausta panopealis feed on this plant.
Medical use is made of the plant in some countries:
- Philippines: a decoction of the leaves is used to clean wounds, and of the roots to stimulate menstruation;
- Mindanao: for dry coughs and toothache, gas pains in infants and convulsions in children;
- Malaysia: for stomach ache: the young leaves are pounded into a paste and applied to the affected areas;
- Martinique: used as a tonic;
- Antilles: used as a stimulant;
- Costa Rica: for toothache;
- Bangladesh: leaf juice is taken for malaria; a root and leaf paste is applied to cuts and abrasions to prevent infection.
Information about medicinal qualities of plants, or about their use as medicines, is for interest only, and is not intended to be used as a guide for the treatment of medical conditions.
Photographs taken in Horseshoe Bay 2013, 2014
Page last updated 15th December 2016