Common and Scientific Names
Marrubium vulgare L.
Origin and Distribution
Horehound is native to temperate Eurasia, Europe, the Middle East, and the Mediterranean region including North Africa. It is widespread throughout Victoria and is common on sheep camps and in waste places.
A bushy perennial plant, 30 to 80 cm high, sharply aromatic when crushed, covered with dense whitish hairs.
Stems - Four-sided, up to 60 cm high, stout, branched, upright to trailing, densely hairy, whitish, and woody at the base.
Leaves - Opposite, broadly oval to round, 1 to 3 cm diameter, wrinkled, the margins irregularly lobed, the upper surface bluish-green, the lower surface white-woolly, the stalks at least half as long as the blades. Veins are sunken on upper surface and prominent underneath.
Flowers - White, 6 to 10 mm long, arranged densely around the stems in the leaf axils. Flowering occurs mainly in spring, sometimes through to autumn (September to March).
Seeds - Brown or black, ovoid or triangular, slightly roughened, 1 to 2 mm long, up to 4 per capsule.
Roots - Branched woody taproot or rootstock with numerous fibrous lateral roots.
Horehound is a weed of pastures and crops in southern Australia where it thrives on poor soil and in waste places. It invades poor pastures which provide little competition. Horehound contains a bitter alkaloid which makes it unpalatable for grazing livestock. Horehound burrs contaminate wool, reducing the value of the fleece. The meat of animals which are forced to eat horehound is tainted by the plant's strong flavour, and it takes about 7 grazing days on clean pasture for animals to lose this.
There are reports of horehound causing stomach impaction in sheep and acting as an alternative host to pest insect species. As well as being an agricultural weed of pastures horehound has become an important environmental weed because of its ability to invade disturbed native vegetation.
Seeds are primarily dispersed by stock, as the fruit or burr readily attaches to wool, fur, clothing and similar materials. Water is also an effective dispersal agent, and horses are known to pass the seeds, after ingestion, in a viable condition.
Prescribed measures for the control of noxious weeds
- Application of a registered herbicide
- Physical removal
Important information about prescribed measures for the control of noxious weeds.
Other management techniques
Changes in land use practices and spread prevention may also support horehound management after implementing the prescribed measures above.
A biological control program commenced in 1990. The horehound plume moth, Pterophorus spilodactylus, was first released in 1994 and is now established at a number of localities in Victoria. It is specific to horehound and the caterpillar (larva) feeds on the growing tips of the plants and then work their way down the shoot, progressively defoliating the stem. This weakens the plant and reduces the number of seeds and flowers produced.
The horehound clearwing moth, Chamaesphecia mysiniformis, was first released in March 1997. Larvae feed on the growing tissue inside the roots and stems. This affects the flow of water and nutrients through the plant, weakens it, reduces growth and increases the likelihood of the plant dying. Additional biological control agents are under investigation.
The brightly coloured orange and black horehound bug, Agonoscelis rutila, a native insect often seen on horehound in great numbers, does not provide any worthwhile control.
Biological control is a long term program which is best used on large, chronic infestations with a low priority for control due to inaccessibility, remoteness or low threat of spread.
- Contact your local landcare or friends group for further assistance and advice.
- Call the DEPI Customer Service Centre on 136 186.
- Weeds Australia (external link)
Parsons, W.T. and Cuthbertson, E.G. (1992) Noxious Weeds of Australia. Melbourne, Inkata Press.