Horehound (Marrubium vulgare)

Photo: (c) H. Zell GNU license

Family: Lamiaceae

Growth Habit: Horehound is a bushy spreading plant that grows to 75cm in height. The base of the stems is woody. The leaves are grey-green, with long hairs on the upper and lower surface and wavy edges. The veins are deeply impressed above and prominent below.  giving it a crinkled appearance.

The mature plant is erect and bushy in habit with stems which branch from the base and along their length and become woody.  The top growth dies back, partially or completely, over winter but dead stems may persist for a considerable time.

Photo: (c) Stan Shebs GNU License

Flowers: Fairly insignificant white flowers arranged in rings around the upper part of the stems.

Fruit/Seed: Flowers die off leaving a spiny burr. Each burr contains 4 small brown or black seeds.

Dispersal: Horehound was sold in Tasmanian nurseries as a garden or medicinal herb from as early as 1845. The plant now spreads when the burrs attach themselves to passing animals in fur and wool and on to peoples clothing. It is spread by water along drainage lines and creeks.

Distribution: This plant is found throughout Tasmania often around old farm building. It is most troublesome in the Midlands grazing areas. It is drought tolerant and is able to quickly increase its range when hot dry conditions limit the growth of other plants. It does best in alkaline soils.

Status: Horehound is a Declared Secondary Weed in Tasmania. It is a declared Noxious Weed in Victoria, South Australia parts of New South Wales and Western Australia. The importation, sale and transportation of the weed is therefore illegal. Containment to properties is the minimum legal requirement in the Tamar under the Horehound Weed Management Plan.

Weed Impact:

  • Horehound is very bitter. Grazing animals tend to concentrate on other plants in the paddock cutting down the competition and leaving Horehound to spread.
  • If animals are forced to eat Horehound, their meat has a strong offensive smell and flavour.
  • The most significant effect of the weed is that the burrs attach themselves strongly to wool and are difficult to remove. The sale price of the wool will then be down graded.
  • Because the plant is drought tolerant, it can use the occasional drought to suddenly increase its range.
  • Horehound is also an important environmental weed due to its ability to invade disturbed native vegetation.
  • Historically, Horehound was used for a number of purposes in the early days in Tasmania. These included its use as a herbal medicine for humans and stock and as a bitter flavouring for beer making. Horehound honey was highly prized.

For further information on Horehound contact DPIPWE

Control Methods:

  • Wash down machinery and equipment between sites to reduce chance of weed seed spread.
  • Small infestations can be controlled by grubbing plants.
  • Dense infestations can be controlled through cultivation, where appropriate. The area should be burnt (to stimulate seed germination) and then ploughed to bury the plants. Alternatively un-rooted plants can be removed completely as partially buried plants can continue to grow.  Cultivation is best done in summer when uprooted plants will be killed by the heat. Check if you need a permit to burn. Repeated cultivation is necessary to up-root any new horehound growth, followed by sowing to crop or pasture.
  • A number of herbicides are registered for use on horehounds in Tasmania. See Herbicides for Horehound Control for more information.
  • For best results, apply herbicides when the plants are actively growing (usually spring and autumn). Poor results will result if horehound plants are suffering moisture stress at the time of application.
  • Spring spraying will reduce the formation of flowers and seed, reducing the soil seed bank and fleece contamination.
  • Horehound shows herbicide symptoms relatively slowly; it can take between 6 and 20 weeks for symptoms to develop.
  • All herbicides require the addition of a surfactant as the hairy leaf of the plant is difficult to wet.

For further information visit the DPIPWE website

N.B. Always check the herbicide label before use.