Kangaroo Island
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Horehound (Marrubium vulgare)

Horehound (Marrubium vulgare)

Medium Risk Weed for Kangaroo Island

Horehound is an Alert Weed for Kangaroo Island. Notify the Kangaroo Island Landscape Board. Early detection and control are the best way to prevent the next widespread weed. 

It is a declared plant for South Australia. 

The Situation and What to do

Horehound is a perennial herb native to Europe, North Africa and parts of Asia that was introduced to Australia in the 1800’s as a medicinal plant. It has since become widely naturalised and is now a significant pest plant of pastures, rangelands, roadsides and disturbed areas across South Australia. 

Horehound is a declared pest plant in South Australia under the Landscape South Australia Act 2019 

Horehound is commonly found on non-arable land such as roadsides, disturbed areas, waste land, open disturbed bushland and grazing country. It thrives on dry, degraded alkaline soils and often invades overgrazed pastures where competition from other plants is low.

On Kangaroo Island horehound has a scattered distribution and is locally abundant in the Hundreds of Menzies and the Dudley. The Kangaroo Island landscape board has had a long running control program that focused on reducing the movement of horehound along roadsides. 

You can help stop the spread of horehound by:

  • Controlling horehound on your property.
  • Practicing good hygiene.
  • Maintain healthy pasture.
  • Not overstocking your land.
  • Ask for a vendor declaration when purchasing stock and fodder.

Preventing the spread of horehound is the best control measure. Be aware of the risks of purchasing stock, fodder and machinery from contaminated areas. 

Description

  • Growth form: Bushy, many-branched perennial herb 30–80 cm tall with square stems densely covered in fine white hairs.
  • Leaves: Opposite, oval to round, wrinkled and hairy, grey-green, 2–5 cm long with scalloped edges.
  • Flowers: Small white flowers in dense clusters in leaf axils, flowering from late spring to summer.
  • Fruit: Burr-like calyx with hooked teeth that attach to wool, fur, clothing, and machinery.
  • Seed: Each burr contains several small brown seeds. A single plant can produce up to 20,000 seeds per year, and seeds remain viable in the soil for 7–10 years, allowing infestations to persist and re-establish.

Impacts

Horehound is unpalatable to livestock, allowing it to dominate degraded pastures and reduce productivity. Its burrs can contaminate wool, clothing, pets, and vehicles, creating extra work. 

Large unmanaged infestations can reduce visual amenity and restrict access to areas. It can also invade native grasslands and disturbed bushland, competing with native species and reducing biodiversity. Well established infestations can take many years to get under control and is very costly. 

Control options

Early identification and control before fruiting are key to success, along with good hygiene practices, avoid moving contaminated stock, fodder, soil and machinery from infested areas. 

Control requires an integrated approach. Small infestations can be removed by hand, ensuring the root crown is taken. Larger infestations in bare areas or broken vegetation can be treated with selective herbicides. In pastures it is best to use a selective herbicide to main healthy, competitive pastures. Grazing can supress growth of young plants but can cause spreading to new areas in mature infestation and over grazing can create less competition from pasture.

Biological control using the horehound plume moth (Wheeleria spilodactylus) can reduce flowering and seed production but does not kill the plant. 

Follow-up monitoring over many seasons is needed due to long-lived seeds.

Permitted and on label herbicides and rates can be found on the PIRSA Weed Control Website.

Declarations 

Permitted and on-label herbicides and rates can be found on the PIRSA Weed Control Website:

  • 186(2) Prohibiting movement on public roads
  • 188(1) Prohibiting sale of the plant
  • 188(2) Prohibiting sale of contaminated goods
  • 192(2) Landowners to control the plant on their properties
  • 194 Recovery of control costs on adjoining road reserves

Further Information

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