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Horehound (Marrubium vulgare)

Horehound is a widespread, naturalised weed in Australia, especially troublesome in alkaline soils and degraded or overgrazed pasture. Stock generally avoid it, but may eat it when other feed is scarce, tainting meat or milk.

Key facts

Type: Weed, shrub.

Native to: Europe, where it is native across almost the whole continent..

Established in: QLD, NSW, ACT, VIC, TAS, SA.

Other names: Common horehound, White horehound.

Best time to act: Look for flowering plants from July to December and treat plants before they set seed.

Identification

  • A soft-wooded, many-branched plant growing to about 60 cm high, often branching from near the base.
  • Branches and the lower leaf surfaces are grey and woolly-haired.
  • Leaves are roughly rhombic-ovate to rounded, about 1–3.5 cm long, with toothed edges and short to long stalks.
  • Small white flowers grow in dense clusters around the stems from July to December.
  • The calyx has narrow, hooked teeth, and the small fruits are enclosed within it.
  • Look-alike: Other grey, hairy mint-family plants may look similar; check for horehound’s white flowers in dense clusters around the stem and its hooked calyx teeth.

Where it grows

In Australia it grows particularly on alkaline soils, in degraded or overgrazed pasture, and on bare, eroded ground.

Why it matters

  • It is a widespread and troublesome weed in pasture and other disturbed ground.
  • Stock generally avoid grazing horehound, but may browse it when alternative feed is unavailable.
  • Animals that eat it can taint meat or milk.

How it spreads

  • Seeds are enclosed in the persistent, hooked calyx, which can catch on wool, animal fur, clothing and equipment.
  • Seed-bearing plants and contaminated soil or fodder can carry horehound to new sites.

Control options

  • For small infestations, hand-pull or dig plants, removing the crown and roots; collect and dispose of seed-bearing material appropriately.
  • For larger infestations, use only herbicides registered for horehound and the site, and follow the product label and applicable local requirements.
  • Revisit treated areas and remove regrowth and seedlings before they set seed.
  • Check current advice from the relevant state or territory biosecurity agency, including whether reporting is required.

Hygiene and washdown

  • Clean seeds and plant material from clothing, boots, vehicles and machinery before leaving an infested site, and avoid moving contaminated soil or fodder.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Not a priority weed. Not a state or regional priority weed in NSW. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Regionally controlled / restricted weed (varies by region). Where it is regionally controlled, landowners must take all reasonable steps to prevent its growth and spread on their land. Where it is a restricted weed, trade in it (including the plant, its seed or contaminated material) is prohibited.
  • South Australia: Declared plant. Depending on the declaration, it can be an offence to move it on public roads or sell it, and landowners may be required to control it on their land.
  • Western Australia: Permitted, not declared. Listed as a permitted organism in WA, so it can be brought in, but it is not a declared pest.
  • Tasmania: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • Northern Territory: Not declared. Not on the NT A to Z weed list, and Weeds Australia does not list it as declared in the NT. Check the NT declared weeds list before moving or planting it.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Australian Capital Territory: Biosecurity Act 2023 (ACT), administered by ACT Biosecurity
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania
  • South Australia: Landscape South Australia Act 2019 (SA), administered by the regional landscape board

Sources