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Mint weed (Mesosphaerum suaveolens)

This introduced weed forms dense thickets that can make infested pasture unproductive and compete with grain and peanut crops. It also invades woodlands, open forests and watercourses across parts of northern Australia.

Key facts

Type: Weed, herb.

Native to: Tropical Central and South America.

Established in: QLD, WA, NT.

Other names: Hyptis, Horehound, Wide spikenard, Chinese mint.

Best time to act: In northern Australia, look for germination mainly in October and November after the opening rains, and treat before flowering, which occurs from about February to May.

Identification

  • An erect, strongly aromatic annual or short-lived perennial herb, usually about 1.5 m tall but sometimes reaching 3 m.
  • Its green or reddish-green stems are very hairy and square in cross-section.
  • The opposite, hairy leaves are usually oval to egg-shaped, about 2 to 10 cm long and 1 to 7 cm wide, with shallow teeth along the edges.
  • Pinkish or lavender-blue flowers, about 5 to 7 mm long, grow singly or in clusters of up to five in the upper leaf axils.
  • After flowering, the brown tubular calyx persists, with each of its five lobes topped by a bristle about 5 mm long; its dark brown to black, flattened, shield-shaped nutlets are about 3 to 4 mm long.
  • Look-alike: Hyptis capitata can look similar, but it has white flowers in globular clusters and hairless or sparsely hairy leaves.

Where it grows

In Australia it occurs across much of northern Western Australia, the northern Northern Territory as far south as Barrow Creek, much of northern Queensland and south-east Queensland, including the Wide Bay region, often on fertile coarse-textured soils, roadsides, overgrazed pastures, around stockyards and watering holes, and along floodplain margins.

Why it matters

  • Hyptis is unpalatable to stock and can form dense thickets in overgrazed areas, making them unproductive.
  • It can be a serious competitor in grain and peanut crops.
  • It invades woodlands, open forests and watercourses, forming dense thickets on floodplain margins.

How it spreads

  • It spreads by seed, which usually remains in the bristly fruit and readily attaches to animals and clothing.
  • Water, machinery and vehicles also spread it, and it is a common contaminant of hay.

Control options

  • Remove small infestations by hand before they set seed.
  • In pasture, light to moderate grazing with perennial climbing legumes such as Calopo can provide effective control; mowing or slashing gives only temporary relief.
  • Where appropriate, use registered herbicides, with spraying best applied before flowering begins.
  • An early application in November followed by another in February can help kill late-germinated seedlings.

Hygiene and washdown

  • Check clothing for bristly fruits and clean vehicles and machinery to reduce the risk of spreading seed.
  • Take care when moving hay, as it is a common contaminant.
  • 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: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • 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: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Declared weed, Class B. Class B: growth and spread must be controlled. All declared weeds are also Class C, so they must not be brought into the NT.
  • 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)
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources