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Knobweed (Hyptis capitata)

Knobweed is an introduced weed in northern Australia that reduces grazing area because stock avoid its bitter, strongly scented foliage. It persists in disturbed and degraded areas, where it can out-compete pasture plants and invade native vegetation.

Key facts

Type: Weed, herb.

Native to: Tropical Central America, from southern Mexico to Panama.

Established in: QLD, NT.

Other names: Knob Weed.

Best time to act: Find and treat plants before flowering; flower-heads form around late autumn and seeds mature from about mid-winter.

Identification

  • An aromatic perennial herb or sub-shrub with several upright, branching stems, growing up to 2.5 m tall.
  • The stems are square in cross-section, with bright green, sparsely hairy leaves arranged in opposite pairs.
  • Leaves are usually egg-shaped to narrowly diamond-shaped, with toothed edges and numerous oil glands on the underside.
  • Compact, spherical flower-heads contain many small white tubular flowers, with violet dots on the upper lip.
  • The round heads enlarge as fruit develops; small dark brown to black seeds rattle inside the persistent capsules when shaken.
  • Look-alike: Hyptis suaveolens has a strong minty smell and pinkish or lavender-blue flowers in loose clusters in the leaf axils, rather than Knobweed’s compact globular heads.
  • Look-alike: Lesser Roundweed (Hyptis brevipes) is very similar, but has smaller flower-heads and shorter flower stalks.

Where it grows

In Australia, Knobweed occurs mainly in north-east Queensland, especially the wet tropical lowlands and foothills from Cape York to near Mackay, and has also been recorded in Kakadu National Park in the Northern Territory.

Why it matters

  • Stock avoid Knobweed because of its peculiar odour and bitter taste, reducing the area available for grazing.
  • Preferential grazing of palatable pasture plants can encourage Knobweed to spread, particularly in low-nitrogen or overgrazed pastures.
  • Its rootstock allows it to resprout, and it can persist in degraded pastures and after slashing or mowing.
  • It can invade disturbed openings in open eucalypt forest and rainforest, and its seeds have been found contaminating Siratro and stylo seed.

How it spreads

  • Knobweed spreads mainly by seed held in a rigid, persistent fruiting calyx whose stiff tips readily attach to wool, fur, clothing and other fibrous materials.
  • Seeds can also stick to mud and be carried by farm machinery, vehicles and water.
  • Rootstocks disturbed during cultivation can establish in previously clean areas.

Control options

  • Manually remove new plants or small groups before flowering, cutting the root well below the crown to prevent regrowth from the rootstock, and burn the removed material.
  • Slash larger colonies before flowering, then treat the young regrowth with registered herbicides before it flowers.
  • In good pasture, follow treatment with nitrogen fertiliser to encourage pasture growth that can out-compete Knobweed.
  • Degraded pasture may need resowing; after slashing, cultivation, spraying regrowth on the fallow, fertilising and sowing pasture suited to the district are described as management steps.

Hygiene and washdown

  • Check and clean machinery and vehicles after working in infested areas, as seeds can adhere to them and to mud.
  • Check clothing, boots and other gear for seeds that may cling to fibrous materials.
  • 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)
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources