Quick Facts

Quick facts

• Knobweed (Hyptis capitata) is an upright, aromatic, perennial herb or sub-shrub growing up to 2.5 m high with globular flower-heads comprised of numerous small white tubular flowers.
• A native of Central America, it is now a widespread weed of tropical regions of the world.
• It is spread mainly by seed dispersed by animals, humans, machinery and vehicles (when attached to fur, clothing and mud), as well as by water.
• Knobweed occurs mainly in disturbed areas and is a weed of pastures, cultivated areas, roadsides, waste places, creekbanks and native vegetation.
• It is avoided by stock and reduces grazing area by out-competing palatable pasture species, particularly in disturbed conditions.
• Plant material can be manually removed and burnt; larger infestations can be slashed, chemically treated and where appropriate cultivated.

What Does It Look Like?

What is it?

Knobweed (Hyptis capitata) is an aromatic long-lived (perennial) herb or sub-shrub, with several upright branching stems growing up to 2.5 m high that arise from a short, robust rootstock. The stems are square in cross section. Bright green leaves are arranged in opposite pairs that are widely spaced along the stem. The leaf blade is usually ovate (egg-shaped) to narrowly ovate or narrowly rhombic (diamond shaped), up to 15 cm long and 7.5 cm wide, sparsely hairy, with irregularly toothed to serrated margins and dotted with numerous oil glands on the undersurface. The leaf stalk is up to 3 cm, rarely up to 6 cm, long. Spherical flower-heads (about 10-15 mm in diameter, increasing up to 25 mm when in fruit) occur on the stalks that are generally up to 6 cm long, or sometimes up to 10 cm long, and borne in the forks of the leaves (resulting in pairs of leaves and corresponding flower-heads along the stems). The compact ball-shaped heads are composed of many small, white, tubular, 2-lipped flowers with the upper lip violet-dotted. The fruit is a 4-lobed capsule dividing into 4 seed-like nutlets. The seeds are small and 1-2 mm diameter, dark brown to black, and rattle in the capsules when shaken. The seeds are held inside the rigid, persistent fruiting calyx (Backer & Bakhuizen van den Brink Jr 1965; Cardenas et al. 1972; Kleinschmidt & Johnson 1977; Keng 1978; Henty & Pritchard 1982; Friend 1983; Barker 1993; Flanagan 1998; Parsons & Cuthbertson 2001; PIER 2006).

For further information and assistance with identification of Knobweed, contact the herbarium in your state or territory.

Flower colour

White

Growth form (weed type/habit)

Shrub, Herb

Where it currently grows? Preferred habitat

Knobweed occurs in the humid tropics and subtropics, in high rainfall areas, growing best in disturbed areas on heavy soils retaining moisture. It is a weed of roadsides, waste places, creek banks, ditches, headlands, cultivations and degraded pastures, especially in damp low-lying areas. It is also found in forest clearings, along margins and tracks of rainforests and in open forests where soils may be sandy and well drained (Kleinschmidt & Johnson 1977; Friend 1983; Flanagan 1998; Parsons & Cuthbertson 2001; Smith 2002; National Herbarium of New South Wales 2008).

Are there similar species?

Hyptis (Hyptis suaveolens) is a widespread weed across northern Australia, which is distinguished by its strong minty smell and its pinkish or lavender-blue flowers that occur in loose clusters in the leaf axils, rather than in compact globular heads like Knobweed (Flanagan 1998; DEWHA undated).

Comb Bushmint, also known as Comb Hyptis, Mint Weed, Purple Top or Wild Mint (Hyptis pectinata) and Marubio, also known as American Bushmint or Black-sesame (Hyptis spicigera), are rare tropical weeds introduced in Australia. They differ from Knobweed in having their flowers arranged in clusters along the tops of the stems or in dense spikes, respectively (Kodela 2008, pers. comm.).

Lesser Roundweed (Hyptis brevipes), which is not known to occur in Australia presently, could potentially become a weed of tropical areas if introduced, is very similar in appearance to Knobweed, but has smaller flower-heads (up to 14 mm in diameter at fruiting) on shorter stalks (up to 1.5 cm long in Lesser Roundweed versus up to 10 cm long in Knobweed) (Backer & Bakhuizen van den Brink Jr 1965; Waterhouse & Mitchell 1998).

Why Is It A Weed?

What are its impacts?

Knobweed is generally a problem in pastures that are low in nitrogen or are overgrazed (Flanagan 1998). It has a peculiar odour and bitter taste which making it unattractive to stock and thus reduces available grazing area. This results in the more palatable species being grazed preferentially, which encourages the spread of Knobweed. It is particularly persistent in degraded pastures, since the large reserves in the rootstock permit the weed to resprout faster than pasture species, even if damaged, which also reduces pasture competitiveness (Parsons & Cuthbertson 2001).

Knobweed invades cultivated areas and is a common weed of other disturbed areas such as roadsides and creek banks (Friend 1983; Parsons & Cuthbertson 2001).

Once established, Knobweed's resilience to slashing and mowing and its prolific seeding ensure that it will remain troublesome (Flanagan 1998). Because it produces large quantities of seeds, when areas are cleared to establish pasture or other uses, Knobweed infestations may arise in these disturbed areas (Parsons & Cuthbertson 2001).

Knobweed seed has been found as a contaminant in seed of Siratro and stylo (Friend 1983).

Impacts on biodiversity in native vegetation can occur where Knotweed has invaded open eucalypt forest and rainforests. In these situations, Knobweed usually invades in openings, e.g., along tracks and edges of forest or after disturbance (Smith 2002; Kodela 2008 pers. comm.)

How does it spread?

Knobweed reproduces and is spread mostly by seed. Seed is held inside the persistent, rigid fruiting calyx, which is well-adapted for dispersal by animals and humans as their stiff tips readily adhere to wool, fur, clothing and other fibrous materials, as well as sticking to mud and adhering to farm machinery like slashers and other vehicles. Seed is also spread by water, as indicated by creek line colonies of the weed. Local spread can occur when rootstocks are disturbed, e.g., during cultivation and they establish in previously clean areas (Parsons & Cuthbertson 2001; Smith 2002).

What is its history in Australia?

Knobweed has spread to most of the tropical world including east and south-east Asia, principally as a result of commercial activities. In Australia, it was first recorded on the South Johnstone River near Innisfail, Queensland, in 1937, possibly introduced as a contaminant in agricultural seeds. On the wet tropical coast of northern Queensland, it spread north and south. In the Northern Territory it was discovered in Kakadu National Park in 1997, where it is now subject to an eradication program (Flanagan 1998; Parsons & Cuthbertson 2001).

How To Manage It?

Best practice management

New infestations of single plants or small groups can be manually removed. They should be grubbed before flowering and burnt, taking care to cut the root well below the crown (where stem and root join) to prevent re-establishment from the perennial rootstock (Parsons & Cuthbertson 2001).

Larger colonies on good pasture should be slashed before flowering and the young regrowth treated with herbicides, followed with nitrogen fertiliser to promote rapid pasture growth that can then out-compete Knobweed. Degraded pastures will require resowing. After slashing Knobweed, the area can be cultivated ready for sowing. Weed regrowth can be sprayed on the fallow, followed by the application of fertiliser and sowing of a pasture suited to the district. Chemical spraying should be carried out before flowering (Parsons & Cuthbertson 2001).

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au 

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Seeds germinate at almost any time of the year if sufficient moisture is available, with summer the main germination period in north Queensland. Seedlings establish rapidly and growth is continuous, although it occurs at a slower rate in winter. Flower-heads are formed around late autumn and the seeds mature from about mid-winter. Seedlings that germinated in late summer may not flower in the first year of growth. New shoots form on the rootstock in the succeeding/next wet season (Parsons & Cuthbertson 2001).

Where Is It Found?

Which states and territories is it found?

NT, QLD

What areas within states and territories is it found?

In Australia, Knobweed predominantly occurs in north-east Queensland from near the tip of Cape York Peninsula south to near Mackay and with a concentration of infestations on the tropical wet lowlands and foothills of the Ingham-Cairns region. It has also been recorded from Kakadu National Park, Northern Territory (Flanagan 1998; AVH 2008).

Where does it originate?

Knobweed is native to tropical Central America, from southern Mexico to Panama but is now widespread as a weed (Holm et al. 1979; Parsons & Cuthbertson 2001; PIER 2006).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

NT, WA

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Hyptis capitata

Other scientific names (synonyms)?

Hyptis rhomboidea Mart. & Gal. (misapplied by Henderson, R.J.F. 2002, Names and Distribution of Queensland Plants, Algae and Lichens. 95.)

Does it have other known common name(s)?

Knob Weed