Hardheads

Common name:

• Hardheads

Scientific name:

• Rhaponticum repens (L.) Hildalgo

Other scientific name:

• Acroptilon repens (L.) DC.

Other common names:

• Hardhead thistle
• Creeping knapweed
• Russian knapweed

Plant status

Catchment management authority boundaries

Regionally prohibited in the Glenelg Hopkins, Corangamite, North East, Port Phillip and Western Port catchments.

Regionally controlled in the Mallee, Wimmera, North Central, and Goulburn Broken catchments.

Restricted in the West Gippsland and East Gippsland catchments.

Plant biology

Appearance

Herbaceous plant —  Forb (flowering herbaceous plant — not a grass)

Description

Hardheads are erect perennial herbs growing to a height of 30 to 90cm (mostly 45cm). Plants leave a bitter tasting substance on hands when handled.

Stems

Stems of Hardheads are erect from the centre of the rosette, stiff, branched and with a slightly woolly covering of soft grey hairs.

They are dark-brown to black underground and at the base of the plant, while silvery-grey to dull grey-green above ground.

Leaves

Leaves of hardheads are greyish or silvery-green with short hairs.

The rosette leaves grow up to 15cm long, 2-5cm wide and are lance-shaped and toothed with stalks present.

They have irregular-shaped lobes and are sparsely covered with glandular hairs. Stalks are covered in a fine down.

Stem leaves alternate along the stem and are 5 to 7cm long.

Lower stem leaves have no stalks and are slightly dissected. Upper stem leaves are progressively smaller and not divided or lobed.

Flowers

Hardheads flowers appear in heads up to 2.5cm in diameter when open. Heads are in solitary clusters on the ends of short, leafy branches.

Florets are purple, pink or occasionally white and tubular. They are surrounded by broad, thin, spineless white or pale-yellowish bracts, with a thin papery tip.

Fruit

Hardheads bear no fruit.

Seeds

Seeds are whitish or ivory and sometimes mottled. They grow 3 to 4mm long and 2 to 3mm wide and are wedge-shaped and often slightly curved.

Seeds have a pappus of numerous, stiff, barbed, white hairs which are loosely attached.

Seed heads remain tightly closed and most seed is retained within the head. Seed can remain viable for a number of years under dry conditions.

Growth and lifecycle

Method of reproduction and dispersal

Hardheads are spread mainly by the movement of root fragments through cultivation.

Segments as short as 2.5cm moved by cultivation equipment, are capable of establishing new plants wherever it is deposited.

Grazing stock may also spread the weed in paddocks by pulling stems from the ground, with the attached root segments capable of growing.

Hardheads reproduce by creeping roots and seed. Seeds may be dispersed by water (particularly along channels), contaminated farm machinery and in fodder, seed and grain.

Grazing animals are also important in dispersal as seeds can pass through the digestive tract of livestock in a viable condition and also adhere to their coats.

Seeds are not dispersed by wind.

The seed of Hardheads is largely retained in the flower head. It is relatively heavy and has a weakly attached pappus.

Rate of growth and spread

Plants usually flower in their second year of growth in late spring and summer, with aerial growth dying off again in autumn.

The weed's root system extends both laterally and vertically and the top growth dies off in autumn. Buds on the roots develop into new aerial shoots in spring and summer. The longevity of the root system enables patches of hardheads to survive adverse conditions for many years.

Seedbank propagule persistence

Seeds of Hardheads can remain viable for a number of years under dry conditions.

Preferred habitat

Hardheads prefer semi-arid to subhumid temperate regions in the 300-600 mm annual rainfall belt.

They can tolerate all soil types and drought conditions for many years because of the longevity of their extensive root system.

Growth calendar

The icons on the following table represent the times of year for flowering, seeding, germination, the dormancy period of Hardheads and also the optimum time for treatment.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Flowering
Seeding
Germination
Dormancy
Treatment

Impact

Agricultural and economic impacts

Hardheads are one of the most competitive of all weeds. They can virtually exclude all other vegetation and reduce yield in dry land cropping by as much as 80 per cent.

Management

Prescribed measures for the control of noxious weeds:

• application of a registered herbicide
• physical removal.

Read about prescribed measures for the control of noxious weeds.

Other management techniques

Changes in land use practices and spread prevention may also support hardheads management after implementing the prescribed measures.