Cotton Thistle (Onopordum acanthium)

Family: Asteraceae

Growth Habit: Cotton Thistle, a plant commonly growing to about 1.5 metres in height, gets its name from the characteristic greyish white colour of the foliage. This is most noticeable in mature plants and those reaching maturity.

In the cotyledon stage, seedlings closely resemble slender and spear thistles in size and shape. In early rosettes, the leaves are oval in outline, shallowly lobed, and light green with a light covering of hair which gives them a greyish tinge and a velvety appearance. Rosettes may grow up to 800 mm or more in diameter with leaves up to 400 mm long and 150 mm wide. During this development, leaves grow longer in proportion to their width and become more deeply lobed. With increasing density of leaf hairs the foliage progressively becomes greyer in colour. Stem growth normally begins in early October. The stems, which are branched in the upper part, bear prominent wings throughout their entire length.

Type of Plant: Cotton Thistle is normally biennial, producing a rosette in one season and flowers and seed heads in the next. In Tasmania, however, the life cycle is shortened and is usually completed within a twelve month period.

Photo: (c) H. Zell

Flowers: Flower buds are first formed at the tips of the stems and later at the tips of branches growing out from the axils of the stem leaves, later ones overtopping the earlier ones on the same branch. Flower buds are at first globular in shape with a large number of spiny bracts all pointing outwards.

The main flowering period usually starts in December and continues through to autumn. However, in some seasons, flowering may commence as early as mid-October. Mature flower heads are large (30 to 40 mm in diameter) with purple florets.

Fruit/Seed: The seed, which is brown with grey markings and is about 3 mm long, is large and heavy in relation to the size of the pappus.

Dispersal: Seed spread is believed to be mainly by livestock, particularly sheep, either entangled in the wool or by passage through the digestive system. Seed may be carried by water and may also be spread by birds. In the ground, seeds can remain dormant and viable for a long period. Seed buried below a depth of 75 to 100 mm does not germinate until brought nearer the surface.

Distribution: The principal infestations of cotton thistle in Tasmania are on improved pastures in the lower rainfall areas of the Midlands.

Status: Cotton Thistle is declared a Noxious and Prohibited Weed under the Noxious Weeds Act 1964. Measures must be taken to eradicate all infestations and prevent seed production.

Weed Impact:

  • Cotton Thistle grows best on high nitrogen soils and, therefore, favours clover and lucerne pastures and fertile river flats more than areas of unimproved native vegetation.
  • Cotton Thistle competes well with perennial ryegrass and cannot be controlled by traditional pasture improvement techniques which encourage the establishment of perennial ryegrass-clover pastures to increase soil fertility.
  • Newly sown pastures are often overrun by a high germination of thistle seeds and, later, its relatively large leaves smother other plants; as it survives well into summer, there is little chance of other species recovering.
  • Cotton Thistle is rejected by stock because of the dense spines and, therefore, heavy grazing favours its survival. If it is eaten the spines cause mechanical damage, particularly around the mouths and eyes of animals. It also contributes to vegetable fault in wool.

Control Methods:

  • Grazing. Although most animals reject the plant, goats graze flowering plants in summer and autumn which can be important in reducing seed production. Merino sheep, grazing heavily on seedlings may weaken plants to the extent that, at flowering, seed production may be halved.
  • Grubbing. For single thistles or small patches, grubbing with a hoe or mattock is a simple, cheap and effective control measure, provided that care is taken to sever the root below ground level to ensure that all growing points are removed. Once flowering has started all grubbed material should be collected and burnt to make sure that the most advanced flower heads do not mature and produce seed, even after the stem has been severed.
  • Cultivation such as ploughing is a satisfactory control method provided it is carried out early enough, the land is suitable, and it is done thoroughly. Because of the expense involved, cultivation should be used only in an area intended for cropping, or where pasture needs to be re-sown. A substantial seedling germination can be expected to follow cultivation of a previously infested area: these will have to be controlled by herbicide application to the crop or pasture.
  • Slashing and mowing of Cotton Thistle are too unreliable to be classed as control measures on their own.
  • Prevention of infestation or re-infestation. For any eradication campaign to be successful, two things are important: There should be a complete prevention of seeding every year. Nothing less than 100 per cent kill is acceptable. Great care should be taken to ensure that fresh seed is not introduced into clean areas or into those from which the weed is being eradicated.
  • Farm hygiene – Implements which have been used on infested areas should be cleaned on leaving. All feed grains or hay should be free of Cotton Thistle seed. Any livestock suspected of carrying seed on their bodies or in their digestive system should be held in a suitable area for approximately two weeks before being put on clean paddocks. Special care should be exercised when buying sheep from other properties as seed is readily carried in wool.
  • Chemical Control – In most situations chemical spraying has been found to be the most satisfactory control method. Cotton Thistle is often referred to as Scotch Thistle on herbicide labels. For specific information regarding recommended herbicides consult the Department of Primary Industry, Parks, Water and Environment.

N.B. Always check the herbicide label before use.