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| Cotton Thistle (Onopordum acanthium L.) Control GuideRevised 11/02
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 Europe, where it is a native, cotton thistle is normally a 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.
Germination may occur from late summer to early spring with the bulk of the seedlings appearing during the autumn and winter. 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. 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.
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.
The principal infestations of cotton thistle in Tasmania are on improved pastures in the lower rainfall areas of the Midlands.
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.
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. Because of this, it often establishes where pasture improvement has been attempted and in cropping paddocks that have been previously under improved pasture.
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.
All Onopordum thistles are declared weeds in Tasmania, largely due to actual and potential impacts on agricultural productivity. As such, importation, sale and distribution of these plants are prohibited. The legal responsibilities of landholders and other stakeholders for these declared weeds are specified in a statutory weed management plan available from DPIW.
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
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
Slashing and mowing of cotton thistle are too unreliable to be classed as control measures on their own. In a dry hot summer, a high percentage kill can be achieved. However, if the soil is moist or if rains follow treatment, many thistles will recover and produce seed. Slashing or mowing may be considered as methods of handling an infestation which has passed the flower bud stage without being treated. Neither operation should be delayed for more than a few days after the first flowers open or some of the several heads will continue to mature and produce viable seed. If regrowth occurs from the base of the plants, this can be treated successfully if sprayed with dicamba before the buds reach about 10 mm diameter. This usually occurs two to four weeks after slashing or mowing. Boom or spot spraying should be used as recommended in the attached table.
Since 1987 scientists at CSIRO have been involved in searching for insects that can be introduced to specifically control cotton thistles.
The seed weevil, Larinus latus, released in 1992 and the stem-boring weevil, Lixus cardui, released in 1993 have been confirmed as established in the field. A large scale redistribution program for these two species has resulted in over 30 releases of the former and over 180 releases of the latter. Both insects are increasing in population and spreading from early release sites and evaluation of their impact has commenced. At present there are no release sites in Tasmania.
Work is progressing on other possible biological control agents for cotton thistles with host specificity studies currently being carried out. Possible agents include the adults and larvae of the plant hopper (Tettigometra sulfurea); the larvae of the weevil Trichosirocalus horridus and the larva of the fly Botanophila spinosa.
In most situations chemical spraying has been found to be the most satisfactory control method. The most used herbicides are dicamba, MCPA and clopyralid.
Dicamba
When used as recommended, dicamba gives a complete kill of cotton thistle, even up to quite an advanced stage of development. A disadvantage is that at the minimum required dose rate it damages or kills clover and lucerne and may damage seedling grasses.
Dicamba can be used in cereal crops. In order to cover the range of weeds commonly found in cereals it is normally used in a mixture with MCPA.
Dicamba is a residual herbicide which may impact on following crops. For more detailed information refer to the product label.
MCPA
MCPA is a more selective herbicide which at recommended rates is tolerated by established white clovers and to some extent by subterranean clover and young white clover. It cannot be relied upon to kill cotton thistles after they have reached the small rosette stage (i.e., up to 150 mm diameter). Dicamba and MCPA can be used for either boom or spot spraying.
Clopyralid
Clopyralid is a selective herbicide active on numerous thistle species including cotton thistle. This herbicide can be used alone or in combination with MCPA to target other broadleaf weeds. Clopyralid does damage clover which needs to be considered when spraying in pasture situations. It is also a residual herbicide which may affect the planting of following crops. For specific information it will be necessary to refer to plant back guidelines on the product label.
Diquat
Diquat plus wetting agent can be used but it is a non selective herbicide quickly killing green growth with which it comes into contact.
Both boom and spot spraying give satisfactory kills, the choice depending on the crop or the age and composition of the pasture, the terrain and the extent and density of the weed infestation.
Boom Spraying
Boom spraying is usually the cheapest method for dense or extensive infestations and on land where this method of application is practicable. Depending on seedling germination, treatment can be carried out in autumn or spring. Recommendations covering the material and rate appropriate for the growth stage of the thistle are given in the attached table.
Spot Spraying
Spot spraying using a knapsack sprayer, power operated spray gun or hand held boom can be employed in virtually any situation. It is particularly recommended for infestations of scattered plants, for small patches and for any follow-up work which may be necessary in areas where boom spraying carried out earlier in the season has not given a complete kill or where individual plants have been missed.
Wiping
An alternative to spray application is the use of a wiper. Wipers may be small hand-held units, or larger frame units that can be mounted on or towed by three and four wheel bikes and tractors. The hand held units usually have a thick rope wick at the end. Larger wipers come in a variety of styles, using materials such as rope wicks, felt strips or rolling carpet.
Wiper application of herbicides, permits the operator to apply the herbicide directly onto the foliage, with minimal risk of run-off or drift. It applies an even coat of herbicide to the required area of the target plant and leaves non target plants untouched.
No herbicides are registered for use by wiper application in Tasmania at the time of printing. As wiping offers many advantages over spraying, DPIW should be contacted prior to undertaking chemical control of cotton thistle for information on any new herbicides which have subsequently been registered for wiper use.
For any eradication campaign to be successful, two things are important;
1. There should be a complete prevention of seeding every year. Nothing less than 100 per cent kill is acceptable.
This means that before it is too late in the season a thorough check should be made of all areas which have been treated earlier and any survivors grubbed or spot sprayed. This check should be carried out irrespective of the control treatment employed initially. Cotton thistle is a tall conspicuous plant and any surviving plants are easily seen.
2. 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.
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. | Some herbicides are toxic to bees.
As a general rule, avoid applying herbicides when and where bees are foraging.
Always read the label. |
Note:
These herbicide recommendations are made subject to the product being registered for that purpose under relevant legislation. It is the user’s responsibility to check that registration or an off-label permit covers the proposed use. Always read the herbicide label. If in doubt, check with the Registrar of Chemical Products, Department of Primary Industries and Water. Statewide Freecall 1300 368 550.
Boom Spray Application in Pasture Situations
Herbicide
(Active ingredient) | Commercial Product (Content of active ingredient) | Application Rate of Commercial Product per Ha | Withholding Period after Application | Comments | | MCPA | MCPA (250g/L)
MCPA (500g/L) | 4.4 L
2.2 L | 7 days
7 days | For seedlings and rosettes up to 150 mm diameter only.
As above. | | Dicamba | Banvel (200g/L) | 0.7 to 2.8 L | 7 days | Lower rates for seedlings. Use the higher rate for more advanced plants. | | MCPA + Dicamba | Banvel M (340g/L + 80g/L) | 2.8 to 4.0 L | 7 days | Grass pastures - Use higher rate on larger weeds. | | Clopyralid | Lontrel (300g/L) | 50 or 70 mL + 1.0 to 1.5 L MCPA amine (500g/L) | 7 days | Treat rosette stage. Use higher rate on larger plants. Apply during active growth. | | MCPA + Clopyralid | Thiselex (500 g/L + 15 g/L) | 1 to 1.4 L | 7 days | As above. | | Glyphosate | Roundup CT (450g/L) | 1.2 L | 1 day | This product is non selective - use prior to establishing a crop or pasture. | | Diquat | Reglone (200g/L) | 300 mL | 1 day | Add surfactant. Use for seedlings and small rosettes only. |
Spot Spray Application
Herbicide
(Active ingredient) | Commercial Product (Content of active ingredient) | Application Rate of Commercial Product per Litre | Withholding Period after Application | Comments | | MCPA | MCPA (250g/L)
MCPA (500g/L) | 5 mL
2.5 mL | 7 days
7 days | For seedlings and rosettes up to 150 mm diameter only.
As above. | | Dicamba | Banvel (200g/L) | 3 mL | 7 days | For plants up to the early bud stage. | | MCPA + Dicamba | Banvel M (340g/L + 80g/L) | 4 mL | 7 days | As above. | | Clopyralid | Lontrel (300g/L) | 2.5 mL | 7 days | Treat rosette stage prior to stem elongation. Apply during active growth. | | Diquat | Reglone | 2.5 mL | 1 day | Add surfactant. Use for seedlings and small rosettes only. |
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