Scotch thistle

Scientific name: Cirsium vulgare

Scotch thistle flower head

Scotch thistle invading a field

Scotch thistle rosette
  • Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • Biennial thistle with large reddish, prickly thistle flowers, either singly or in clusters of two or three flowers at the tops of the branched stems
  • Can form a large ‘shrub’ in late summer
  • Erect branched stems with spiny wings grow from a basal rosette to 1.5 m tall
  • Flowers from November to March, producing reddish-purple flower heads which grow solitary or in clusters of two or three flowers
  • Outer bracts of flower heads are slightly hairy with strong sharp spines up to 4 mm long
  • After flowering each seed head contains small, narrow seeds (about 4 mm long), each with a feathery pappus (thistledown) about 20 – 25 mm long
  • Leaves are large (up to 30 cm long by 10 cm across), dark green, divided almost to the midrib, with sharp prickles at the ends of the leaf lobes. Prickly hairs are on the upper surface and white cottony hair on the lower surface
  • Distributed very widely throughout New Zealand; common in pasture, wastelands, forests, and roadsides.

Origin

  • Scotch thistle originates from Southern Europe, Central Asia and Asia Minor
  • Now distributed throughout New Zealand, it is considered one of the country’s commonest thistles
  • Wind-blown seeds may travel some distance, and can germinate in large numbers where the ground has been disturbed
  • In New Zealand, this thistle can reach a maximum height of about 1.5 m.

Life cycle

  • Seeds mostly germinate in gaps in pasture in the autumn, but some can germinate at other times of the year
  • Most Scotch thistles growing in pastures appear to be biennial, germinating in one year, surviving their first summer in rosette form and not flowering and bolting until their second summer
  • However, where thistles have no competition they may act as annuals, germinating in autumn and flowering the following summer
  • In poor growing conditions, Scotch thistles can flower when as small as 30 cm tall.

Impact on pasture

  • Scotch thistles can take up a good deal of pasture space, and thus reduce the availability of quality pasture to livestock.

Impact on livestock

  • Livestock, especially cattle, are reluctant to graze too close to thistles so pasture space taken up by thistles is, in effect, greater than the area occupied by them
  • Very dense stands can restrict stock access to pasture and water, as cattle are reluctant to push through thick thistle masses
  • Goats will graze the plant once it is in flower, but most sheep will rarely graze this thistle at   any stage.

Pasture species/cultivars

  • Dense, vigorous pastures help stop thistles from establishing and reduce their growth and survival
  • Choose grass species or cultivars appropriate to the district and infected with the right endophytes
  • In areas where cocksfoot grows well it may be a better competitor than ryegrass.

Chemical control

Scotch thistle seem to be susceptible to MCPB and 2,4-DB, both of which will kill Scotch thistles without damaging clovers, So far no resistance to these herbicides has been found in New Zealand. But farmers should make sure they are sure of the identification of the thistles in their paddocks before they decide on the herbicide to use.

  • The most cost-effective control is achieved by spraying young thistles
  • Best time to spray is autumn and winter when thistles are still small
  • Spraying small thistles (up to six leaf stage) in autumn and winter will be less damaging to clover than spraying in spring
  • Grazing before spraying will reduce the amount of clover leaf present and thus reduce clover damage
  • In spring and summer, spot spraying and weed wipers can be used for larger thistles, although nearby clover will also be damaged.

ALWAYS READ PRODUCT LABELS BEFORE APPLYING

Active ingredient When to apply Residual effect Grass damage Clover damage
2,4-D Up to small rosette Slight No Slight
2,4-DB Up to small rosette Very slight No None
dicamba Up to large rosette Severe No Severe
triclopyr/picloram Up to large rosette Severe No Severe
metsulfuron-methyl Up to large rosette Yes Moderate Severe
picloram Up to large rosette Severe No Severe
MCPA Up to small rosette Moderate No Slight
MCPB Up to small rosette Very slight No None
MCPA/MCPB Seedling Moderate No Very little
glyphosate Any stage No Severe Severe
clopyralid Up to large rosette Severe No Moderate – severe
mecoprop/dichloroprop/MCPA Up to small rosette Yes No Severe

Consult your farm consultant, industry rep or the New Zealand Agrichemical Manual for more information about chemical control.

Biocontrol

Mowing / grubbing
  • Thistles are killed by mowing only after flowering has started
  • If thistles are mown too early, flowering is delayed and thistles can produce multiple crowns
  • Mowing is recommended in summer.

References

  • Popay I, Champion P, James T 2010. An illustrated guide to common weeds of New Zealand. New Zealand Plant Protection Society, Christchurch, New Zealand. 416 p.
  • Young S 2013. New Zealand Novachem agrichemical manual. Agrimedia Ltd., Christchurch, New Zealand. 767 p.