Nodding thistle

Scientific name: Carduus nutans

Nodding thistle flower

Nodding thistle

Nodding-thistle-before-flowering

Nodding thistle in pasture
  • Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • Leaves are dark green, deeply divided into triangular lobes with spiny tips, and do not have large, distinct white markings, although the leaf margins are white at the base of the marginal spines
  • Flowering stems that arise from the often large rosettes have spiny wings except just below the flower heads
  • Flowerheads are globose, usually 2 – 6 cm across, the largest being solitary at the top of the flower stalk. Smaller heads are found on lateral branches
  • Flower petals are red-purple, very rarely white
  • Seeds have thistle down, soft feathery attachments which help the seeds disperse for short distances
  • Nodding thistles have an annual or biennial life-cycle, living for one or two years.

Origin

  • Nodding thistle originates from Europe, north-west Africa and Asia Minor
  • It is now abundant in New Zealand, and is especially predominant in drier eastern regions. It is found on dry open faces and ridges, pastures, crops, roadsides and waste places
  • Drought, overgrazing and pasture pests such as grass grub and porina open up the pasture making it easier for thistles to establish
  • Nodding thistle spread quickly across New Zealand in the 1950s, possibly as the result of a series of dry summers.

Life cycle

  • Some nodding thistle plants flower in the first year and have an annual life-cycle, others have a biennial life cycle, flowering only in their second year
  • Most plants germinate in autumn, form a rosette during winter, then bolt (send up a tall flower stalk) in spring, and flower, seed and die in summer
  • Seedlings that germinate in autumn are more likely to survive and produce flowers
  • Germination usually occurs in bare patches; pasture cover suppresses germination
  • Strong pasture competition reduces thistle growth rates and increases their mortality. Small thistles are more vulnerable to damage than large ones
  • Thistle infestations are worse after droughts; most nodding thistles germinate within two months after a drought: a summer drought nearly always leads to a ‘thistle year’ the next summer
  • Each nodding thistle plant can produce from 7000 to 30,000 seeds, but some seeds will not be viable
  • Stock (especially goats) may eat flowering thistles and destroy the seeds, but will only eat them when they are flowering
  • Most seeds are not carried far by the wind, and most fall within 20 m of the parent plant
  • If seeds become buried 50 mm deep in the soil, over 40% of the seeds may be viable 4 years later. Seed can remain viable in the ground for 15 – 20 years.

Benefits

  • Bee-keepers value nodding thistle flowers as they provide copious pollen and nectar for bees and other insects.

  • Nodding thistle contributes to scabby mouth in stock (a disease caused by a parapox virus that infects the lips and other parts of an animal’s face)
  • As thistle numbers increase, animal production declines. On one study, the growth rate of sheep grazing pastures with high thistle numbers (2+ thistles per m2) was 30% lower than sheep grazing pastures with low thistle numbers (0.5 thistles per m2) at the same stocking rate
  • Sheep growth rates can also be reduced indirectly through herbicide damage to clovers when thistles are sprayed.

  • The best returns from thistle control come from managing the most productive land
  • Relatively low returns for sheep and beef make thistle control marginal in hill country, except after a drought
  • High livestock value and gross margins increase the value of the pasture and make weed control more worthwhile
  • Many principles which apply to nodding thistle and variegated thistles are also relevant for other thistles such as Scotch and winged thistles.

Grazing management

  • Goats will eat and control thistles, but only when the plants are flowering
  • Careful grazing in autumn, spring and summer to ensure strong pasture cover, especially in autumn, will give most effective control of the weed
  • Pasture damage caused by overgrazing or pugging encourage thistles and other weeds.

Pasture species/cultivars

  • Dense, vigorous pastures stop thistles from establishing and reduce their growth and survival
  • Pasture cover is most important in autumn
  • Where new improved cocksfoot varieties perform well they may to keep thistles out.

Chemical control

  • The most cost-effective control is achieved by spraying young thistles in late autumn or in winter after drought
  • Thistles are most susceptible to herbicides when small, that is in autumn and winter
  • Blanket spraying small thistles (up to six leaf stage) in autumn and winter is less damaging to clover than spraying in spring
  • Grazing before spraying in spring will reduce the amount of clover leaf present and reduce clover damage
  • In spring and summer, spot spraying or weed wipers can be used for larger thistles, although clover may also be damaged
  • Check the tap root diameter before spraying; two year old plants may have small rosettes but a large root system with a larger diameter tap root. They look similar above ground to one year old plants, but two year old plants need more herbicide to kill them
  • For low use rates, or to use clover-safe herbicides like MCPB: root diameter below the crown must be smaller than a five cent piece
  • In some areas, thistles have developed resistance to phenoxy herbicides such as MCPA, MCPB, 2,4-D; use non-phenoxy herbicides such as metsulfuron-methyl, clopyralid or glyphosate to control them.

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
dicamba Up to large rosette Severe No Severe
triclopyr/picloram Up to large rosette Yes No Severe
metsulfuron-methyl Up to large rosette Yes usually moderate Severe
picloram Up to large rosette Yes No Severe
MCPA Up to small rosette Moderate No Slight
glyphosate Any stage No Severe Severe but temporary
clopyralid Up to large rosette Severe No Moderate – severe
MCPA/MCPB Seedling Moderate No Very little
mecoprop/dichloroprop/MCPA Up to small rosette Moderate No Severe

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

Mowing / grubbing

  • Thistles are killed by mowing only after flowering has started, but some may regrow and flower if soil moisture is adequate
  • If mown too early, flowering is delayed and multiple crowns can be produced
  • Mowing is recommended in summer.

Biocontrol

Nodding thistle crown weevil (Trichosirocalusspp.)
  • Well established throughout New Zealand
  • Attacks several different thistles including plumeless, winged and slender-winged thistles (Carduus spp.), Californian and Scotch thistles (Cirsium spp.). and cotton thistle (Onopordum acanthium)
  • The adult weevils are small, greyish-brown, 3 – 4 mm long
  • Eggs are laid in leaf cavities
  • White grub larvae feed on the crown and can stunt growth, reduce the number of flowering stems and lateral growth, and kill plants
  • Adults feed on the foliage but don’t usually cause a lot of damage
  • Thistle numbers have declined at infected sites.
Nodding thistle gall fly (Urophora solstitialis)
  • Well established throughout New Zealand
  • Attacks thistles including Carduus spp.
  • Adults are black, 5-8 mm long
  • Adults lay eggs inside the green flower buds
  • Larvae feed on the flower which produces a gall in which the larvae pupate
  • Each larva destroys about 6 seeds
  • Strongly prefers nodding thistle and plumeless thistle (Carduus acanthoides).
Nodding thistle receptacle weevil (Rhinocyllus conicus)
  • Widespread throughout New Zealand
  • Adults are dark brown with lighter specks and 6 mm long
  • Adults lay eggs on the thistle flower buds
  • Larvae remain in the buds and flower heads until fully grown
  • Adults do not cause much damage; larvae cause the greatest damage
  • Larvae can destroy most seeds from primary flowers, but cause less damage to secondary and tertiary flowers
  • The weevil prefers nodding thistle but will also attack other thistles.