Thistle: nodding and plumeless

Nodding and plumeless thistles are agricultural pests in the Waikato region. They have unpalatable foliage and reduce pasture production. Their seeds can remain viable in the soil for many years. They can form dense stands that obstruct livestock movement and inhibit and suppress the growth of desirable pasture species. Their seeds are spread between properties mainly by contaminated hay and machinery, uncertified seed and stock feed and also via wind dispersal.

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.

Some options for managing thistles are included below, but there are many others. For more information contact your herbicide representative.

Physical control

Grubbing plants is an effective method of controlling thistles, provided they are grubbed at least 5cm below the crown and control work takes place before the onset of seed. Mowing (or topping) thistles is less effective as the plant is able to regrow from the crown. You will need to mow repeatedly to make sure the plants do not reach flowering/seeding stage.

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 this 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 grass varieties perform well they may keep thistles out.

Mowing or 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.

Herbicide control

There are many herbicides that will control nodding and plumeless thistle. The herbicide best suited to your property will depend on the level of infestation, the application equipment you have available and the stage of growth the plants are at. Some suggested herbicides are included in the table in this factsheet. Contact your local biosecurity plant pest officer for free advice on what herbicide and application method would best suit your situation on the freephone 0800 800 401.

Spray application

Spot spraying plants should be done before flowering, as mature plants are less susceptible to herbicides. Spraying after seed has set is ineffective, so make sure that you do not allow nodding or plumeless thistle to go to seed. Boom spraying in autumn and early winter with a herbicide such as MCPA, 2,4-D or Batton provides effective control of seedlings and reduces the likelihood of pasture damage compared to a spring application.

Because plumeless thistle germinates at a later stage it is difficult to control both plumeless and nodding thistle with one autumn/winter application.

Small block owners who may not have access to spray equipment could consider grubbing out plants (see ‘physical
control’ above) or selecting a herbicide such as picloram granules. Picloram is a dry application powder which does not require mixing or spray equipment.

Note: using non-selective herbicides (which you may have already purchased for other plant control jobs) will destroy surrounding pasture species and reduce competition against new thistle seedlings.

Herbicide control tips

• 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, the 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 and 2,4-D. Use non-phenoxy herbicides such as metsulfuron-methyl, clopyralid or glyphosate to control them.

Safety when using herbicides

• Follow the instructions on the manufacturer’s label.
• Always wear protective clothing.
• Always minimise the risk to your other plants.
• Contact the supplier for further advice.
• Consult your farm consultant, industry rep or New Zealand Novachem Agrichemical Manual for more information about chemical control.

Summary of herbicides and application methods for control

Disclaimer: Any product names mentioned below are not an endorsement nor are they a criticism of similar products not mentioned.

Management

After initial control, it’s important to clean out the site again at least annually to control regrowth.

Biological control

Biological control involves importing insects or fungi that feed on plants in their native countries. Biological control agents reduce infestations but do not eradicate plants from an area. All the following biocontrol agents have been released throughout the Waikato and have established themselves in all suitable areas. However, if you have nodding or plumeless thistle on your property you will still need to undertake control work.

Four biological agents have been released in the Waikato to feed on nodding thistle:

• receptacle weevil
• crown weevil
• gall fly
• green thistle beetle.

Each of these agents will also feed on plumeless thistle. The receptacle weevil has had the most significant impact on nodding thistle in the region. The larvae feed on the receptacles, reducing the amount of seed being produced. The effect is obvious only long term, as this agent does not reduce plant growth.

Landcare Research runs a national biological control programme. Waikato Regional Council supports the programme and maintains a local biological control programme for the Waikato region.