Weed Watch – Nodding thistle

Nodding thistle

Nodding thistle

Nodding thistle (Carduus nutans) is one of the most competitive thistle species causing problems in New Zealand pastures. It is common throughout New Zealand and is especially abundant in pastures, crop plantings, along roadsides and in wasteland areas in summer.

 

The species gets the name “nodding thistle” because of the large seed-heads which “nod” over due to their weight.

 

Nodding thistle can be distinguished from Scotch thistle at the vegetative stage, as Scotch thistle has spines on both the upper leaf surface and the leaf margins, whereas nodding thistle only has spines around the margins. It can be a bit difficult to distinguish from Californian thistle rosettes, though the latter tend to grow in clusters due to being connected underground by creeping roots.

 

Nodding thistle can form dense stands of up to 150,000 plants per hectare. It obstructs livestock movement and prevents access to neighbouring pasture plants. Mature plants can produce up to 10,000 seeds per plant, which have a high germination viability (60-80%) and can survive in the soil for long periods.

 

In the field

In the field

Nodding thistle is classified as a Containment Contro Pest in the Canterbury Regional Pest management Strategy (2005-2015). Strategy rules require land occupiers to clear nodding thistle at least 40m from neighbouring boundaries, stock water and irrigation races.

 

There are a number of insect biocontrol agents that can reduce nodding thistle vigour. These include the nodding thistle crown weevil, receptacle weevil and the gall fly. Phenoxy herbicides are also typically used to control nodding thistle selectively in pasture. Thistle germination in autumn should be minimised by trying to keep the pasture competitive at this time, not over-grazing in summer and introducing drought-tolerant pasture species such as cocksfoot and phalaris. Goats will eat the weed once it is growing taller than surrounding vegetation.