Nodding thistle

TRAVELLING around the North Island is one of the best parts of my job.

I am privileged to see very wild parts of the country: the East Cape, the forgotten highway areas of Whanganui and Tauramunui, the wild west coast of the Taranaki, and the wondrous Northland.

There are a few weeds that seem to flourish everywhere, from temperate climates on sedimentary soils to the subtropical north and its volcanic soils.

Thistles are one that colonise everywhere, including my own block in the Hunua Ranges, south-east of Auckland.

Nodding thistle ( Carduus nutans, musk thistle) is one of the most competitive species of thistle in New Zealand. It takes its name from the large seed heads that cause the tall plant to 'nod' under their weight.

Like the Scotch thistle, the ‘nodder' usually germinates in autumn and spends its first year in a vegetative state, growing leaves and storing energy in a deep taproot. In its second spring-early summer, it flowers, sets seed and dies. This growth habit makes controlling it more difficult than annual thistles.

Nodders do look similar to Scotch thistles, especially in the vegetative stage. The main difference is that Scotch thistles have prickles on the leaf surface and around the leaf margin. Nodders have spikes only around the leaf margin.

How to control it

This can be tricky due to its biennial growth habit. In many parts of NZ you are obliged by your regional council to control it. Some assist landowners.

Maintaining thick, healthy pasture will stop nodding thistle from germinating.

Plants can be grubbed out. You need to cut the tap root at least 5cm under the ground to stop it growing again, before the plant sets seed.

Goats grazing nodding thistle when it's in flower can help prevent its spread.

You can mow it, just after it flowers and before it sets seed. However, what looks like a small seedling, with three to four leaves in spring, can be over a year old and have a big tap root. Mowing it can cause it to regrow with multiple crowns.

Three insects (two types of weevil and a fly) have been released over the last 30+ years to give some biological control.

Spraying in autumn often gives the best control. Products containing MCPB are the most clover-safe but are only effective on seedlings. Once the rosette gets bigger, you'll need sprays containing MCPA and 2,4-D but these can damage clover. Spraying these chemicals in autumn does less damage to clover than if you do it in spring.

There are growing numbers of plants that are resistant to these sprays. You may need to use ones containing clopyralid or metsulfuronmethyl. These are not clover-safe and best applied with a weed wiper.