Peter Joynt, a Dargaville-based Biosecurity Officer for the Council, says targeting thistles at this time of year will also help avoid damage to other valuable pasture species.
Mr Joynt says thistle control in Northland is a perennial problem, although the numbers and spread of the four main species found in Northland vary depending on a number of conditions including fertility, soil type and the way pasture is managed.
The two most common thistle species in Northland are ‘Scotch thistle’ and ‘winged thistle’.
“Scotch thistle a biennial, germinating in winter or spring, surviving its first summer as a rosette and then flowering in its second summer. Winged thistle is an annual that germinates in autumn or early winter, over wintering as a rosette and then bolting up to a flower head in spring or early summer.”
Mr Joynt says unchecked both species can occupy significant areas of pasture land, although good pasture management and a well planned control programme will minimise their effects on pasture production.
The other two thistle species found in Northland – ‘nodding’ and ‘Californian’ - are relatively limited in their distribution, but are more difficult to control successfully.
‘Nodding thistle is a biennial is similar in both size and growth habits to the Scotch thistle. However, nodding thistle is more open with bright red/purple flower heads that droop over long before maturity and give it its name.”
Mr Joynt says careful attention to the timing and application rate of herbicides is needed to control nodding thistle once it’s established in pasture.
“Californian thistle is a perennial and is the only true thistle in New Zealand that spreads from rhizome development. New shoots become evident about November when they form rosettes with small purple flowers.”
Mr Joynt says Californian thistle characteristically grows in patches that often stem from a single plant.
“Because all flowers on the one plant are of the same sex, opportunities for pollination are often poor, resulting in low levels of fertile seed being produced over a large area.”
He says the extensive underground root system of Californian thistle makes successful control very difficult to achieve.
Mr Joynt says there a number of key requirements to ensure a successful thistle control programme:
- Assess the situation and identify your target weed.
- The timing and choice of chemical to be applied.
- Calibration of the boom and accurate herbicide application.
- Following up treatment with spot spraying or grubbing.
Special conditions, such as the presence of new grass or thistles seeding in crops, must also be taken into account and expert assistance sought if required.
People wanting information on the best control methods and management for thistles and other pest plants can contact Biosecurity staff at Regional Council offices in Whangarei, Dargaville and Kaitaia.