Aspects of the biology of nodding thistle ( Carduus nutans L.) in Canterbury, New Zealand

Aspects of the biology of nodding thistle ( Carduus nutans L.), with special emphasis on seed production and germination are described. The principal flowering period in Canterbury is December through January. Inflorescences maturing outside this period produced significantly less seed. All of a series of marked seedlings completed their life cycle within 12 months from the date of germination. The majority of seed is produced over an 11‐week period. An average of 63 seed‐bearing inflorescences were produced per plant. Inflorescence production per plant per week increased with plant age but inflorescence size and seeds per inflorescence declined over the same period. The average production of 6 200 viable seeds per plant was consistently less than the potential of 15 300. Excluding pollinating insects by caging immature inflorescences reduced viable seed production by 97%. Dark‐coloured seeds, freshly collected from primary inflorescences germinated better than pale coloure seeds from primary inflorescences and all seeds from secondary inflorescences. Adding extra mucilage improved germination of all seeds by 15%. Ninety‐four‐day‐old seed under daylight conditions germinated more successfully than did 12‐ and 843‐day‐old seed. Shading severely depressed germination. Twenty‐five different species of insects were recorded as visitors to nodding thistle plants during the period of this study.