Weed Research, 1993, Volume 33. 69-78 Influence of plant age on the alielopathic potential of nodding thistle (Carduus nutans L.) against pasture grasses and legumes D. A. WARDLE, K. S. NICHOLSON AND A. RAHMAN Ruakura Agricultural Centre, Ministry of Agriculture and Fisheries, Private Bag, Hamilton, New Zealand Received 3 December 1991 Revised version accepted 20 May 1992 Summary: Resum6: Zusammenfassung The allelopathic influence of four developmen- tal stages of nodding thistle (small rosette, large rosette, bolting and dead plants) on eleven test species (six grasses, four legumes and itself) was investigated using aqueous extract bioassays, aqueous leachate bioassays, and bioassays based on addition of ground material to soil. The aqueous extract and leachate bioassays in- dicated that nodding thistle was most allelo- pathic at the rosette stages, while the bioassays involving amendment of soil with thistle tissue showed bolting live and dead plants to be the most allelopathic. It is proposed that nodding thistle is allelopathic at two phases of its development, i.e. at the early bolting stage when the larger rosette leaves are decomposing and releasing soluble inhibitors, and at the stage when bolting plants are dying and releasing in- soluble inhibitors. Nodding thistle seedlings appear to be stimulated by addition of thistle tissues to soil, indicating that thistle plants may weaken pasture and simultaneously encourage recruitment of its own species. This is consistent with previous studies detecting high densities of thistle seedlings in the vicinity of deceased parent plants. The six grass species were generally more tolerant to thistle allelopathic effects than were the four legume species, but no consistent differences emerged between different grass or legume species. Influence de I'age de la plante sur le potentiel allilopathique du chardon penche (Carduus nutans L.) vis a vis des prairies de graminies et de ligumineuses L'influence allelopathique de 4 stades de developpement du chardon penche (petite rosette, grande rosette, montaison, plantes mortes) sur 11 espfeces tests (6 graminees, 4 legumineuses et lui-meme) a €t€ etudi6e en utilisant des bioessais d'extraction acqueuse, des bioessais de lessivage acqueux et des bioessais fondes sur l'addition au sol d'amende- ment. Les bioessais d'extraction acqueuse et de lessivage ont montre que le chardon penchd 6tait plus alldlopathique au stade rosette tandis que les bioessais impHquant l'amendement de sol avec des tissues de chardon ont montr6 que la montaison et les plantes mortes 6taient les plus allelopathiques. 11 est propos6 que le chardon pench6 est allelopathique a 2 phases de son developpement, par exemple au stade montaison pr6coce quand les plus grandes feuilles de la rosette se decomposent et re- lachent les inhibiteurs solubles, et au stade oii les plantes montees meurent et relachent les inhibiteurs insolubles. Les plantules de chardon pench6 apparaissent etre stimul6es par l'addi- tion de tissus de chardon au sol, montrant que les plantes de chardon peuvent affaiblir une prairie et simultan6ment favoriser sa propre r^implantation. Ceci est conforme avec des etudes pr^cedentes montrant des fortes densit6s de plantules de chardon au voisinage de plantes parentes mortes. Les 6 especes de graminfies ont 6te de fagon gen^rale plus tolerantes aux effets allelopathiques que les 4 especes de