Effect of Field Application of Growth Regulators on Secondary Dormancy of Common Ragweed ( Ambrosia artemisiifolia ) Seeds

Winter chilling in the field removed the primary dormancy in common ragweed ( Ambrosia artemisiifolia L.) seeds and allowed them to germinate in both light and darkness. A secondary dormancy was induced in the spring and seeds again failed to germinate in light or darkness. Kinetin (K) + ethephon (E) + gibberellin A 4 + A 7 (GA 4+7 ) applied to the soil in December prevented secondary dormancy. The following May only 20% of the seeds at the 15-cm depth remained viable. The rest had germinated or decayed. In contrast, 89% of the untreated seeds remained viable. Application of K + E + GA 4+7 on April 22 and May 2 also broke the secondary dormancy. Gibberellin A3 (GA 3 ) was less effective than GA 4+7 in preventing or breaking secondary dormancy.