Survival of nodding thistle (<i>Carduus nutans</i>) seed buried at different soil depths in four soils

The viability of nodding thistle (Carduus nutans) seed buried at three depths in four different soils was evaluated over a 16 year period.Seed viability was affected more by burial depth than by soil type.The amount of viable seed retrieved from the top 2 cm soil layer after 1 year was less than 16% of that buried, while up to 71% and 84% of seed buried at the 5 cm and 20 cm depths respectively remained viable.Sixteen years after burial, up to 20% and 32% viable seed was retrieved from the 5 and 20 cm depths respectively.It is estimated that if left undisturbed, it would take between 14 to 80 years for viable seed numbers to be reduced to 1% of the original seed numbers, depending on soil type.