A claim was placed on the website of the British Horse Society a few years ago that ragwort plants could produce up to 150,000 seeds and that after a period of 20 years 70% of them could still germinate. These individual claims about the number of seeds and the germination figure are in themselves misleading see Ragwort Seed production and Ragwort Seed germination. Taken together they are a myth as the combination of the claims is clearly untrue. The source of the seeds surviving for 20 years in the soil would appear to be a paper in The New Zealand Journal of Experimental Agriculture (Thompson and Makepeace 1983) where extrapolating from figures over a 6 year period they estimated that the time to where there would only be one percent of the ragwort seeds able to germinate was 20 years. Clearly this is not a 70% figure, and it would in any case be highly exceptional for any seed to retain this kind of germination power after this period in the soil.

Reference Thompson A, Makepeace W. Longevity of buried ragwort (Senecio jacobaea) seed. New Zeal J Exp Agric. 1983;11:8990.