Ragwort + 150,000 seeds per plant

Ragwort is a pernicious weed.  It can kill a horse or cow if cut with hay and given as winter fodder.  It used to be an offence not to pull and burn it but once it took hold on motorways councils were not able to enforce the rule.  It flowers in its second year and then the plant dies, but if it is damaged the roots respond and turn it into a perennial.  Even with poisoning with chemicals the plant can survive for 15 years.