There is a great deal of misinformation around about the ability of ragwort seeds to spread.
This example from Horse and Hound's forum recently is a good example.
The plant is a real survivor and is Worldwide, each plant can produce up to 120, 000 seeds per year that are light enough to travel the globe in the upper atmosphere.
This is nonsense!
Firstly the 120,000 seeds figure is a maximum and not typical. but the research hows very clearly that the seeds do not spread like this. They are heavier than air but they have what is in effect a tiny parachute. Parachutes do not lift they merely slow the rate of descent.
In fact the seeds have been shown to mostly fall within a few metres of the plants.