This is actually one of the more difficult pages on this site to write. The reason being is that there is no opportunity to use scientific references. The reality is that a search of the scientific literature shows not one single case of ragwort poisoning reported in dogs. There are plenty of scare stories in the press from time to time but there is no evidence that ragwort poses any serious risk at all to dogs.
It is unfortunate that the quality of journalism in some parts of the press is so dreadfully bad that facts are not checked before they are printed and this often leads to unneccessary panic for people who own animals. The facts are however extramely clear on this subject. The risk just does not exist to any significant degree at all.
Ragwort is of course poisonous to mammals. So if a dog were to be force fed this intensely distasteful plant it would be poisoned. However, the poisonous dose for the grazing animals where it is a risk if it is in hay is measured in percentages of body weight so it would be safe to assume that the same sort of dosage risk would apply to dogs. There would be no risk to them from just encountering the plant.
The risk could be summed up like this. Ragwort is poisonous to dogs and it is also poisonous to dolphins but they don't eat it either so there is no risk to either species.
The message here is just don't panic about this scare story. Look at the facts of which you will find plenty on this website, and make your decisions on the basis of the evidence.