I don't know about Tansy, but here in the UK ragwort is considered a noxious weed. It is poisonous to livestock and spreads like mad if allowed to seed. Regardless of it's companion properties, or lack of, I wouldn't consider letting it get generally established anywhere.
Even if you don't have livestock now you may want some in the future, and you don't want your land to be a source of airbourne seeds for others.
Moderator, Treatment Free Beekeepers group on Facebook.
Don't plant ragwort! It is highly toxic to most livestock, and to people, and bees love it, but if they have too much of it as a proportion of their forage, their honey is toxic! It's a constant battle on our land.
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