If you google "milkweed" you'll get all sorts of USA sites about how and where to grow different kinds of milkweed which, it seems, are great for Monarch butterflies. The photos will help you identify what you have and the info will tell you how best to grow it.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
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Pink flowers.....New pic helps and rules our Asclepias .
Structure of flower seems to put it in Asteraceae family.
Possibly Eupatorium maculatum atropurpureum?
https://www.google.com/search?q=Asclepias+incarnata&client=firefox-b-d&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiA9Nq9-8TrAhWKTBUIHbRMCrkQ_AUoAXoECBgQAw&biw=1280&bih=579#imgrc=RqVvuWSs8Ku8GM
https://www.google.com/search?q=eupatorium+maculatum+'atropurpureum&client=firefox-b-d&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj6m_iCgMXrAhUxtHEKHWwHC2IQ_AUoAXoECA0QAw&biw=1280&bih=579#imgrc=1KyXqBoyNr9uiM