"I thought that all herbs were medicinal, but if some are poisonous how do you define what a herb is? Have you a good definition please?"
Photo Asian Medicinal Herbs - Text courtesy of atlantic-acupunctu re
You are right, Patricia. Plants as well as spices, seeds, bark, roots, bulbs,fungi, sea weeds etc. - are called herbs because at one time or another in the history of humankind they were used for healing/medicinal purposes.
This morning, while I was thinking about an answer to your question, I came across an article written by Cori Young and her statement should partially answer your question:
"We've all heard the alarming research showing that a specific herb has been found to be toxic - comfrey, ephedra, kava, etc. Yet somehow Native peoples managed to use these herbs very successfully for many generations."
Photo courtesy of asylumoffunk
Plants haven't become something else from what they used to be in ancient history. But big corporations that are selling us synthetic nutrients and vitamins for big profits - they emerged more recently.
Everything in nature is there for a purpose, including the so-called poisonous or toxic plants which require caution and expertise when used.
Indigenous people from around the world were much more connected to the nature than we are today. Plants "talked to them". They learned when was the best time to harvest, what was the proper way to gather certain part of an herb, they only picked as much as they needed - not everything they could get their hands on, they knew what time of the day and year certain herb was at the peak of its healing strength - and so much more.
Photo courtesy of job_earth
Western civilization managed to destroy that connection and replace it with profit-oriented mind-set. A large portion of that invaluable native knowledge got lost and forgotten and we became dependent on laws and rules, regulations and prescriptions.