Introduction to Herbal Magick

Herbs have been an important thing in magick since the beginning of time. Herbs have been used to heal ailments in medicine, used for spiritual purposes such as divination, smudging, banishing. Some herbs have been used even to poison foes.
They were widespread over the Earth and different regions had access to different herbs. Egyptians, for example, had access to Myrrh. Most of our botanical knowledge comes from various Native American tribes and immigrants from Europe.

Medicinal use

Using herbs to heal an ailment, requires a great deal of care. You could have an allergic reaction to them and some are also poisonous if you ingest or take too much of them. Check with your physician to see if you won't have any problems or issues with taking herbal supplements or drinking tea made from different herbs, better safe than sorry.

Some herbs are used in culinary fields such as Basil and Rosemary. You can find many herbs such as Bay leaves, Rosemary, and Basil at your local grocery store, so no need to run to the spiritual store. With some herbs you can make tinctures, infusions, ointments and teas. Which will help heal ailments, increase mood, energy, etc.

Magical use

All herbs can be used magically, just as they can be used medicinally, different herbs have different properties and effects. You can find lists of properties online or in books, many will recommend Scott Cunningham's Encyclopedia of Magical Herbs. 
There's also a video with Scott Cunningham on Herb Magick.

Herbs can be used for purposes such as divination, spiritual banishment or conjuring/summoning, to bring a desired effect to you. They can be made into oils, which have magical properties as well. Herbs can smoked for different effects as well. Others can be ingested to cause an altered state or trance. Herbs can be combined into a powder for a desired result such as love, wealth, protection, etc.

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Sources: Herbal Magick: A Witch's Guide to Herbal Folklore and Enchantments by Gerina Dunwich.