Traditional herbalism is the use of herbs for healing. Yes, the most common ones are St. John's Wort, Valerian, and Chamomile, which is also used for folks sensitive to St. John's Wort. Yes, schizophrenia and organic mental disorders. Acupuncture is very effective for stress disorders. Also aromatherapy works well for stress and uplifting spirits in general.
Herbal treatments can be as effective as medications, but they take longer to be effective.
For severe mental illness, I would default to medications, and as always, any treatment should be discussed with your doctor. I am not sure how psychiatrists in general feel about herbal medications, though. It might be hard to find one who will work with herbals. The specialist really depends on the preferences of the main psychologist working with the client. Osteopaths are generally more holistically inclined.