If the truth be known, St. John's Wort works like antidepressants do...by inhibition of the Tryptophan Pyrrolase (oxygenase) pathway. (see Badawy A.A. on medline)
St. Johns wart words well in mild to moderate depression, but those with severe and chronic depression, stronger meds/therapies are needed.
Unfortunately it's difficult to work out a reliable dose with the stuff. Eli Lilly and other pharmaceutical manufacturers have what you need.