Please talk to your doctor before mixing trazadone, (or any prescribed medication), with St John's Wort, (or any other herbal medication.) There *can* be severe side effects due to mixing the two.
Many times, the pharmacist is even a better person to ask. It is their specialty, afterall. Doctors only have a few hours training in pharmacology.
In any case, get professional guidance. Don't depend upon strangers on the internet for your medical care, ok?
Shalom!