One of the most useful books in Herbal Medicine is Rosemary Gladstar's Herbal Recipes for Vibrant Health : 175 Teas / Tonics / Oils / Salves / Tinctures and Other Natural Remedies.
Recently, I noticed my St. John's Wort had berries and flowers. I missed the day for collecting the herb. According to Rosemary's book, traditionally June 24th is the day to pick the bud. If ready, it will stain your fingers red. Why the 24th of June – it is the anniversary of the day St. John the Baptist was beheaded. How appropriate!
You can use both the flowers and the leaves although the flowers are preferred in about 70% of the case.
St. John's Wort (Hypericum perforatum) is useful for depression and doesn't require a prescription. Please discuss with a health care provider if you decide to take it. I use this homeopathic remedy for neuropathy secondary to shingles, lead intoxication, and even diabetic neuropathy.