St. John's Wort is an ancient medicinal herb and naturalised wildflower — the bright yellow five-petalled flowers with their distinctive fringe of stamens bloom around the summer solstice. It spreads readily by self-seeding and rhizomes and can become weedy; grow it in a designated area or contained situation.
The flowers are attractive in an informal way and the berries that follow are ornamental, transitioning from red to black. Both can be used in naturalistic cut arrangements. Direct sow in spring; it germinates readily and establishes quickly.
Hypericum Rust
Orange to brown powdery pustules on undersides of leaves and along stems. Heavily infected leaves yellow and drop, weakening the plant.
Remove and destroy infected foliage. Improve air circulation by thinning stems and avoid overhead watering. Apply a sulfur or copper fungicide if infection is widespread.
Powdery mildew
White or grey powdery coating on leaves — usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Root and Crown Rot
Wilting despite moist soil, blackened or mushy roots, and collapse at the crown. Common in heavy or waterlogged soil.
Plant in well-draining soil and avoid overwatering. Remove and discard affected plants. Do not replant in the same spot without improving drainage.