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.
Rust
Orange to brown raised pustules on the undersides of leaves, with yellow spotting on the upper surface. Heavy infections cause leaves to yellow and drop.
Remove and destroy infected leaves. Avoid overhead watering and improve air circulation. Apply a sulfur or copper-based fungicide if it spreads. Clear plant debris in fall.
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 rot
Plants wilt, yellow, and stunt even in moist soil; lower stems and roots turn brown, soft, and mushy and the plant may collapse. Most common in heavy, wet, or poorly drained ground.
Improve drainage and stop overwatering, and never plant too deep. Lift and destroy badly affected plants along with the soil around the roots. Grow in raised or free-draining beds to prevent it returning.