St. John's wort (Hypericum perforatum) is a naturalised wildflower from Europe whose bright yellow five-petalled flowers, each with a fringe of stamens, open around the summer solstice. Hold a leaf to the light and it is dotted with tiny translucent glands, which is the "perforatum" of the name. It grows 12 to 24 inches and takes poor, dry ground in full sun to part shade.
It spreads by seed and rhizome and can become weedy; in parts of the West it is a listed noxious weed of rangeland. Grow it in a designated patch or a container, and deadhead before the seed capsules ripen if you do not want volunteers. Direct sow in spring; it germinates readily and establishes fast, and needs no water once it is up.
The flowers are attractive in an informal way and are worked by bees for pollen. The capsules that follow turn from red to black and hold up in naturalistic cut arrangements. Grazing animals avoid it, as do deer. For a garden shrub with the same flower on a tidier plant, the native golden St. John's wort or 'Albury Purple' are better choices.
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.