Hypericum perforatum aka Saint John's Wort

Hypericum perforatum is a perennial herb called St. John's wort. It belongs to the Hypericaceae family. Its native range spans Europe to China and northwest Africa. It is native to Britain and Ireland too. People introduced it to North America, where it has spread widely. Wild plants grow in grassland, open woods, and roadside ground.

Plants stand upright, with many branches and a woody rootstock. Most reach 1–3 feet [0.3–0.91 metres] tall in gardens. They spread about 0.75–2 feet [0.23–0.61 metres] wide. RHS gives larger limits of 4.9 feet [1.5 metres] tall. Its listed spread reaches 3.3 feet [1 metre]. Green stems have ridges when young and become round with age. The small, oblong leaves grow in pairs without leaf stalks. They have smooth edges and no hairs. Clear dots make the leaves look pierced when held against light. Dark dots may line their lower edges. The plant loses its leaves in winter.

Yellow flowers appear from May to August, depending on location. Each flower has five petals and many clustered stamens. Black dots mark the petal edges. Groups of blooms form at the tips of upper stems. Bees visit the flowers. Brown seed capsules follow, with three chambers and many black seeds. The seeds are flat, narrow, and pitted. Seeds ripen from July to September. Flowers and leaves also yield dyes in several colours.

Full sun gives the best bloom, though plants tolerate part shade. They grow in sand, loam, clay, and chalk. They accept acid, neutral, and alkaline soil. Moist soil with good drainage suits them best. Established plants withstand some drought. Sources place them in USDA zones 3–8 and RHS class H6. Garden uses include borders, slopes, meadows, and woodland edges. Routine pruning is limited to a tidy before spring growth. Serious pests and diseases are scarce, though honey fungus can occur. Spreading roots and heavy seed crops make this plant invasive. It can crowd out native plants and is a listed noxious weed in several US states.

The plant has a long history of herbal use for depression. PFAF records herb and fruit teas, plus flowers used in mead. RHS warns against eating the fruit. These uses do not mean the plant is safe to consume. Sap contact or ingestion can make human skin sensitive to sunlight. It can also cause stomach upset, allergic reactions, and fatigue. It can weaken prescription drugs and interact with antidepressants. Dogs, cats, horses, sheep, and goats face poisoning risks too.