Hypericum calycinum aka Aaronsbeard St. Johns Wort

Hypericum calycinum is a low shrub in the Hypericaceae family. Gardeners call it Aaron's beard or creeping St John's wort. Its native range runs from southeast Bulgaria into northern Türkiye. It grows in grassland and open woods. People also grow it outside its native range. It has spread into parts of the United States.

This fast-growing shrub makes a broad mat of stems. Sources give garden heights of 1–3.3 feet [0.3–1 m]. Reported spreads range from 1.5–4.9 feet [0.45–1.5 m]. Creeping stems and rhizomes can carry it beyond those widths. Some stems lie flat, while others rise from the ground. Young stems have ridges; older stems become smooth and round. The simple, oval leaves grow in opposite pairs. They have smooth edges and dark green tops. Their undersides have a blue-green tint. Leaves may turn purple in fall. They stay on in mild winters but may fall in cold weather. Hard winters can kill the top growth. The plant grows back in spring and flowers on new shoots.

Large yellow flowers open alone or in small groups. Most have five petals around a dense cluster of stamens. Flowers can reach about 0.25 feet [0.075 m] across. Bloom dates vary between sources and growing regions. Missouri Botanical Garden lists July and August. RHS describes flowers through summer and into autumn. The flowers draw butterflies. NC State describes the fruit as a capsule with several chambers. RHS instead reports red, berry-like fruit that turns black in autumn.

Give this plant moist soil that drains well. It accepts clay, loam, sand, and chalk. It also grows in acid, neutral, or alkaline soil. It tolerates sun and shade, but sun gives the best bloom. Established plants withstand drought and dry soil. Sources list USDA zones 5–9, with NC State extending to zone 10. RHS rates it H5. Use it beneath trees, along borders, or in rock gardens. Mass planting on banks can help hold soil. Spring cutting can keep the ground cover dense. Its flowers also yield a yellow-orange dye.

Routine care is light, but spreading growth needs control. It can crowd out small plants. Some parts of Washington and California regard it as invasive. Rust may attack it, and honey fungus occurs rarely. Wilt and root rot can cause trouble in hot, humid climates. No edible or medicinal uses are recorded by PFAF. NC State warns that eating it can cause severe illness. Contact can also cause dermatitis. RHS warns against fruit consumption by people and pets.