Blackstonia perfoliata

The most useful field mark is the pair of upper leaves, which are fused at their base so that each pair encircles the stem. The leaves are glaucous, meaning they have a bluish, waxy bloom, and they grow in opposite pairs with smooth edges.

The plant is slender and stiff. Its stems are erect and branch from the upper part, reaching 10–50 cm tall.

The flowers are bright yellow and stalked, borne in terminal clusters called cymes. Each flower measures 1–1.5 cm across. The calyx is divided into 6–10 narrow lobes, which are shorter than the corolla, and the petals number six to ten and form a short tube.

Inside each flower are six to ten stamens joined to the corolla, and a two-lobed stigma.

Yellow-wort is native from Europe to the Mediterranean and Iraq. Its native range includes Spain, Portugal, France, Italy, Greece, Turkey, Cyprus, Morocco, Algeria, Tunisia, Lebanon and Syria, and parts of central Europe such as Germany, Austria and Hungary. In northwestern Europe it occurs in Great Britain, Ireland, Belgium and the Netherlands.

It grows on chalk and limestone turf, where open grassland supports its low, branching habit. It also grows on coastal dunes.

Observations of the plant cluster in early summer, peaking in June and then in July. Few observations fall in the winter months.

Outside its native range, the plant is established in Argentina's northeast and northwest regions, the Azores, the North Island of New Zealand and Uruguay.

Yellow-wort is an annual, so each plant completes its life cycle within a single year. It grows as an erect herb with stiff stems that branch, and it reaches 10–50 cm in height.

In Britain, the plant flowers from June to October. Its flowering period is long relative to its single-year life, so the stems keep producing flowers through the summer and into autumn.

Yellow-wort supports a range of insects. Moths are its main leaf-eating partners, and the moth Stenoptilia zophodactylus is the best-established among them. A second moth, Gynnidomorpha permixtana, feeds on the plant only occasionally.

Its flowers attract insects, especially bees and wasps of the order Hymenoptera. Among these, several sweat bees of the genus Lasioglossum and the bee Halictus tumulorum visit most often, alongside the hoverfly Episyrphus balteatus. Other flies, beetles and bugs visit only occasionally.

The plant is also host to the downy mildew Peronospora chlorae, a pathogen that affects its leaves.