Epilobium tetragonum aka Square Stalked Willowherb

Epilobium tetragonum is a perennial called square-stalked willowherb. It belongs to Onagraceae, the willowherb family. The RHS also lists the Welsh name helyglys pedrongl. Its native range covers Europe, including Britain, and reaches western Asia. Plants For A Future places Sweden within that range. The RHS confirms that it is native to Britain and Ireland. An older name for this species is Epilobium adnatum.

This plant grows to about 2 feet (0.6 metres). It flowers from July to August. Each flower has both male and female organs. The species is self-fertile, so it does not need another plant for fertilisation. The supplied species accounts give no further detail on flower form. They also give no mature spread or fruit description. The RHS describes leaf and stem traits for the genus, rather than this species.

In the wild, it grows in damp woodland clearings. It also occurs along hedge banks, in ditches, and on cultivated ground. These habitats fit its preference for moist soil. Plants For A Future recommends a sunny site for cultivation. It also reports that the plant can grow in light shade. Woodland gardens and shaded edges are listed as suitable settings. A hedgerow offers another possible place for it in a garden.

It grows in sandy, loamy, or clay soils. The soil should hold moisture while allowing excess water to drain. It accepts mildly acid, neutral, and mildly alkaline conditions. The evidence gives no hardiness zone or winter temperature limit. Sow seed where it will grow in early spring. Seed can also be sown as soon as it ripens. Divide established plants in spring or autumn. Large divisions can go straight into their final sites. Grow small divisions in pots under a cold frame first. Plant them out in spring once they have rooted well.

The leaves are edible when cooked and serve as a vegetable. Plants For A Future gives this use a low rating. It records no known medicinal uses or other uses. It also lists no known hazards, though this is not a full safety assessment. The supplied evidence names no pests or diseases for this species. Plants For A Future does not flag it as a weed risk.