1 / 15

Epilobium tetragonum

square-stemmed willowherb

Onagraceae Europe, western Asia, North Africa

Attracts Pollinators

SunFull Sun – Part Shade

WaterHigh

HardinessZones 4–9

Overview

Epilobium tetragonum is an erect perennial of the willowherb family, growing 1-2.5 feet (30-80 cm) tall from a basal rosette that overwinters. The stems carry raised lengthwise lines or ridges that give a four-angled look, the feature behind the common name. Narrow, lance-shaped leaves 1.2-3 inches (3-8 cm) long are mostly opposite, stalkless, and shallowly toothed, their bases running down the stem as the ridges. Small four-petalled flowers about 0.24-0.3 inch (6-8 mm) across open pale rose-pink in summer, each petal notched at the tip; the club-shaped, undivided stigma helps separate it from related species. Flowers give way to long, slender capsules 1.6-3 inches (4-8 cm) long that split to release many seeds, each carried on a tuft of white hairs. The plant spreads readily by wind-blown seed and behaves as a colonising weed of damp, disturbed ground. Foliage dies back in autumn.

Native Range

Native across much of Europe and into western Asia and North Africa, and naturalised in North America and elsewhere. Grows in damp open habitats including ditch sides, stream banks, marshy field edges, waste ground, and cultivated soil, mostly in lowland areas.

Suggested Uses

Occurs in damp wildlife areas, pond margins, and naturalistic plantings where its seed spread is acceptable. It is seldom planted on purpose and is more often managed as a volunteer in moist ground. Its colonising habit makes it unsuited to formal or low-maintenance borders.

How to Identify

Identified by the four raised ridges running down the stem, the narrow toothed leaves with bases decurrent onto those ridges, and the small pale pink four-petalled flowers about 0.24-0.3 inch (6-8 mm) across. The club-shaped, undivided stigma sets it apart from Epilobium hirsutum and other willowherbs with four-lobed stigmas. Its nearly hairless, ridged stem separates it from the densely hairy E. parviflorum.

Appearance

Size & Dimensions

Height1' - 2'6"

Width/Spread8" - 1'4"

Colors

Flower Colors

pink

Foliage Colors

green

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~8 weeks

Summer

Flowers from July to August, sometimes into September in mild seasons. Individual flowers are short-lived, but a plant opens new flowers in succession over about 6-8 weeks. Flowering begins later than in many willowherbs. Cool, wet summers extend the period, while drought shortens it.

Detailed Descriptions

Foliage Description

green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 4-8 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.5 - 7.5(Neutral)

357912

Soil Types

loam clay silt

Drainagemoist

Water & Climate

Water Needs

High

Frost Tolerance

hardy

Time to Maturity

1-2 years

Care & Maintenance

Care Guide

Grows in sun to partial shade on consistently moist to wet soils and tolerates a range of soil types. It needs no attention to persist and readily colonises bare, damp ground from seed. In gardens it appears as a self-sown weed and can be removed by hoeing or hand-pulling before the capsules ripen. Established plants tolerate short dry spells but flower and seed less on dry sites. Heavy seedling flushes follow soil disturbance near parent plants.

Pruning

Cut stems to the ground in autumn, or remove flowering shoots before the capsules open to limit self-seeding. No pruning is needed for plant health. Taking out plants before seed set is the main way to curb their spread.