perennial

Square-stemmed willowherb

The Story

Plant Bio

Epilobium tetragonum, the square-stemmed willowherb, is an erect herbaceous perennial native to Europe, Western to Central Asia, and North Africa. The species is distinguished by its square-appearing stems featuring two to four raised lines running down from the leaf bases, and its narrowly lanceolate, slightly toothed leaves.

Small, pale pink to rose-purple flowers develop in leafy racemes during summer and fall, each featuring a floral tube with an internal ring of hairs and a four-lobed stigma. In fall, the plant produces a basal rosette of green foliage that persists into the next season.

Uses & Benefits:

Produces pale pink to rose-purple flowers on upright, square-angled stems.

Thrives in difficult, damp, calcareous clay soils where other perennials struggle.

Provides forage for generalist beneficial insects, small bees, and flies.

Essentials

At a Glance

Plant Type

Perennial

Height

40"

Width

20"

Light

Full Sun, Partial Sun

Zones

5, 6, 7, 8, 9

Water

High

Humidity

Medium

Lifespan

Perennial

Native To

Europe, Western to Central Asia, and North Africa

Toxicity

Non-toxic

Bloom period

Summer – Fall

In flower about 6 months of the year

Planting

How to Grow

Select a site with full sun to partial shade and moist, loam, clay, or sandy soil.
Ensure the soil is neutral to alkaline, as this species is highly adapted to calcareous conditions.
Water regularly to maintain consistent moisture, preventing the soil from drying out completely.
Provide moderate drainage to prevent total stagnation and root rot in excessively wet conditions.

Pro Tip

Look for the characteristic raised stem lines and the ring of hairs inside the floral tube to distinguish this species from similar willowherbs that easily hybridize in the garden.

Keep It Thriving

Care Guide

Do

Cut the stems back hard to ground level in late winter or early spring.
Apply a balanced organic mulch or fertilizer in spring to support new growth.
Divide the basal rosettes in spring to propagate and control spread.

Don't

Do not allow the soil to dry out completely, as drought tolerance is low.
Do not prune into old, dead woody stems during the active growing season.
Avoid planting in poorly drained, stagnant areas where root rot can develop.

Seasonal Care Calendar

Task	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Fertilize
Pest Check
Propagate
Spring Cleanup

Watch Out For

Pests & Diseases

Common Pests

Common Diseases

In the Garden

Landscape Uses & Companions

Landscape Uses

Rain gardensDamp bordersNaturalized areasWild gardens

Companion Plants

Carex Juncus effusus

Multiply

Propagation Methods

Good to Know

Common Questions

Square-stemmed willowherb questions

What zones can Square-stemmed willowherb grow in?

Square-stemmed willowherb is hardy in USDA Zones 5–9. Inside that range it survives winter in the ground; outside it, grow it as an annual or a container plant you protect.

When does Square-stemmed willowherb bloom?

Square-stemmed willowherb typically blooms in summer through fall. Exact timing shifts a week or two with your zone — Sow's bloom calendar maps it to your garden.

Does Square-stemmed willowherb need full sun?

Square-stemmed willowherb does best in full sun to part shade. In hot climates, afternoon shade keeps blooms fresher longer.