Stream Willowherb

Botanical name: Epilobium macropus
Synonyms: None
Common name(s): Stream Willowherb, Willowherb

About:
Stream Willowherb is a distinctive mat-forming perennial native herb adapted to life beside flowing mountain streams. Its creeping stems root freely along their length, allowing it to form dense carpets that often extend into shallow running water.

Delicate white flowers appear throughout summer, followed by slender seed capsules containing silky wind-dispersed seeds. It is one of New Zealand’s most specialised willowherbs and is highly regarded by alpine plant enthusiasts.

Natural habitat:
Occurs naturally along the margins of mountain streams, seepages, springs and permanently wet alpine soaks.

It frequently grows partially submerged in flowing water or on constantly saturated gravels and moss-covered rocks in montane and alpine environments.

Growing environment:
Requires permanently moist to wet conditions with cool temperatures and excellent water quality. It performs best in full sun or light shade where its roots remain constantly damp.

Unlike most willowherbs, this species does not tolerate drought or prolonged heat and performs poorly in humid lowland climates without moving water.

Endemic distribution:
Endemic to New Zealand.

Found in the central North Island mountains and southern North Island ranges, and throughout much of the eastern South Island in suitable montane and alpine habitats.

Height:
2–10 cm tall

Width:
Spreads widely by creeping stems, often forming mats exceeding 50 cm across

Flowering:
November–March

Fruiting:
January–May

Uses:
• Alpine gardens
• Stream margins
• Bog and seepage gardens
• Wetland restoration
• Native water gardens
• Botanical collections
• Pollinator plant

Appearance:
Growth habit:
A dense, creeping perennial herb forming extensive mats by slender rooting stems.

The stems spread across wet gravel, moss and shallow flowing water, rooting wherever they contact suitable substrate.

Stems:
Slender green to purplish stems creep along the ground and root freely at the nodes.

Unlike many related species, the stems are evenly covered with short, stiff hairs throughout their length, giving them a slightly rough texture.

Leaves:
Leaves are mostly opposite and closely arranged, becoming alternate only near the flowering tips.

They are bright yellowish-green, elliptic to narrowly ovate and usually 6–15 mm long, with smooth to very finely toothed margins. The undersides are often slightly glaucous and paler than the upper surface.

Flowers:
Delicate white flowers arise singly from the leaf axils throughout summer.

Each flower has four deeply notched petals that may develop a soft pink flush after pollination. Bright yellow anthers contrast beautifully against the white petals.

Fruit:
Long, slender seed capsules develop after flowering and are held on remarkably elongated stalks that may reach over 10 cm in length.

The smooth capsules eventually split open to release numerous tiny seeds.

Seeds:
The seeds are small, orange to orange-brown and equipped with a persistent tuft of silky white hairs.

This unusually long-lasting coma assists wind dispersal and distinguishes the species from most other New Zealand willowherbs.

Ecology:
Stream Willowherb is an obligate wetland species almost exclusively confined to permanently wet alpine habitats.

Its creeping mats help stabilise stream margins and seepage areas while providing habitat for aquatic and semi-aquatic invertebrates. The flowers attract native pollinating insects, and the lightweight seeds are dispersed by wind to newly disturbed wet habitats.

The foliage is occasionally browsed by introduced mammals including deer, chamois and possums.

Associated habitats:
Mountain streams, alpine seepages, springs, bog margins, mossy gravels, herbfields and permanently wet alpine flushes.

Associated species commonly include Ourisia, mountain buttercups, Carex species, Plantago species, native mosses, liverworts and other moisture-loving alpine herbs.

How to grow:
Plant in full sun or light shade where the roots can remain permanently moist. The ideal position is beside a flowing stream, pond overflow or seepage, or in a container standing partially submerged in clean water.

Use a gritty, moisture-retentive growing medium and ensure water is refreshed regularly if grown in pots. Protect plants from drying winds, hot summers and prolonged drought.

This species is best suited to cool alpine gardens or specialist wetland collections.

Propagation:
Propagation is readily achieved from fresh seed or by dividing rooted stem sections.

Creeping stems root naturally and can be separated to establish new plants. Fresh seed germinates well when sown onto constantly moist growing media under cool conditions.

Cultural use:
Although no specific traditional Māori uses have been widely documented, Stream Willowherb is an important component of New Zealand’s alpine wetland ecosystems and contributes to the remarkable diversity of the country’s native mountain flora.

Garden value:
Stream Willowherb is one of New Zealand’s most attractive moisture-loving alpine herbs. Its lush green mats, elegant white flowers and unusual streamside growth habit make it a rewarding species for specialist alpine gardens, seepage beds and water features.

Although more demanding than most willowherbs, it is an exceptional choice for gardeners able to provide the permanently moist, cool conditions that closely resemble its natural habitat.