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Crassula helmsii

New Zealand pygmyweed

SunFull Sun – Part Shade

HardinessZones 7–10

Overview

Crassula helmsii is a low, succulent, mat-forming perennial in the family Crassulaceae that grows submerged, as a floating mat, or as a creeping land form on wet mud. The slender stems root at the joints and bear pairs of narrow, fleshy, bright green leaves 0.2–0.8 inch (4–20 mm) long, joined in a small sheath around the stem. Submerged shoots are lax and elongated; emergent and terrestrial shoots are shorter and denser. From summer to autumn it produces tiny solitary white to pale pink four-petalled flowers about 0.1 inch (3 mm) across on short stalks from the leaf axils. The plant spreads aggressively from the smallest stem fragments, each of which can root and form a new colony, and it grows year-round in mild conditions. It forms dense mats that blanket the surface and shallow margins of ponds, ditches, and lake edges, crowding out other plants and lowering oxygen levels in the water. It is listed as an invasive non-native species and is banned from sale in the United Kingdom and the European Union.

Native Range

Native to Australia and New Zealand, where it grows in and around still or slow fresh water. It has been introduced widely in Europe and elsewhere through the aquatic plant trade and is now a damaging invasive species across much of north-western Europe.

Suggested Uses

Crassula helmsii has no permitted horticultural use in the United Kingdom or European Union, where its sale, planting, and transport are banned. It was formerly sold as an oxygenating pond plant before its invasive behaviour was recognised. It now serves only as a subject for research into the control of invasive aquatic plants.

How to Identify

Recognized by dense, low mats of slender stems with paired, narrow, fleshy bright green leaves 0.2–0.8 inch (4–20 mm) long joined by a small sheath at the base. The tiny white to pale pink four-petalled flowers sit singly on short stalks from the leaf axils through summer and autumn. The succulent paired leaves and mat-forming growth in and beside water separate it from native water-starworts (Callitriche spp.) and stoneworts, which have softer or whorled foliage.

Appearance

Size & Dimensions

Height1" - 5"

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~14 weeks

Summer Fall

Small white to pale pink flowers appear from June to September, each about 0.1 inch (3 mm) across and held singly on short stalks in the leaf axils. Flowering and seed set are limited in cooler regions, where the plant spreads mainly by stem fragments rather than seed. Even short pieces of broken stem can root and start new growth.

Detailed Descriptions

Flower Description

white to pale pink

Foliage Description

bright green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 4-10 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 Range5.0 - 8.0(Neutral)

357912

Soil Types

silt loam sand

Drainagewet

Water & Climate

Water Needs

High

Frost Tolerance

hardy

Time to Maturity

1 year

Care & Maintenance

Care Guide

Crassula helmsii grows in full sun to part shade in shallow fresh water and on wet mud, tolerating water from a few inches deep to damp ground and a wide range of temperatures. It survives mild frosts, low light, and brief drying, and it grows actively through much of the year. Because it spreads from tiny fragments and is extremely hard to remove, it is not planted in gardens or ponds, and its sale and movement are prohibited in the United Kingdom and European Union. Where it has invaded, control relies on careful physical removal, shading, or treatment, with strict disposal of all plant material. Cut or pulled fragments are bagged and destroyed rather than composted or returned to water.

Pruning

Routine pruning does not apply. Any cutting, raking, or pulling releases fragments that can each grow into new plants, so all removed material is collected and destroyed, and tools and boots are cleaned to keep fragments from reaching other water.