Nau

Botanical name: Lepidium oleraceum
Synonyms: Lepidium oleraceum var. acutidentatum, Lepidium oleraceum var. frondosum, Lepidium oleraceum var. serrulatum
Common name(s): Nau, Cook’s scurvy grass, Ngau, Naunau, Heketara

About:
Lepidium oleraceum is a distinctive endemic New Zealand coastal herb in the cabbage family, Brassicaceae. It is a strongly aromatic, much-branched perennial with fleshy toothed leaves and small fragrant white flowers.

Historically it was far more widespread, but today it is largely restricted to offshore islands, rock stacks and exposed coastal headlands where seabirds maintain nutrient-rich, regularly disturbed soils.

The species is culturally and historically significant. It was used by Māori as an edible green, while visiting European crews collected it as an antiscorbutic vegetable because of its high vitamin C content. It is particularly associated with the voyages of Captain James Cook, giving rise to the common name Cook’s scurvy grass.

Despite this long history of human use, the species is now highly threatened and is classified as Threatened – Nationally Endangered.

Natural habitat:
Strictly coastal in its modern distribution.

Nau usually grows in friable, highly fertile soils associated with seabird roosts, nesting colonies and burrows. It is particularly characteristic of guano-enriched ground, rock crevices, exposed headlands, offshore stacks and small predator-free islands.

Plants may also occur beneath taller vegetation near petrel or shearwater burrows, on sand or gravel beaches and occasionally on disturbed coastal structures such as seawalls.

Historical records from the upper Waitaki Valley indicate that the species was formerly capable of growing well inland and probably occupied a much broader ecological range before human settlement and associated environmental change.

Growing environment:
Full Sun, Coastal Exposure, Wind, Salt Spray, High-fertility Soil, Disturbed Ground & Free-draining Soil tolerant.

Nau performs best in open, well-aerated soils with high nutrient levels, especially nitrogen, phosphorus and potassium.

It is adapted to repeated disturbance and nutrient enrichment associated with seabirds and generally performs poorly in stagnant, heavily shaded or nutrient-poor sites.

Despite its coastal toughness, it can be difficult to maintain in cultivation because of its susceptibility to insect pests and fungal or fungus-like diseases.

Endemic distribution:
Endemic to New Zealand.

Occurs naturally through the Kermadec Islands, Three Kings Islands, North Island, South Island, Stewart Island/Rakiura and Bounty Islands.

Plants traditionally included under the name Lepidium oleraceum from the Chatham, Antipodes, Snares and Auckland Islands differ somewhat from the typical form, and their taxonomic status has been subject to further study.

Today most surviving populations are highly localised and concentrated on offshore islands, sea stacks and other coastal refuges.

Height:
Usually up to approximately 50 cm tall, occasionally approaching 1 m under favourable conditions.

Width:
Usually less than 1 m across, though vigorous plants may approach approximately 1 m in spread.

Flowering:
Flowers may be produced throughout much of the year but are most abundant from September–March.

Small fragrant white flowers are borne in terminal and lateral racemes.

Each flower has four white petals and four yellow stamens.

Fruiting:
Mainly December–April, although fruit may occur outside this period.

Seed development is rapid, so flowers, immature capsules and ripe fruit are often present on the same plant at the same time.

The fruits are small flattened silicles approximately 3–5 mm long.

Uses:
• Historic food plant
• Traditional Māori vegetable
• Vitamin C-rich edible green
• Conservation planting
• Offshore island restoration
• Seabird-colony restoration
• Coastal ecological restoration
• Specialist native plant collections
• Educational and historical gardens
• Research and ex-situ conservation

Appearance:
Growth habit:
A glabrous, much-branched perennial herb with erect to decumbent stems.

Plants are usually compact but may become broad and spreading under favourable conditions.

The lower stems can become somewhat woody with age, while the upper stems remain more flexible and succulent.

All parts of the plant have a strong pungent smell and flavour when bruised.

Rhizomes/Stems:
Stems are stout, flexible and much branched, ranging from upright to spreading or decumbent.

The lower portions may become slightly woody.

Unlike many related Lepidium species, the stems and flower stalks are hairless.

Leaves:
Leaves are dark to bright green, fleshy and somewhat succulent.

They measure approximately 2–10 cm long and 1.5–4 cm wide and become progressively smaller toward the shoot tips.

Shape ranges from narrow-oblanceolate to obovate or elliptic.

The margins are deeply and evenly toothed, particularly toward the outer portion of the leaf.

Leaf bases narrow gradually into winged petioles.

The leaves are strongly pungent when crushed.

Flowers:
Flowers are small, fragrant and arranged in terminal and lateral racemes that may reach approximately 15 cm long when in fruit.

Sepals are approximately 1–2 mm long.

The four petals are white, obovate to spoon-shaped and approximately 2.5–3.5 mm long.

There are four conspicuous yellow stamens.

Fruit:
The fruit is a broadly ovate silicle approximately 3–5 mm long and 2.5–5 mm wide.

It is generally truncate at the base and acute at the tip.

The margins are not winged, a useful feature for distinguishing the species from several other native and introduced Lepidium species.

Seeds:
Seeds are approximately 1.5–2 mm long, ovoid and orange-brown to brown.

When wet they become mucilaginous, allowing them to adhere to animals, soil and other surfaces.

Dispersal is mainly through attachment, with wind and water also likely contributing.

Ecology:
Lepidium oleraceum is closely associated with seabird-driven coastal ecosystems.

Seabirds enrich soils with guano, disturb the ground through nesting and burrowing, and maintain the open, nutrient-rich environments in which nau thrives.

The decline of seabird colonies across much of mainland New Zealand has therefore contributed directly to the decline of the plant.

Nau is also vulnerable to a wide range of introduced threats, including rats and other rodents, snails, livestock and insect herbivores such as aphids, leaf miners, diamondback moth and cabbage white butterfly.

White rust caused by Albugo candida can cause severe damage, and other fungal diseases may also become problematic in cultivation.

Its continuing dependence on small, isolated offshore populations makes the species especially vulnerable to localised disturbance.

Associated habitats:
Seabird nesting colonies, petrel and shearwater burrow systems, offshore islands, sea stacks, coastal cliffs, windshorn headlands, guano-enriched rock crevices, gravel beaches, sand beaches and disturbed coastal margins.

It commonly occurs where vegetation is kept open by seabird activity, salt exposure and wind.

How to grow:
Nau is fast growing but can be demanding to maintain for long periods.

Grow in full sun in loose, free-draining but fertile soil.

Soils should contain relatively high levels of available nutrients, particularly nitrogen, phosphorus and potassium, to mimic the guano-enriched substrates of natural seabird colonies.

Good airflow is important to reduce fungal disease.

Avoid dense shade, stagnant humidity and heavy waterlogged soils.

Plants should be monitored regularly for aphids, caterpillars, leaf miners and signs of white rust.

Although technically perennial, it is often most practical to grow the species as a short-lived perennial or annual and maintain populations through regular seed propagation.

For conservation planting, only appropriately sourced material should be used to avoid compromising the genetic integrity of wild populations.

Propagation:
Very easy from fresh seed.

Sow mature seed onto a fertile, freely draining propagation mix and cover lightly.

Germination is usually rapid under warm, bright conditions with consistent moisture.

Semi-hardwood cuttings can also be used.

Because plants can be short-lived and vulnerable to disease, maintaining multiple generations from fresh seed is generally the most reliable cultivation strategy.

Cultural use:
Nau has a long history of Māori use as an edible leafy plant.

The Māori name is of ancient Polynesian origin, with related names and food-use traditions occurring elsewhere across the Pacific.

Young leaves may be eaten raw or cooked and have a hot, cress-like flavour.

The plant later became historically important to European explorers because its vitamin C-rich foliage was collected and eaten to help prevent scurvy.

During James Cook’s voyages to New Zealand, plants described as “scurvy grass” were gathered for this purpose, giving rise to the enduring English name Cook’s scurvy grass.

Garden value:
Nau is more valuable as a conservation, educational and historical plant than as a conventional ornamental.

Its bright green fleshy foliage, compact branching habit and numerous small white flowers can nevertheless be attractive in exposed coastal gardens and specialist native collections.

Its greatest value lies in representing one of New Zealand’s most threatened coastal herbs and demonstrating the close ecological relationship between native plants and seabird colonies.

Because wild populations are vulnerable, cultivated material should be maintained responsibly and never collected directly from natural populations without appropriate authority.