Coastal Peppercress
Lepidium banksii
Coastal Peppercress (scientific name: Lepidium banksii ), known to Māori as Nau, is New Zealand's rarest and most critically endangered native peppercress. Endemic to the northern South Island, this fleshy-leaved perennial herb holds deep historical significance, having been collected by Captain Cook's crew in 1770 to combat scurvy. Today, it survives only in a handful of managed sites around Tasman and Golden Bays. Its disappearance from much of its former range is a stark reminder of the fragility of coastal ecosystems. This guide provides an essential overview of its botanical characteristics, its unique ecological requirements, and the intensive efforts required to ensure its survival in Threatened Species .
Plant Description
Lepidium banksii is a low-growing, rambling perennial herb that typically reaches a height of 20 to 50 centimetres. It forms sprawling clumps of stout, upright to spreading stems that can become quite woody at the base as the plant matures. The foliage is dark green and distinctive; the leaves are fleshy, often coarsely serrated along the margins, and emit a sharp, pungent aroma when crushed. A deciduous herb, it dies back to its persistent rootstock each winter, with fresh growth emerging in early spring. In summer, from November to January, the plant produces small white flowers. These are followed by relatively large, dry, and deeply notched seed pods, which are significantly larger than those of its more common relative, Cook's Scurvy Grass ( L. oleraceum ). The entire plant exhibits adaptations to the high-nutrient, salt-laden environments of the New Zealand coastline.
Quick Facts
| Scientific Name | Lepidium banksii |
|---|---|
| Height | 20-50 centimetres |
| Spread | 40-80 centimetres |
| Water Needs | Moderate (Moist) |
| Light | Full Sun |
| Frost Tolerance | Moderate (Dies back) |
| Salt Tolerance | High |
| Growth Rate | Fast (Seasonal) |
| Lifespan | Short-lived (Often treated as annual) |
Climate Best Suited to
Coastal Peppercress is a maritime specialist, perfectly suited to the temperate coastal climates of the northern South Island. It thrives in high-light, high-nutrient environments with clean, breezy air. It requires a climate that remains relatively frost-free during its peak growing season in spring and summer. It does not tolerate high humidity or stagnant air, which can encourage the many fungal diseases to which this species is exceptionally prone. In a garden or restoration setting, it needs a position that mimics the fresh, salt-laden air of an estuary or rocky coastal point.
Regional Suitability
| City | Climate Suitability |
|---|---|
| Nelson | Excellent |
| Blenheim | Good (Requires coastal breeze) |
| Wellington | Good |
| Auckland | Moderate (High disease risk) |
| Christchurch | Moderate (Requires humidity) |
Plant Habitat
Historically, Lepidium banksii was found along the rocky coastlines and estuaries from Karamea on the West Coast to the Marlborough Sounds. Its preferred habitat is extremely specific: it thrives on boulder banks and shell banks, often in areas where seabirds or penguins congregate. These sites are characterized by very high nutrient levels (guano-rich) and frequent salt spray. It is a true coastal specialist, often growing just above the high-tide mark where it can access the minerals and nutrients provided by marine life. It requires open, sun-drenched ground with minimal competition from taller vegetation.
Plant Conservation
Coastal Peppercress is currently classified as "Nationally Critical" and is one of New Zealand's most threatened plants. It is effectively functionally extinct in the wild, with its survival entirely dependent on intensive human management. Threats are numerous: predation by cabbage white butterflies and diamondback moths, browsing by feral pigs and deer, and a high susceptibility to diseases like powdery mildew and white rust. Habitat loss through coastal development and the stabilization of gravel banks has also played a role. Current conservation efforts involve hand-weeding, manual pest control, and the establishment of new populations in highly protected, guano-rich sites.
Growing Requirements
Soil Requirements
Must be extremely nutrient-rich and free-draining. Thrives in guano-affected ground.
- High nitrogen and phosphate levels
- Gritty or shelly texture
- Well-drained but stays moist
Light Requirements
Requires full, direct sun to maintain its vigor and produce flowers.
- Maximum solar exposure
- Clear, breezy position
- Dislikes any overhanging shade
Water Requirements
Needs consistent moisture during the growing season. Salt spray is tolerated.
- Regular watering in summer
- Avoid stagnant water at roots
- High humidity is beneficial
Planting Guide
Planting Lepidium banksii is a significant challenge even for experienced gardeners. Select a sun-drenched coastal spot or create a specialized raised bed. Use a mix of shell grit, coarse sand, and a high-nitrogen fertiliser (to mimic guano). Because the plant is so vulnerable to pests, it is often best grown in a large pot or a dedicated trough where it can be monitored daily. Plant in early spring to take advantage of the main growing season. Be prepared to treat it as an annual, as it is often very short-lived in cultivation. Provide consistent moisture and ensure the site has the best possible air movement to ward off fungal diseases.
Ecological Role
In its natural state, the Coastal Peppercress is a vital part of the specialized flora associated with seabird colonies. It acts as a "nutrient cycler," taking up the high levels of nitrogen and phosphorus from bird droppings and incorporating them into its fleshy tissues. It provides habitat and food for specialized coastal invertebrates. As a pioneer of the shell and boulder banks, it helps to stabilize these mobile environments. Its flowers are a nectar source for native flies and moths, which are the primary pollinators in the windy coastal environments it inhabits.
Uses & Significance
Garden Uses
Primarily grown for conservation purposes or in specialized native botanical collections.
- Feature plant for coastal restoration projects
- Educational subject for threatened species
- Unique addition to a "Captain Cook" themed garden
Known as Nau to Māori, it was once a widespread and valued food source.
- Traditionally eaten as a leafy green vegetable
- Historical link to Captain Cook's first voyage
- Symbolic of the extreme rarity of NZ coastal flora
Ecological Value
A key species for the biodiversity of northern South Island coastal margins.
- Highly adapted to bird-colony ecosystems
- Supports native coastal invertebrate communities
- Indicator of healthy, high-nutrient coastal habitats
Cultural Significance
While specific records are limited for this species, Lepidium banksii sits within a wider native plant tradition valued for its ecological role and local stewardship.
Landscaping Uses
Due to its extreme rarity and difficult nature, Lepidium banksii is rarely used in general landscaping. However, in specialized coastal restoration or educational landscapes, it is used to demonstrate the unique beauty of New Zealand's endangered flora. Its dark green, coarsely toothed leaves provide a rugged texture that looks best in gravel-heavy or coastal rockery designs. It can be an effective sprawling highlight among other salt-tolerant natives if its high-maintenance needs can be met by a dedicated gardener.
Seasonal Care Calendar
Spring
Rapid new growth emerges from the rootstock.
- Apply high-nitrogen fertiliser (e.g., blood and bone)
- Monitor daily for cabbage white butterfly larvae
- Ensure consistent moisture as growth peaks
Summer
The peak growing and flowering period.
- Continue intensive pest monitoring
- Water deeply during dry spells
- Enjoy the small white flowers
Autumn
Growth slows and seed pods mature.
- Collect fresh seeds for immediate sowing
- Reduce watering as the plant begins to die back
- Tidy up any diseased or yellowing stems
Winter
The plant is dormant below ground.
- Protect the rootstock area from excessive wet
- Ensure the site remains free of aggressive weeds
- Minimal above-ground maintenance required
When to Prune and How Much
Minimal pruning is needed, as the plant naturally dies back to its base each year.
- Remove all dead top growth in late autumn or early winter
- Snip off any diseased leaves immediately throughout the season
- Trim back any leggy stems to encourage more compact growth
Always use sterilized snips to avoid spreading diseases like white rust. Pruning should focus on maintaining hygiene and airflow within the sprawling mat of foliage, which is the best defense against the various rots and mildews that plague this species.
How to Grow Coastal Peppercress (Nau)
Growing Coastal Peppercress ( Lepidium banksii ) is a labor of love and a true commitment to conservation. This is perhaps one of the most high-maintenance native plants you can attempt to grow. The secret to success lies in two factors: nutrients and protection. In the wild, this plant only grows where seabirds provide a constant supply of rich guano. In your garden, you must replicate this with regular applications of high-quality organic fertiliser. More critically, you must be a "bodyguard" for the plant. Without daily monitoring for cabbage white butterfly caterpillars and aphids, the plant can be stripped to the ground in a matter of days. Traditionally, successful conservation growers often use fine netting to exclude pests. Furthermore, the plant is extremely susceptible to soil-borne and airborne fungal diseases. Providing perfect drainage and the best possible air movement is the only way to keep the foliage healthy. It is a challenging but vital species to keep in cultivation.
From Seed
Seeds are the primary way to propagate this species, but they have a short window of viability.
- Collect seeds from notched pods as soon as they turn brown.
- Sow fresh seeds immediately into a gritty, nutrient-rich mix.
- Keep in a warm, bright, and well-ventilated spot.
- Maintain consistent moisture without allowing the mix to become sodden.
Fresh seeds usually germinate within 1-2 weeks.
From Cuttings
Semi-hardwood cuttings can be taken, but they are often prone to rot.
- Take small tip cuttings from non-flowering shoots in spring.
- Use a very clean, sharp blade to make the cut.
- Insert into a mix of sharp sand and perlite.
- Keep in a high-humidity, airy, and bright environment.
Additional Methods
Self-seeding can occur if you create a "safe zone" of shell grit around the parent plant.
- Allow some seed pods to mature and drop naturally.
- Look for small, fleshy seedlings emerging in the grit.
- Carefully protect these from slugs and snails immediately.
Pests & Diseases
Lepidium banksii is a magnet for pests. Cabbage white butterfly caterpillars, diamondback moth larvae, and aphids are the most significant threats. Slugs and snails will also devour the fleshy leaves. Diseases are just as problematic: powdery mildew, white rust ( Albugo candida ), and various root rots ( Fusarium , Verticillium ) are common. Regular applications of organic fungicides and daily hand-picking of pests are often required to keep this species alive in a garden setting.
Bonus Tip
To truly replicate its natural home, try top-dressing your Coastal Peppercress with a layer of crushed oyster shells and a little bit of seaweed. This provides the minerals and the high-drainage environment the plant needs to survive.
Related Plants
Explore other native plants that share similar characteristics or habitats with Coastal Peppercress ( Lepidium banksii ), offering further insights into New Zealand's diverse flora. These related species provide context and opportunities for complementary planting in your garden.
