<i>Carmichaelia williamsii</i>(Carmichaelia williamsii) hero image

Giant Flowering Broom

Carmichaelia williamsii

Not specified

This native plant, known as Giant Flowering Broom (scientific name: Carmichaelia williamsii ), is a remarkable species endemic to New Zealand. It is characterized by its unique features, ecological role, and cultural significance. This comprehensive guide provides detailed information on its care, propagation, and importance within the New Zealand ecosystem. Understanding this plant contributes to the appreciation of our rich biodiversity and heritage, and supports conservation efforts for Browse All .

<i>Carmichaelia williamsii</i>(Carmichaelia williamsii) showing <i>Carmichaelia williamsii</i>growth habit
Image credit: Giant Flowering Broom ( C. williamsii ) - Photo via Wikipedia

Plant Description

Identification and Physical Characteristics

Carmichaelia williamsii is a robust, many-branched shrub that typically grows between 2 and 4 metres in height and width Discover more about native ferns. It is immediately distinguished by its stout, woody base and its striking, flattened green branches (cladodes). These branches are unusually wide for a native broom, measuring 5 to 12 millimetres across, and often feature a characteristic wavy edge. While adult plants are predominantly leafless, they may produce small, short-lived leaves in shaded areas or on young reversion shoots. The crowning glory of this species is its floral display; the pea-like flowers are the largest of any native broom, reaching up to 25 millimetres in length. They are typically creamy-yellow or pale yellow, often adorned with delicate purple or red veins. Flowering primarily occurs from July to October, followed by the development of dry, flattened pods containing distinctive orange and black mottled seeds.

Quick Facts

Carmichaelia williamsii quick facts
Scientific Name Carmichaelia williamsii
Height 2-4 metres
Spread 2-4 metres
Water Needs Low to Moderate
Light Full Sun to Partial Shade
Frost Tolerance Moderate
Salt Tolerance High
Growth Rate Medium
Lifespan 5-15 years

Climate Best Suited to

Carmichaelia williamsii is well adapted to various New Zealand regions.

Regional Suitability

City suitability for Carmichaelia williamsii
City Climate Suitability
Auckland High
Wellington High
Christchurch Medium

Plant Habitat

Natural Occurrence and Ecology

Naturally, Carmichaelia williamsii is a strictly coastal species. Its primary stronghold is on the northern offshore islands of New Zealand, including the Poor Knights, Alderman, and Mercury Islands. On the mainland, it is now extremely rare, with only a few remnant populations persisting near East Cape. Its preferred habitat includes the margins of coastal forests, open scrubland, steep cliff faces, and stable rocky slopes (talus). It is a hardy pioneer of the coastal edge, capable of growing in thin, rocky soils where it is often exposed to the full force of the Pacific Ocean.

Growing Requirements

Soil Requirements

Prefers well-draining soil rich in organic matter.

  • Well-drained
  • Loamy

Light Requirements

Grows best in partial to full sun.

  • Full Sun
  • Partial Shade

Water Requirements

Requires regular watering during dry periods.

  • Regular
  • Drought tolerant once established

Planting and Establishing

Establishment Guidance

Plant Carmichaelia williamsii in a hole twice the width of the root ball.

Proper establishment is key to long-term success. Ensure the planting hole is twice as wide as the root ball and backfill with a mix of native soil and organic compost. Initial watering should be thorough, followed by regular monitoring during the first two growing seasons to ensure the plant becomes well-anchored and resilient to local climate fluctuations.

Ecological Role

Biodiversity and Ecosystem Impact

Provides habitat and food for native fauna.

This species plays a vital role in its local ecosystem, providing essential habitat and food sources for native biodiversity. Its presence supports complex ecological interactions, contributing to the overall health and stability of the environment. By planting this native, you help maintain the intricate web of life that defines New Zealand's unique natural heritage.

Uses & Significance

Garden Uses

Ideal for rock gardens and native borders.

  • Rock gardens
  • Ground cover

Ecological Value

Supports local pollinators.

  • Birds
  • Insects

The versatility of this plant makes it a valuable addition to many New Zealand gardens. Beyond its aesthetic appeal, it offers practical benefits such as soil stabilization and shelter for smaller plants. Its historical and modern uses highlight its enduring importance in both traditional practices and contemporary landscape design, making it a thoughtful choice for any restoration or gardening project.

Landscaping Uses

Adds texture and form to modern landscapes.

In a landscape setting, this plant provides excellent structure and year-round interest. Its unique form and foliage can be used to create focal points or as a cohesive element in a larger native planting scheme. It pairs well with other New Zealand natives, allowing for the creation of diverse and resilient garden environments that reflect the natural beauty of Aotearoa.

Seasonal Care Calendar

Spring

Mulch and apply balanced fertilizer.

  • Fertilize
  • Mulch

Summer

Water deeply during dry spells.

  • Water
  • Check for pests

Autumn

Tidy up any dead foliage.

  • Prune lightly
  • Plant new specimens

Winter

Protect from heavy frosts if necessary.

  • Frost protection

When to Prune and How Much

Minimal pruning required to maintain shape.

  • Remove dead branches

Always use clean, sharp tools.

Regular maintenance through careful pruning ensures the plant remains healthy and maintains its desired shape. Focus on removing dead or diseased wood to promote better air circulation and prevent the spread of pathogens. Proper pruning techniques help the plant direct its energy toward new growth, resulting in a more robust and aesthetically pleasing specimen in your garden.

How to Grow Giant Flowering Broom

Cultivating Carmichaelia williamsii ( Carmichaelia williamsii ) is a rewarding endeavor for any gardener interested in New Zealand's unique flora. This species can be propagated through several methods, each requiring specific conditions to ensure healthy growth and establishment. Successful propagation depends on mimicking the plant's natural environment and providing consistent care during the early stages of development.

From Seed

Propagation from seed is often the most effective way to produce a large number of Carmichaelia williamsii plants. Seeds should be collected when they are fully mature. Freshly collected seeds generally have higher germination rates than those that have been stored for long periods.

Sow fresh seeds in a seed-raising mix.

  1. Collect seeds
  2. Sow in trays

Keep moist until germination.

From Cuttings

For those wishing to replicate specific characteristics of a parent plant, vegetative propagation via cuttings is the preferred method. This technique ensures that the new plants are genetically identical to the source. Semi-hardwood cuttings taken during the appropriate season usually yield the best results for this species.

Take semi-hardwood cuttings in late summer.

  1. Take cuttings
  2. Use rooting hormone

Additional Methods

Depending on the growth habit of Carmichaelia williamsii , other methods such as division or layering might also be viable options. Division is particularly suitable for plants that form dense clumps or have spreading root systems, while layering can be used for species with trailing or flexible stems.

Division can be done in early spring.

  1. Divide clumps
  2. Replant immediately

Pests & Diseases

Generally resistant to major pests.

While generally resilient, maintaining optimal growing conditions is the best defense against pests and diseases. Regular monitoring allows for early detection of any issues, which can often be managed through cultural practices or organic interventions. Ensuring good air circulation and appropriate watering will significantly reduce the plant's susceptibility to common environmental stresses and pathogens.

Cultural Significance

While specific records are limited for this species, Carmichaelia williamsii sits within a wider native plant tradition valued for its ecological role and local stewardship.

  • Traditional medicine
  • Crafts

Bonus Tip

Excellent for low-maintenance gardens.

For an extra boost, consider applying a thin layer of high-quality organic mulch around the base of the plant. This helps retain soil moisture, suppresses weeds, and gradually improves soil structure as it breaks down. This simple step can make a significant difference in the plant's overall vigor and resilience, especially during periods of extreme weather.

Plant Conservation

Conservation Status

The Giant Flowering Broom is currently classified as "At Risk - Relict." This status reflects a species that has suffered a significant decline and is now largely confined to island refugia. The primary threats include habitat loss and modification, as well as browsing by introduced animals such as sheep, goats, and cattle. A critical threat is seed predation by rats, which can significantly reduce the recruitment of new plants. Furthermore, the decline of native pollinating birds, such as the Bellbird and Tūī, which are attracted to the large flowers, can impact the plant's reproductive success.