Kermadec Poplar
Botanical name: Homalanthus polyandrus
Synonyms: Carumbium polyandrum
Common name(s): Kermadec poplar
About:
A fast-growing, broad-leaved small tree endemic to the Kermadec Islands, Homalanthus polyandrus forms a light, open crown of large, thin, often triangular leaves on slender brittle branches. New foliage may be bright green, yellow-green or flushed red, while mature leaves are often broadly ovate to rhomboid and carried on unusually long stalks.
It is a classic pioneer and successional species, rapidly colonising slips, disturbed forest and open scrub. The species was once severely reduced by browsing from feral goats on Raoul Island but recovered strongly following goat eradication.
Its stems contain a white milky sap, typical of many members of the Euphorbiaceae.
Natural habitat:
Coastal scrub, lowland forest and upland forest of the Kermadec Islands.
It is especially associated with disturbed or regenerating vegetation and commonly establishes on slips, forest clearings and other open ground.
In established forest it may form part of the understorey or occur as a scattered emergent, while on recently disturbed sites it can temporarily dominate the canopy.
Growing environment:
Sun, Partial Shade, Coastal, Humidity & Free Draining tolerant. Extremely frost tender and best suited to warm, sheltered, frost-free sites.
Endemic distribution:
Endemic to the Kermadec Islands of New Zealand.
Naturally recorded from Raoul Island and Macauley Island.
The species became extremely scarce during the period of heavy browsing by introduced goats but has recovered dramatically on Raoul Island since the last goats were removed in 1986.
Height:
Up to approximately 10 m tall.
Width:
Approximately 4–7 m depending on site and growing conditions.
Flowering:
Throughout the year.
Produces slender green terminal flower spikes up to approximately 20 cm long, usually containing female flowers toward the base and numerous male flowers above.
Fruiting:
Throughout the year.
Produces reddish, three-angled fruits approximately 10–12 mm across. When mature, the fruit splits to expose seeds surrounded by a yellowish fleshy aril.
Uses:
• Kermadec Island restoration
• Warm-climate native gardens
• Coastal planting in frost-free areas
• Fast-growing pioneer tree
• Successional restoration planting
• Shelter for slower-growing forest species
• Botanical collections
• Wildlife planting
Appearance:
Growth habit:
A small, completely hairless tree with slender, rounded and relatively brittle branches.
Young stems may be slightly ribbed, with conspicuous scars left by fallen leaves.
The crown is generally broad and open, with foliage concentrated toward the ends of new growth.
Like many fast-growing pioneer trees, its overall form may be somewhat irregular where exposed to wind or repeated disturbance.
Leaves:
Leaves are thin, broad and carried on very long stalks that may equal or exceed the length of the leaf blade.
Adult leaves are approximately 30–200 mm across and usually broadly ovate, rhomboid or triangular in appearance. Their bases vary from wedge-shaped to truncate, while the tips narrow abruptly into an acute or acuminate point.
The upper surface is dark green, bright green or yellow-green, while the underside is paler and slightly glaucous.
New foliage may develop conspicuous red colouring.
Juvenile leaves can become much larger, occasionally approaching 300 mm across, and may be broadly ovate with cordate or truncate bases.
Flowers:
Flowers are produced in long, slender racemes at the ends of new shoots.
The flower spikes may reach approximately 200 mm long.
Male and female flowers are usually produced on the same inflorescence, although occasionally racemes are entirely male or female.
Female flowers are generally positioned near the base on longer stalks, while the numerous male flowers occur above on shorter stalks.
Male flowers contain approximately 30–35 tightly packed stamens.
Female flowers have a three-celled ovary and three styles.
Individual flowers are small and inconspicuous, giving the entire inflorescence a green appearance.
Fruit:
Fruit is leathery, reddish and distinctly three-angled, approximately 10–12 mm in diameter.
As the fruit matures it splits apart to expose the seeds.
Seeds are approximately 2.3–4.8 mm long and are almost completely enclosed by a yellow fleshy aril.
This conspicuous aril attracts fruit-eating animals responsible for seed dispersal.
Ecology:
Homalanthus polyandrus is an important early-successional species within Kermadec Island forests.
Its rapid growth and ability to establish on slips, disturbed ground and forest openings allow it to quickly occupy newly available habitat.
The canopy created by established plants can provide shelter and improved growing conditions for slower-growing forest species, helping drive succession toward more mature forest.
The fleshy arils surrounding the seeds are adapted for dispersal by fruit-eating animals.
Its recovery following the eradication of feral goats from Raoul Island is a notable example of the rapid ecological response possible after browsing pressure is removed.
The species remains naturally uncommon because of its very restricted island distribution.
Associated habitats:
Coastal scrub, lowland forest, upland forest, regenerating forest, slip scars, forest gaps, disturbed slopes and successional vegetation.
How to grow:
An easily cultivated tree where sufficient warmth is available.
Plant in a warm, sheltered, frost-free position in full sun or partial shade. A fertile, moist but free-draining soil encourages rapid growth.
Young plants should be protected from cold winds and any possibility of frost.
Even slight frost may severely damage or kill plants, making the species unsuitable for exposed or cold inland gardens without protection.
Once established in suitable conditions it grows rapidly and may self-seed around cultivated plants.
Because branches are relatively brittle, some protection from severe winds is beneficial.
Propagation:
Easy from fresh seed and cuttings.
Fresh seed can be sown into a warm, free-draining propagation medium and kept evenly moist.
Semi-hardwood or suitable young stem cuttings may also root successfully.
Plants can grow rapidly once established.
Cultural use:
No specific traditional Māori use is widely documented for Homalanthus polyandrus.
Its principal significance is ecological and conservation-related as a distinctive endemic component of the Kermadec Islands flora.
Garden value:
A striking foliage tree for warm, frost-free gardens.
The combination of broad triangular leaves, long petioles, red-flushed new growth and open tropical-looking form gives it a distinctive appearance unlike most mainland New Zealand native trees.
It is particularly valuable for specialist collections, subtropical native gardens and Kermadec-themed plantings.
Its very low frost tolerance is the main limitation in cultivation, but in suitably warm conditions it is fast-growing, attractive and relatively easy to establish.