Metrosideros: Planting and Growing

Description and botany

The genus Metrosideros includes around fifty species of trees, bushes and lianas belonging to the myrtle family, like theEucalyptus, theCallistemon, and the Leptospermum. They are mainly native to New Zealand but some have become naturalised in many subtropical regions and Pacific islands where they grow in scrub vegetation, forests, on volcanic coasts, having colonised lava and altitudes up to 1,000 m. From their origins, Metrosideros have retained some sensitivity to cold. TheMetrosideros excelsa (nicknamed New Zealand Christmas Tree or pohutukawa in Māori) and its forty or so cultivars and hybrids is the most widespread species in our gardens that experience mild winters. TheMetrosideros kermadecensis or Kermadec Metrosideros is another species for mild climates, native to the volcanic Kermadec Islands. The Metrosideros umbellata is the most cold-resistant species (-12 °C) and therefore the best suited to our cool climates and moist soils. As for the Metrosideros carminea, it is a tender climbing species reaching 15 m in height.

Metrosideros robusta, botanical plate from 1840

In their natural habitat, Metrosideros form bushes or beautiful trees sometimes reaching 20 m in height with a trunk up to 2 m in diameter. They are rarely very large. In our regions and gardens in the south or on the Atlantic coast, they will remain much smaller, rarely exceeding 6 m in height and 5 m in width when mature. Specimens grown in containers, as they make good pot plants, show even more modest growth. Growth is slow (about 3 m in 10 years), making them easy to accommodate in pots.

They live a long time, over 200 years! The habit varies depending on the species and varieties but the foliage is naturally dense. Upright and compact when young for most, they develop an umbrella-shaped habit with age. Some have a creeping or climbing habit. They develop on a powerful and deep root. They have beautiful corky bark, grey-brown in colour with a gnarled trunk and branches. The wood is dark and very hard, which earned these trees their name derived from the Greek “metra” (heartwood) and “sideros”, iron. For centuries, Māori have used it to make their carvings. The Metrosideros excelsa and its descendants have the particularity of possessing aerial roots, which sometimes end up rooting in the ground. Some species are even epiphytic, starting life on the branches of the host tree before generating roots.

Metrosideros are beautiful evergreen bushes decorative in all seasons. They bear small, leathery evergreen leaves, downy and white on the underside. They are single, opposite, lanceolate or oblong, with slightly undulate margins, long and narrow, 2 to 10 cm. They sometimes take on a silky appearance, pubescent then after the spring bud burst, become glabrous. Dark olive green in most, they take on an irregularly variegated yellow robe in some cultivars such as ‘Sunninghill’ and ‘Aureomarginata’ (syn. ‘Variegata’).

Metrosideros are remarkable for their flowering as original as it is dazzling, occurring at Christmas time in the southern hemisphere, which earned them the vernacular name “New Zealand Christmas Tree”. No doubt also because of their flowers in the shape of large scarlet pompoms resembling Christmas baubles. Setting the whole tree ablaze, these upright inflorescences give the plant a spectacular appearance. From May to August, in our latitudes, small flowers gathered in cymes at the ends of the shoots display their bouquets of prominent and colourful stamens most often scarlet, about 3 cm long. Like compact bottlebrushes, they evoke those of the Callistemon. These feathery floral spikes, bordering on fluorescent, more rarely display softer or different shades, ranging from white to orange.

Their long stamens prettily dusted with golden yellow pollen are melliferous and nectariferous, attracting many pollinating insects like bees as well as some birds. The flowers then turn into small 8 mm diameter fruits in the form of dehiscent capsules containing very many winged seeds carried by the wind.

The flowering pōhutukawa symbolises summer in New Zealand. The most famous in the country is said to be around 800 years old! The M. excelsa is a sacred tree for the Māori for whom the spirits of the dead descend through its roots to reach the island of their ancestors.

Metrosideros excelsa on the left, and on the right Metrosideros kermadecensis (© Forest and Kim Starr)

Main species and varieties

[product sku=”16684″ blog_description=”This variety has a very compact creeping habit! Magnificent when planted in a seaside garden, or grown as an orangery plant in cold climates.” template=”listing1″ /]

[product sku=”89991″ blog_description=”This is the typical species with its beautiful flowering in large scarlet pompoms! Perfect for a greenhouse, conservatory or garden in an exotic setting.” template=”listing1″ /]

[product sku=”100739″ blog_description=”A particularly decorative variety, with bright red flowers and green foliage maculate with yellow. Its fine pruning and tropical appearance make it ideal for exotic borders.” template=”listing1″ /]

[product sku=”85136″ blog_description=”Here is a very beautiful hybrid of the Christmas Tree! An excellent small tree for seaside avenues.” template=”listing1″ /]

[product sku=”184591″ blog_description=”Here is another variety with foliage variegated in pale yellow! This Metrosideros is hardier than the typical species, tolerating temperatures down to -4/-5°C.” template=”listing1″ /]

Planting the Metrosideros

Where to Plant It?

Originating from New Zealand, the Metrosideros retains a strong sensitivity to cold. Planting it directly in the ground should be reserved for coastal gardens and southern regions where frost is neither severe nor prolonged. This limited hardiness (generally -3 to -5°C, -6°C at worst) restricts its cultivation to coastal areas of Brittany, the southern Atlantic coast, and the French Riviera. Its hardiness is further compromised in damp soil or when exposed to icy winds. This is why outside the most sheltered gardens, it is best grown as an orangery plant, in large containers so it can be overwintered frost-free. It makes an excellent greenhouse or unheated conservatory plant, or can be kept indoors near a well-oriented glass door, before being moved outside during the warmer months.

Nicknamed “splashed by the sea” by the Māori, this tree is highly resistant to salt spray and adapts effortlessly to coastal gardens. In regions with brief and mild frosts, plant it in the ground in rich, well-drained soil (avoiding excessive lime). While it can tolerate relatively dry conditions, it prefers soil that remains sufficiently moist in summer. Regular watering is essential in dry climates. Once established, it becomes more drought-resistant. It requires full sun to flower well, though it tolerates partial shade in sunnier regions. Choose a warm, sheltered spot away from cold winds.

Plan for its eventual size! Although slow-growing, carefully consider its planting location, as some varieties can reach 6m in height and 5m in spread at maturity. Its extensive and robust root system makes transplanting difficult later on.

In Mediterranean gardens, the Metrosideros can be planted as a focal point or at the back of an exotic border to showcase its beauty. In colder climates, grown in pots on a sunny terrace or balcony, it adds an exotic touch.

When to Plant?

The best time to plant a Metrosideros is in April-May, once the risk of frost has passed, allowing it to establish roots before winter.

Metrosideros excelsa

How to Plant It?

In the Ground

If your soil is very lime-rich, I recommend mixing in some heather soil with your garden soil.

• Soak the root ball in a bucket of water for 15 minutes to rehydrate it thoroughly
• Dig a hole at least 80cm wide and deep
• Add a thick layer of clay pebbles or gravel at the bottom
• Position the bush upright in the centre of the hole
• Backfill with an equal mix of heather soil, compost, and garden soil, adding a few handfuls of river sand
• Firm the soil with your foot
• Water generously after planting

In a Pot

Good drainage is essential.

• Place a thick layer of gravel, pumice, or clay pebbles at the bottom of a pot at least 50cm in diameter
• Plant in a light substrate of river sand, heather soil, and leaf compost
• Water well

Maintenance, Pruning and Care

As the Metrosideros is very sensitive to cold, it is easy to grow in Mediterranean regions or along the mild-wintered Atlantic coast.

• When planted in the ground, it appreciates having its roots kept cool in summer: watering (preferably with rainwater) should therefore be generous and regular (on average once or twice a week) to maintain moisture at the base, without excess. The soil must not dry out in summer during the first few years after planting. A mulch at the base can help reduce watering and retain sufficient moisture. Later on, this tree will tolerate some soil drought well. For the first few winters, protect the stump with a thick organic mulch and, if necessary, shield the aerial parts from frost with a winter fleece.

• In pots, it will require more care and attention. Water very regularly with non-calcareous water throughout summer but without ever saturating the substrate. The soil should never dry out completely either. Don’t hesitate to mist the foliage during very hot weather. In spring and autumn, to encourage growth, apply a good slow-release fertiliser. At the first signs of cold, bring the pot into a cold greenhouse or a bright, frost-free room where temperatures will not drop below 5°C. Being adaptable, it can also spend winter indoors in a heated room. Reduce watering to about once a month, allowing the soil to dry out between waterings. Move pots back outside when frost is no longer a risk. Repot once a year in spring; later, when its size makes repotting too difficult, simply refresh the substrate with an annual top-dressing.

When and how to prune a Metrosideros?

A simple annual maintenance pruning is sufficient for this slow-growing bush. It helps stimulate flowering and growth while maintaining a dense, attractive shape by correcting its habit.

1. In autumn or after flowering, use pruning shears to remove dead or misplaced branches.
2. Overgrown specimens can be controlled with a hard cutback each year, making them easier to move indoors.

A truly stunning flowering!

Possible diseases and parasitic pests

Fairly resistant to pests and diseases, when grown indoors, the Metrosideros can become vulnerable to scale insects. In this case, apply rapeseed oil sprays, repeating two or three times at 15-day intervals. In the event of an aphid infestation, spray with soapy water.

In overly chalky soil or due to excess water, its foliage may turn yellow. Ensuring good drainage and regular applications of heather soil by lightly working it into the base of the bush can prevent this issue.

Propagation

To perpetuate the Metrosideros, propagation can be done by sowing in warmth during spring, but flowering will only occur after many years. We recommend propagation by cuttings in summer as the simplest and quickest method to multiply your tree.

• In late summer, take semi-hardwood cuttings (hardening) of 10 to 15 cm
• Remove the leaves from the lower third of the stems
• Plant them in a light, well-draining mix of sand and compost
• Water
• Place under a humidity cover until rooting occurs
• Keep the substrate moist
• Transplant your cuttings into individual pots once sufficiently rooted
• Plant out in pots or in the ground the following spring
• To encourage establishment, water well during the first year after planting

Pairing

With its incandescent flowering throughout summer and tropical allure, the Metrosideros is an essential bush for Mediterranean gardens, Atlantic climate coastal areas, or exotic-themed landscapes. It will make a striking centrepiece in an original and vividly colourful summer display.

Planted at the back or centre of a bed of low-growing plants, it will look sensational surrounded by sun-loving perennials such as the bird of paradise, coneflowers, and coreopsis, alongside exotic bushes like the Syagrus palm, hardy banana trees, or the Eucalyptus ‘Baby Blue’. Consider ground-covering plants at its base, such as a creeping ceanothus and creeping rosemary.

Pair it with its Australian cousins like Phormium and Grevillea, or New Zealand natives such as Callistemon and Leptospermum, which boast equally vibrant flowering.

Metrosideros excelsa, Phormium ‘Apricot Queen’, Leptospermum scoparium, and Grevillea