<em>Eucalyptus</em>: planting, pruning and care

Description and botany

Les Eucalyptus are trees or bushes originating from a relatively restricted area of the globe despite an impressive number of species — more than 600 — and their almost worldwide distribution. Primarily native to the Australian continent – with a few species in southern Papua New Guinea, south‑east Indonesia and the southern Philippines – Eucalyptus today represents the main introduced tree species, particularly in dry tropical and subtropical regions of South America, Africa, India and the Middle East. These trees have the advantage of growing very fast even in poor, degraded soil, which is why they are used in areas undergoing desertification. Their multiple uses — for construction, as fuel and for pulp production — contribute to their success, not to mention their undeniably attractive ornamental appearance. They were even thought to break height records compared with sequoias, perhaps 135 to 150 m, but today no living specimen can confirm this, victims of massive exploitation in the last decades of the 19^th century. Eucalyptus regnans, named the “mountain ash” because the grain of its wood resembles that of ash, reigns supreme in Victoria province at Australia’s far south, reaching almost 120 m in height. The genus, which appeared in the secondary era, predates lime and plane by a few million years.

Les Eucalyptus belong to the Myrtaceae family like myrtle or Callistemon (bottlebrush). They are generally slender trees (E. gunnii), sometimes with a spreading crown as in E. pauciflora ssp. niphophila which can spread 6 m wide and reach 6–8 m in height, or with a bushy habit. Twisted branches and trunks sometimes highlight their variegated or striped bark as in E. viminalis or E. urnigera, due to peeling in plates or strips.

Eucalyptus gunnii

Eucalyptus deglupta, nicknamed Rainbow Tree and only suitable for frost‑free winters, shows very beautiful bark with richly coloured vertical streaks. The trunk sometimes appears completely smooth and pristine white after the shedding of bark strips as in Eucalyptus neglecta, or with a rough, caramelised‑sugar appearance as in E. sideroxylon ‘Rosea’. Heights in cultivation vary from 60 cm to over 70 m, with E. globulus reaching up to 10 m in girth. Low‑vigour forms are of course recommended for modest gardens, such as E. pauciflora which measures around ten metres. Eucalyptus gunnii ‘France Bleue Rengun’ is ideal in a pot or for a small garden, its size hardly exceeding 2 m. Their growth is generally rapid as the tree does not cease growth over winter. Some species form a lignotuber (tuberised root) which allows the tree to resprout after fire.

All eucalyptus types have aromatic evergreen foliage rich in essential oils contained in small translucent glands, which is why they are well known as medicinal plants. Young smooth shoots sometimes show a pretty red tint that contrasts with the silvery foliage, as in Eucalyptus pauciflora ssp. niphophila. Leaves are often dimorphic, their shape changing as the plant ages. Young eucalyptus, such as gunnii or globulus, have round or rectangular leaves, opposite and stalkless, which with age become sickle‑shaped, 5–45 cm long, and alternate. The top of the tree often bears adult leaves while the base retains juvenile foliage. Frequent pruning helps to maintain juvenile foliage, prized in floral art. The lamina, rather leathery, ranges from olive green to silvery, becoming more or less bluish depending on species, age and growing conditions. Veins are barely visible. Adult, pendulous leaves present little surface to the sun’s rays, which limits their evapotranspiration. They are regularly renewed by décurtation (shedding of shoot tips), so these shoots and strips of bark can be gathered for use as fuel. When wounded, the trunk exudes a resin sometimes red, hence the name gum tree, which has sometimes been used as a shoe polish.

The flowering period is extremely variable depending on species; it is sometimes absent outside native habitat, as in Eucalyptus gunnii, and can occur throughout the year, as in E. parvula. Flowering does not occur before five years of age. They are very melliferous flowers, petal‑less and spherical, composed of a multitude of stamens protected by an operculum that pops off at opening. The densely packed stamens open out into pompom‑like clusters, white or cream but also orange, bright pink to red as in some clones of E. ficifolia, E. leucoxylon and E. sideroxylon ‘Rosea’. They open among the leaves, often grouped along the shoots, and are followed by the formation of large green conical fruits, turning brown at ripeness. These capsules contain numerous very fine seeds dispersed by the wind after about a year. They are also appreciated in floral art.

Eucalyptus flowers: E. gunnii ‘France Bleue Rengun’ and E. parviflora

Latex from the cider gum (E. gunnii) was consumed as a sweet drink by Aborigines, while the gum‑resin of the ghost gum (E. papuana) was used as a powerful antiseptic. E. viminalis provided a sweet substance. Water was extracted from the roots of species from arid areas to quench thirst. Leaves rich in essential oils from cider gum are used in various forms (infusion, tincture, oil, …) to treat many respiratory ailments, rheumatism, migraines, fatigue and as an antiseptic. Different eucalyptus species with menthol, lemon or camphor aromas provide varied therapeutic effects ranging from powerful anti‑inflammatory and mosquito‑repellent (E. citriodora) to antiviral, expectorant and anticatarrhal (E. radiata).

Eucalyptus wood, pale pink to light reddish‑brown, varies in quality depending on species. It has a distinct sapwood and a fairly coarse grain which makes drying difficult, with a tendency to crack and undergo severe warping. Its timber is moderately durable to durable but resists insect attack except for termites. Eucalyptus marginata (jarrah) and E. diversicolor are among the most durable, with very hard wood used for shipbuilding or railway construction as well as garden furniture.

Juvenile foliage of Eucalyptus gunnii, globulus and cinerea, as well as the fruits, are prized in floral art.

Main varieties of Eucalyptus

Large and medium vigour varieties

Small varieties

Cider gum - Eucalyptus gunnii

Tree with broad columnar silhouette reaching 20 m, notable for its smooth off-white bark that flakes in multicoloured patches and for its aromatic grey-blue foliage, round when juvenile then elongating. Tolerates all soil types.

• Flowering time October
• Height at maturity 20 m

See the product sheet

Eucalyptus pauciflora subsp. niphophila

Large evergreen bush 6 to 8 metres tall, one of the hardiest species (-18 to -20°C). Snow gum is ramified from the base, bearing long sickle-shaped green leaves that turn grey. Its reddish-grey bark develops into a whitish-blue patchwork streaked with green and beige. Plant in slightly calcareous, poor, free-draining, rather cool soil; tolerates summer drought.

• Flowering time June, July
• Height at maturity 7 m

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Eucalyptus pulverulenta Baby Blue (Little Boy Blue)

A small specimen with pyramidal habit that will bring an attractive bluish note year-round to small gardens in mild climates. Its dense foliage of small round leaves, covered with a silvery-white bloom, is pleasantly scented and bears numerous creamy-white pom-pom flowers in spring. Hardy to -10°C in well-drained soil.

• Flowering time June, July
• Height at maturity 2,50 m

See the product sheet

Planting

Where to plant eucalyptus?

Ideal climate for Eucalyptus is seaside or southern France, where winters are mild and short. But on closer inspection, Ireland and Brittany host magnificent specimens in much less sunny, cooler and more humid conditions, sheltered from wind.

Choose a well-drained soil, fresh to dry, and a warm sunny position, sheltered from strong cold winds. Eucalyptus niphophila does not like lime-rich soil while gunnii and dalrympleana are indifferent. Hardiness varies considerably between species; Gommier des neiges Eucalyptus pauciflora subsp. niphophila is among the hardiest, tolerating below -18°C and not fearing snow. E. gunnii ‘Azura’ tolerates -14 to -18°C, E. parvula, whose foliage resembles that of an olive, and E. viminalis also tolerate frosts down to -15°C. In most regions, plant in open ground, improving drainage with coarse sand, pumice or non-calcareous gravel. Dwarf cultivars such as Eucalyptus pulverulenta Baby Blue or gunnii France Bleu Rengun tolerate little below -10°C but can be brought indoors into a cool bright room for maximum frost protection and pruned each year in March–April. Planting in a container also ensures moderate growth.

Gum trees are useful to dry out wet ground, as they consume large amounts of water even in winter. Once well established they are, however, drought-resistant and generally do very well in Mediterranean-type climates.

When to plant?

Eucalyptus is preferably planted in early spring to avoid winter cold, unless climate is dry and warm.

How to plant?

Prefer planting young plants which cope better with planting stress.

• Dig a planting hole of 50 cm all round.
• Add a 20 cm draining layer (gravel, coarse sand, pumice…) if soil is heavy and severe frosts are likely.
• For a large tree, drive in four stakes 8 cm in diameter around the hole and tie to trunk with soft ties.
• Place rootball so as not to bury the collar.
• Backfill hole and firm lightly.
• Water thoroughly to eliminate air pockets.
• Fit soft ties to stake without risk of injuring the trunk.

Establishment is easy and quick and only requires monitoring of watering during first two years after planting.

In windy coastal areas such as Brittany, a trick is to plant eucalyptus above a large rock so roots can grow around it. This helps anchor tree which, with a taproot and large crown, can sometimes be uprooted by strong gusts. Then let nature take its course; shaping tree is unnecessary.

Choose a large, deep pot, filled with a substrate that is both light and rich.

Eucalyptus gunnii ‘France Bleu Rengun’ grown in a pot

→ Discover our care sheets: Eucalyptus – how to protect from cold in winter? and Growing eucalyptus in a pot

Pruning and care

Water regularly for first two years during summer then plant will need no care once well established in open ground. By contrast, ensure watering of Eucalyptus in a pot by letting surface substrate dry between waterings because its roots cannot penetrate deeply to draw water. You can overwinter it in a cold greenhouse or in an unheated, bright room in winter, like an oleander.

Pruning Eucalyptus

If severe frost scorches twigs, do not worry: cutting back to ground level often encourages rapid regrowth. Be sure to select 1 to 3 stems to reform a tree. Some gardeners prefer to maintain a fine, bushy shrub about 3 m high by systematic hard pruning in March. Take care not to inflict large pruning wounds! You can also cut some large roots to limit vigour. As a rule, Eucalyptus does not require pruning. Limit yourself to removing small branches from trunk to give a smooth appearance and to removing twigs within the crown while tree is still young.

Read also : How to prune an Eucalyptus ?

Able to draw water and mineral salts from deep down, Eucalyptus needs nothing to develop, let nature take its course. For low-vigour varieties grown in a container, occasionally add compost or potting compost as a top-dressing in autumn.

Potential parasitic pests

Eucalyptus is naturally quite resistant to diseases and pests. Eucalyptus gall sometimes occurs in Mediterranean region, caused by the sting of a small wasp Ophelimus eucalypti and which makes leaves yellow. Cut away affected foliage quickly.

→ Learn more about Eucalyptus diseases and pests with Gwenaëlle !

Eucalyptus display beautiful decorative bark

Propagation

Sowing is the most commonly used method to propagate eucalyptus. However, it is delicate because seeds are very small and require suitable warmth and humidity to germinate.

Sowing

• Keep them below 4°C for 1 month, then sow almost on the surface in a bucket wrapped in a freezer bag and kept at 22–25°C in full light.
• As soon as germination occurs, after about one week, move the bucket to 20°C.
• When seedlings have 4 leaves, transplant the lot into a larger pot without disturbing the rootball and remove the weakest young plants. Gradually open the bag and keep above 20°C, out of direct sunlight.

Uses and companion plants

Eucalypts with bluish‑grey foliage work wonders in scenes dominated by white or silver, adding a metallic sheen combined with the sound brought by the breeze that sets the foliage in motion. Eucalyptus can form a large windbreak that nevertheless lets light filter through, in contrast to the deep green of Provence cypress, for example. It brings an undeniable touch of exoticism to the garden whether planted in groups, as a solitary specimen or in association with bushes from southern lands. Don’t hesitate to mix specimens of Hakea, Leptospermum, Callistemon with our essential oils of Mediterranean garrigue such as myrtles, Arbutus, cistus, Phlomis and rosemary.

An example of a tone-on-tone combination: Eucalyptus gunnii, Echinops ritro, Hydrangea macrophylla ‘Endless Summer The Original’, Picea pungens ‘Glauca Globosa’, Veronica longifolia ‘Blauriesen’

The bluish metallic tinge, the graphic quality and the transparency of the foliage of a specimen placed on a balcony or terrace bathe the scene in a calming softness. If you prune it very short on a repeated basis, its silhouette becomes denser and can highlight the bright flowering of bedding plants (geraniums, dahlias, impatiens…), but it will lose some of its character. These bushes have the advantage of being elegant at all stages of their growth, young or mature.

Eucalypts with a ramified silhouette and little frost sensitivity, such as niphophila, can slip into an informal hedge of fairly hardy bushes with a southern feel, such as Buddleia ‘Lochinch’, ‘Bicolor’ and oleanders ‘Luteum Plenum’, ‘Provence’, ‘Atlas’. But an individual as remarkable as the mountain gum equally deserves a place as a solitary specimen, because of its tortured habit, its bark and its elegant foliage.

→ Discover other ideas for combining with Eucalyptus with Alexandra !

History of Eucalyptus

Discovery of Eucalyptus was miraculous. A storm forced two ships that had sailed from Brest in search of a vessel missing in the Southern Ocean for three years to anchor off Tasmania towards the end of the 18^th century. The botanists on board then discovered a huge tree, Eucalyptus globulus, which would fall into obscurity except among a few enlightened collectors.

Its widespread diffusion owes much to insight of Mueller, a Danish naturalist settled in Australia, and Ramel, a French merchant, both convinced of interest of this tree. Eucalyptus, which enjoys continuous growth and a deep root system, is very thirsty and capable of drying marshes to fight malaria transmitted by mosquitoes while also able to withstand drought. They therefore sent seeds across Europe so it could be acclimatised.

Eastern Corsica benefited quickly and by about 1875 success was such that 4 million trees were planted in Algeria and 200,000 in Egypt. Spain and Portugal, which planted it in wet areas (Cantabrian Coast…) to limit soil erosion and for pulp production, are today less enthusiastic because of ecological consequences of this introduction.

Nothing grows beneath it because of its allelopathic effect, even though light penetrates easily through its pendulous leaves. Its cultivation leads to drying of groundwater, and eucalyptus is an active pyrophyte. It emits highly flammable vapours. Fire spreads quickly, unlike in forests of holm oak and cork oak — the first protected by absence of herbaceous understorey due to darkness, the second protected by fire‑resistant bark.