<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 the impressive number of species—more than 600—and their almost worldwide distribution. Primarily native to the Australian continent – with a few species from southern Papua New Guinea, south-east Indonesia and southern Philippines – Eucalyptus today represent the main tree species introduced, notably in the dry tropical and subtropical regions of South America, Africa, India and the Middle East. These trees have the advantage of growing very quickly even in poor, degraded soils, which is why they are used in areas suffering from desertification. Their many uses, whether for construction, fuel or paper pulp, contribute to their success, especially since their ornamental appearance is undeniable. It was even believed they might rival the tallest sequoias, reaching 135 to 150 m perhaps, but today no living specimen can confirm that performance, victims of massive exploitation in the late 19th century. Eucalyptus regnans, called the “mountain ash” because the wood grain resembles that of ash, reigns supreme in Victoria in far-southern Australia, approaching 120 m in height. The genus, which appeared in the Secondary era, predates the lime and plane trees by a few million years.

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 span 6 m wide for 6 to 8 m high, or they can have a bushy habit. Shoots and trunks are sometimes twisted, highlighting their variegated or striped bark as in E. viminalis or E. urnigera, owing to peeling in plates or strips.

Eucalyptus gunnii

Eucalyptus deglupta, nicknamed Rainbow Tree and reserved for frost-free winters, displays very beautiful vertically striped, richly coloured bark. The trunk sometimes appears completely smooth and pure white after strips of bark shed, as in Eucalyptus neglecta, or has the rough, caramelised-sugar appearance of E. sideroxylon ‘Rosea’. Size of eucalyptus in cultivation ranges from 60 cm to more than 70 m high with trunks up to 10 m in circumference in E. globulus. Low-vigour forms are of course recommended for modest gardens, such as E. pauciflora which reach around ten metres; Eucalyptus gunnii ‘France Bleu Rengun’ is ideal in a pot or for a small garden, barely exceeding 2 m in height. Growth is generally rapid due to the lack of a complete growth halt during winter. Some species form a lignotuber (tuberised root) that allows the tree to resprout after fire.

All eucalyptus species have an aromatic evergreen foliage rich in essential oils contained in small translucent glands, which also gives them a reputation as medicinal plants. Young smooth shoots sometimes show a pretty red hue contrasting with silvery foliage, as in Eucalyptus pauciflora ssp. niphophila. Leaves are often dimorphic, their shape changing with the plant’s age. Young eucalyptus, such as gunnii or globulus, have round or rectangular leaves, opposite and without petioles, which, with age, become sickle-shaped 5 to 45 cm long and alternate. The top of the tree often bears adult leaves while the base retains juvenile foliage. Pruning frequently helps to retain juvenile foliage, much appreciated in floral design. The lamina, fairly coriaceous, ranges from olive green to silvery, more or less bluish depending on species but also on age and growing conditions. Veins are barely visible. Adult, pendulous leaves offer little surface to sun rays, which limits evapotranspiration. They are regularly renewed by defoliation of branch tips, so these shoots and strips of bark can be collected for use as fuel. When wounded, the trunk exudes a resin, sometimes red, hence the name gum, which is sometimes used as a polish.

The flowering period is extremely variable depending on species; it can be absent outside the plant’s natural habitat as in Eucalyptus gunnii and can occur throughout the year as in E. parvula. Flowering does not occur before age five. They are very melliferous flowers, petal-less, spherical and composed of numerous stamens protected by an operculum that pops off at opening. The densely packed stamens unfold in pom-poms, white, 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 formation of large green conical fruits that turn brown at ripeness. These capsules contain numerous very fine seeds dispersed by wind after about a year. They are also valued in floral design.

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

The latex of the cider gum (E. gunnii) was consumed as a sweet drink by Aboriginal people 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 roots of species from arid zones to quench thirst. Leaves rich in essential oils from the cider gum are used in various forms (infusion, tincture, oil, etc.) 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 strong anti-inflammatory and mosquito-repellent properties (E. citriodora) to antiviral, expectorant and anticatarrhal effects (E. radiata).

The eucalyptus wood, pale pink to light reddish-brown, varies in quality depending on species. It has a distinct sapwood and a rather coarse grain that makes drying difficult, with a tendency to split and distort. Its timber is moderately durable to durable but susceptible to insect attack except for termites. Eucalyptus marginata (jarrah) and diversicolor rank among the most resistant, with very hard wood used for shipbuilding, railway sleepers and garden furniture.

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

Main Eucalyptus varieties

Large and medium vigour varieties

Small varieties

Cider gum - Eucalyptus gunnii

Tree with a broad columnar silhouette reaching 20 m, notable for its smooth whitish bark that peels in mottled plates and for its aromatic, grey-blue foliage, round when juvenile then elongated. 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 high, one of the hardiest species (-18 to -20°C), the snow gum is ramified from the base, bearing long crescent-shaped green leaves that turn grey. Its reddish-grey bark transforms into a whitish-blue patchwork striate with green and beige. Plant in slightly calcareous, poor, well-drained, rather cool soil, although it tolerates summer drought.

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

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Eucalyptus pulverulenta Baby Blue (Little Boy Blue)

This small specimen with a pyramidal habit will bring an attractive bluish note year-round to small gardens in mild climates. Its dense foliage of small round leaves, covered in silvery-white bloom, is pleasantly scented, and set with numerous cream-white pompom 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?

The ideal climate for Eucalyptus is coastal or the south of France, where winters are mild and short. But on closer inspection, Ireland or Brittany host magnificent specimens in much less sunny, cooler and wetter conditions, provided they are 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 limestone very much, whereas E. gunnii and E. dalrympleana are indifferent. The hardiness of species is quite variable: Snow gum Eucalyptus pauciflora subsp. niphophila is among the hardiest, tolerating temperatures below -18°C and not fearing snow. E. gunnii ‘Azura’ tolerates around -14 to -18°C, E. parvula, whose foliage resembles that of an olive, and E. viminalis also withstand frosts down to about -15°C. In most regions, plant in open ground, improving drainage with coarse sand, pumice or non-limestone gravel. Dwarf cultivars such as Eucalyptus pulverulenta ‘Baby Blue’ or E. gunnii ‘France Bleu Rengun’ hardly tolerate temperatures below -10°C, but they can be moved 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 for drying out wet ground because they use large amounts of water even in winter. However, once well established they are drought-resistant and generally do very well in Mediterranean-type climates.

When to plant?

Eucalyptus is best planted at the beginning of spring to avoid exposure to winter cold, unless climate is dry and warm.

How to plant?

Prefer planting young plants, which cope better with transplant stress.

• Dig a planting hole 50 cm in every direction.
• Add a 20 cm draining layer (gravel, coarse sand, pumice…) if your soil is heavy and severe frosts are possible.
• For a tree expected to grow large, drive in four stakes 8 cm in diameter around the hole and attach them to the trunk with soft ties.
• Place the rootball so as not to bury the collar.
• Backfill the hole and firm lightly.
• Water generously to eliminate air pockets.
• Fit soft ties so the tree is staked without risk of injuring the trunk.

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

In windy coastal regions such as Brittany, a useful tip is to plant the eucalyptus above a fairly large rock so the roots can grow around it. This strengthens anchorage: with a taproot and a large crown the tree can sometimes be uprooted by strong gusts. Then let nature take its course; formal shaping of the tree is unnecessary.

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

Eucalyptus gunnii ‘France Bleu Rengun’ grown in a pot

→ Discover our advice 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; afterwards plant will need no care once well established in ground. However, take care with watering potted Eucalyptus by letting surface of substrate dry between waterings because its roots cannot sink deeply to draw water. You can overwinter it in an unheated, light-filled greenhouse or room, like an oleander.

Eucalyptus pruning

If severe frost scorches twigs, don’t worry, cutting back to ground level often produces rapid regrowth. Be sure to select 1 to 3 stems to reform a tree. Some gardeners prefer to keep a handsome, bushy shrub about 3 m high by systematically cutting back 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. Simply remove small branches from trunk to give it a smooth appearance and remove twigs within 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, add compost or potting soil occasionally as topdressing in autumn.

Possible parasitic pests

Eucalyptus is naturally quite resistant to diseases and pests. Eucalyptus gall sometimes occurs in Mediterranean regions, caused by sting from a small wasp Ophelimus eucalypti, which turns leaves yellow. Remove affected foliage quickly.

→ Learn more about diseases and pests of Eucalyptus with Gwenaëlle!

Eucalyptus display beautiful decorative bark

Propagation

Sowing is the most commonly used method to propagate eucalyptus. It is, however, delicate because seeds are very small and require adequate warmth and humidity to germinate.

Sowing

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

Uses and companion plants

Eucalypts with grey-blue foliage are wonderful in predominantly white or silvery scenes, adding a metallic sheen mingled with the sound of the breeze setting the foliage in motion. Eucalyptus can form a large windbreak that nevertheless allows light to filter through, in contrast to the dark green of cypress from Provence, for example. It brings an undeniable touch of exotica to the garden whether planted in groups, as solitary specimens or in association with bushes of southern origin. Don’t hesitate to mix specimens of Hakea, Leptospermum and Callistemon with our essential oils of Mediterranean garrigue such as myrtle, Arbutus, rockrose, Phlomis and rosemary.

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

The bluish metallic touch, the graphic quality and the transparency of the foliage of a specimen placed on a balcony or terrace bathe the scene in a soothing softness. If you prune it very short on a repeated basis, its silhouette becomes denser and can set off the vivid flowering of bedding plants (geraniums, dahlias, impatiens…), but it will lose a little 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 sensitivity to frost, such as niphophila, can slip into a loose hedge of fairly hardy bushes, with a southern feel such as Buddleia ‘Lochinch’, ‘Bicolor’ and oleanders ‘Luteum Plenum’, ‘Provence’, ‘Atlas’. But a specimen as remarkable as the mountain gum equally deserves a place as a solitary specimen, because of its twisted habit, its bark and its elegant foliage.

→ Discover more pairing ideas with Eucalyptus with Alexandra !

History of Eucalyptus

Discovery of Eucalyptus is nothing short of a miracle. A storm forced two ships that had sailed from Brest in search of a vessel missing in the southern seas for three years to anchor off Tasmania towards the end of the 18^th century. Botanists aboard then discovered an immense tree, the Eucalyptus globulus, which would fall into obscurity except among a few enlightened collectors. Its widespread diffusion resulted from the intuition of Mueller, a Danish naturalist settled in Australia, and Ramel, a French merchant, both convinced of the tree’s value. Eucalyptus, which enjoys continuous growth and a deep root system, consumes large amounts of water and can dry marshes to combat mosquito-borne malaria while still able to withstand drought. They thus sent seeds across Europe so that it could be acclimatised. Eastern Corsica benefited quickly and by around 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 zones (Cantabrian coast…) to limit soil erosion and to produce paper pulp, are today less enthusiastic because of ecological consequences of this introduction. Nothing grows at its foot because of its allelopathic power, although light penetrates easily through pendulous leaves; its cultivation leads to drying out of water tables. Eucalyptus is an active pyrophyte. It emits highly flammable vapours. Fire spreads quickly, unlike in forests of holm oaks and cork oaks, the former protected by absence of herbaceous understorey due to darkness, the latter protected by a fire-resistant bark.