Eucalyptus regnans

Eucalyptus regnans is one of the tallest trees in the world and its growth is extremely rapid in suitable climates. It forms an imposing tree, with a very straight trunk, fibrous bark in the lower part and smooth bark in the upper part. Its habit is distinctly conical, with evergreen foliage ranging from green to grey-green, which in spring is adorned with small white flowers grouped in umbels. Demanding in terms of light, this species grows in cool, well-drained, neutral to acidic, moderately fertile soil. It thrives in cool and humid areas, where frost is minimal, and quickly becomes an exceptional specimen.

Eucalyptus regnans belongs to the large family of Myrtaceae, like the Callistemon (Bottlebrush), the Feijoa or of course the Myrtle. The genus Eucalyptus includes more than 800 species, practically all of which are native to Australia, except for a few from Southeast Asia. Some form small bushes, while the E. regnans is the giant of the genus. The tallest living specimen reaches 100 m (328ft), and only a tropical tree, the Shorea, reaches 1 m (3ft) more, as does the Sequoia which holds the absolute record with a subject of 116 m (381ft) in height.
Eucalyptus regnans is native to southeastern Australia, in the state of Victoria and Tasmania. It forms large forests there and is commonly used in plantations. Its growth is explosive from the second year onwards. It dedicates the first year of its existence to anchoring itself in the ground with a deep taproot, then it produces lateral roots that extend quite far from the initial taproot and give rise to secondary taproots. The young tree can then begin its ascent towards the sky at an astonishing speed when the growing conditions meet its needs. Tremendous growth rates have been measured in different regions in Kenya, with 2-year-old trees reaching a height of 9 m (29ft), in another stand, 12-year-old trees averaging 34.6 m (113ft), or in an arboretum, 32-year-old subjects reaching 44 to 49 m (144 to 161ft)!
In these tropical or subtropical regions, it forms a very straight trunk, with brown fibrous bark up to 15 m (49ft) high, before becoming smooth and taking on a light colour in the upper parts. In very large specimens, some of which exceed 300 years, the trunk can measure up to 3 m (10ft) in diameter. Its crown remains conical for a long time before slightly opening at maturity, while buttresses develop at the base of the tree to support it. The green to grey-green foliage consists of simple, entire, evergreen leaves, measuring up to 23 cm (9in) long and 5 cm (2in) wide, but sometimes smaller. The flowering consists of small white flowers, grouped in corymbs of 9 to 15 units.
This tree needs light to develop well, it cannot tolerate competition from others and even goes as far as clearing the space around it to grow unhindered. In Australian forests, only an understory of Acacia dealbata manages to survive in the shade of this giant. Eucalyptus regnans grows in a rather cool climate, spared from drought as well as strong frost. It can tolerate occasionally -5°C (23°F), or even -7°C once well established, as well as brief dry periods, but it thrives better in well-watered areas. In Australia, it is found from sea level to 1100 m (3609ft) altitude, but in tropical climates where it has been artificially introduced, it grows from 2000 to 3200 m (6562 to 10499ft) altitude, thus escaping the low-altitude heat. It requires deep, moderately fertile, moist, well-drained soils, with a neutral or acidic pH, and free from salt.

Eucalyptus regnans will mainly interest collectors in cool regions with high rainfall. It will make a monumental subject planted alone on a lawn, or in the background of a bed. Nearby, one can plant the Acacia melanoxylon, Blackwood Mimosa which produces original flowers in the form of sulfur yellow balls, or the more classic Acacia dealbata, with its beautiful cut foliage and highly fragrant bright yellow flowers. These two species will tolerate the proximity of the Eucalyptus and create a beautiful exotic scene at its base.