Trees of San Diego: Flame Trees, Kurrajongs, and Bottle Trees

Considered by many to be Australia’s most spectacular group of flowering trees, the genus Brachychiton includes a number of beautiful trees that are admirably suited to many California climates. Although it is not a big group, it is a diverse one, including trees native from tropical rainforests to dry deserts. This diversity makes Brachychiton an especially rewarding genus for Southern California, where the four most commonly-grown species, in particular, are valuable and showy trees for not only coastal and inland but also desert landscapes.

What’s a Brachychiton?

The genus Brachychiton (pronounced brak-key-KYE-ton) is a member of the Sterculiaceae, a tropical and subtropical family of mostly trees and shrubs that includes a few important edibles such as cocoa and cola nuts, ornamentals such as Dombeya, and the well-known California native flannel bush (Fremontodendron). All thirty-one species of Brachychiton are evergreen or briefly deciduous trees. All but one is native to Australia; twenty-nine are endemic there. In Australia, they are native to tropical and subtropical northern and eastern regions in climates ranging from moist coastal forests to dry inland areas. Their diversity of form and foliage is remarkable, from tall rainforest species with large leaves to smaller desert species with generally smaller leaves and fat, water-storing trunks. Flowering in certain species can be showy in late spring and summer, when trees drop some or all of their leaves to better display the clusters of bell-shaped flowers that may be white, pink or red. Each flower is a five-lobed, often waxy, calyx, with no true petals; inside are numerous stamens fused into a column-like arrangement. The large woody seedpods (properly called follicles) that follow are boat-shaped and conspicuous, opening along one seam to reveal rows of seeds surrounded by a brittle and often hairy coat. These hairs can cause irritation of sensitive skin. The genus takes its name from the Greek brachys, meaning short, and chiton, meaning tunic, referring to this seed coat.

Trees in the genus Brachychiton also have a plethora of colorful common names that describe some aspect of their habit or human use. The name “flame tree” is used for a few of the coastal species with brightly colored flowers of red or pink. “Kurrajong” is derived from the Australian Aboriginal word currajong, which means fiber-yielding plant: the fibrous trunks of some species have historically been used by Aborigines to make fishing nets and twine, and have also been used for food. “Bottle tree” quite aptly refers to the swollen shape of the trunks on some species, particularly the desert-dwellers.

The “Top Four”

The four most common species (well, OK, they’re not all that common!) of Brachychiton in cultivation in the San Diego area are Illawarra flame tree (B. acerifolius), pink flame tree (B. discolor), kurrajong (B. populneus), and Queensland bottle tree (B. rupestris). In my garden are twenty-five-year-old specimens of these “top four” species of Brachychiton, but most visitors would never guess that they are all the same age—much less that they are all related. One is forty feet tall, with a strong thick trunk and large glossy leaves. Two others are half that height with fat trunks, but one has maple-shaped leaves and the other has leaves like an aspen. The fourth one is only ten feet tall, with narrow leaves and a big, barrel-shaped trunk nearly three feet across. They are all excellent garden trees where frosts are not severe, and, although they all appreciate good drainage, they require only a minimum of water and care. A closer look at their individual characteristics will tell us a bit about where they come from, what they do, and how to use them in the garden.

Illawarra Flame Tree

Unforgettable in full bloom, Illawarra flame tree (Brachychiton acerifolius) is one of the most spectacular red-flowering trees in the world. Native to rainforest areas from Illawarra to Cape York on Australia’s eastern coast, it can be a giant tree in the wild, towering over the surrounding forest, which includes king palms (Archontophoenix cunninghamiana) and a number of other plants we grow in California. In the summer months there, this flame tree loses all of its leaves and completely covers itself with bright red flowers, creating a spectacle visible for many miles.

In cultivation in California, Illawarra flame tree is somewhat smaller than in the wild due to our cooler and drier climate. It is an excellent choice for a tropical look, growing to an eventual forty to fifty feet tall with a spread of twenty to twenty-five feet. Its six- to ten-inch, bright, glossy green leaves are deeply lobed on young plants (its species name means “maple-like leaves”) and shallowly lobed to oval on older trees. It has a stout trunk and main branches covered in a gray, wrinkled bark. Although its blooming habits in cultivation can vary from year to year, the Illawarra flame tree usually blooms in late spring or early summer; at that time, all or part of the tree becomes leafless, and the ends of branches are quickly covered with large, open, pendant clusters of one-inch, bright red, waxy, bell-shaped flowers. These flowers fall cleanly from the tree while still fresh, creating a carpet of red on the ground, and are followed by interesting clusters of five-inch black seedpods.

In flower, the bright red color of Illawarra flame tree looks especially stunning in combination with the blue flowers of Jacaranda mimosifolia, which blooms at the same time of year. Trees sold in nurseries are typically seedlings that, unfortunately, may take a number of years before they bloom (grafted plants will bloom sooner), but this is a tree well worth the wait. Although the foliage on young trees is frost-tender, mature specimens are hardy to 25∞F; this species is recommended for Sunset zones 15-24, and also for Hawaii.

The Pink Flame Tree

Sometimes called Queensland lacebark or white kurrajong in Australia, pink flame tree (Brachychiton discolor) is also native to the coastal forests of Queensland and New South Wales. Growing at a moderate rate to an eventual thirty to forty feet tall, it’s shape is pyramidal when young but more spreading in maturity. Pink flame tree has a characteristic gray, bottle-shaped trunk and a “fuzziness” of hairs on its leaves, young branches, flowers, and seedpods. Its six-inch, tropical-looking, maple-like leaves are dark green and deeply lobed on young trees but shallowly lobed on older specimens. In summer, all or part of a tree may lose its leaves immediately before bloom, when the tree produces showy clusters of two-inch, bell-shaped, rose pink flowers with darker pink centers (the species name means “two colors”). Dropping cleanly when fresh, these flowers make a carpet of color on the ground, and are followed later in the year by four-inch brown seedpods.

Pink flame tree makes an effective and unusual street or lawn tree, tolerant of either regular watering or some drought. Although its flowering can be somewhat erratic from year to year, it flowers best in warmer inland areas and can be sensational in a good year. The foliage of young plants can be tender to frost, but mature trees are hardy to 25∞F; it is recommended for Sunset zones 15-24.

The Kurrajong

Often simply called bottle tree in California, kurrajong (Brachychiton populneus) is the cold-hardiest of the commonly grown brachychitons, and is especially popular in cultivation in our low and intermediate desert areas. Native to dry areas in eastern Australia, it features a heavy, moisture-storing trunk that is broad at the base and tapers toward the top. It grows at a moderate rate to around thirty to forty feet tall and twenty-five feet wide, is hardy to at least 20∞F, and is reliably tolerant of heat and drought.

Kurrajong is evergreen, with glossy green two- to three-inch long, pointed leaves of variable shape, which shimmer in the breeze like those of aspens (its species name means “poplar-like”); new growth is tinged with pink. It blooms in late spring and summer with clusters of small, bell-shaped, white flowers flecked with red on the inside; they are partially hidden by the foliage but showy at close range, and are followed by clusters of two- to three-inch, dark brown seedpods, which are quite attractive when young.

In Australia, the foliage of kurrajong is used for livestock feed in times of drought, and was also used by the Aborigines for food and fiber. Seeds can be eaten raw, roasted, or made into a coffee-like drink. More drought-tolerant than it looks, kurrajong is an excellent shade tree for hot-summer areas; it also does well near the coast and is recommended for Sunset zones 12-24. Like other brachychitons, trees planted in deeper soils with adequate water will grow faster and larger than those in more challenging sites.

The Queensland Bottle Tree 

Many desert trees throughout the world develop thick, swollen trunks that store water and enable them to survive unusually dry periods. Native to rocky, inland areas of Queensland and northern New South Wales (its species name means “amongst rocks”), Queensland bottle tree (Brachychiton rupestris) has a giant, barrel-shaped trunk that is an exaggeration in water storage, and a perfect adaptation to life in the dry Australian outback. In cultivation, a mature tree is a real conversation piece and quite unlike the other brachychitons we grow. Queensland bottle tree is a small tree that grows fairly slowly to an eventual twenty feet tall and fifteen feet wide, with a stout gray trunk that grows fatter when rain (or irrigation water) is plentiful and may measure five to six feet in diameter at maturity. Dark green leaves on young trees are deeply lobed with narrow segments; on mature trees, the leaves may be undivided. Foliage is usually evergreen but may drop completely for a brief time in late spring in a dry site or before bloom. Flowering is unpredictable, and usually occurs in summer on older plants only, with the flowers being small, yellowish bells; the fruits are inch-long, leathery follicles.

Queensland bottle tree is often grown in containers by collectors, and makes an interesting dwarfed specimen with its fat trunk and swollen roots. In the landscape, it is a showy accent plant for a dry garden and an interesting companion to other desert plants. It prefers good drainage but needs only moderate to little watering, and is cold hardy to around 25°F. It is recommended for Sunset zones 13, 21, 23, and 24, but as a container plant with protection, it can be grown in colder areas. Although it takes a number of years to develop a mature specimen, this is a tree definitely worth waiting for!

Others To Try

Less-common but occasionally grown in California are a few other brachychitons of note. Pink kurrajong (Brachychiton x acero-populneus, syn. B. x roseus) is a natural hybrid between B. acerifolius and B. populneus. It is a small to medium-sized tree, intermediate between its two parents; it flowers at a young age because it is sold as a grafted plant. Dwarf kurrajong (B. bidwillii) is a small tree to twelve or fifteen feet in height, with deeply lobed, dark green, fuzzy leaves and clusters of fuzzy flowers similar to B. discolor but of a bright red color; it flowers while still young and is well adapted to southwestern gardens. Desert kurrajong (B. gregorii) and northern kurrajong (B. diversifolius) are both drought-tolerant small trees native to dry regions, with lobed leaves and characteristics somewhat intermediate between B. rupestris and B. populneus. For the ardent collector, of course, there are still more interesting species to be tried by obtaining seed from Australia.