How to identify a eucalypt by its bark

There are more than seven hundred species of eucalypt, and almost all of them are Australian. This is an awkward number. It is large enough that the genus resists casual identification, but the trees themselves are so common — street verges, paddock edges, the back of every school oval — that most of us stop looking at them entirely. They become the generic gum tree, a shape rather than a species.

The difficulty is partly structural. Eucalypts are tall, and they hold their flowers high. The leaves hang vertically to avoid the midday sun, which makes them awkward to read from below, and on a mature tree the nearest one may be fifteen metres up. The parts a botanist would want are almost always out of reach.

The bark is not. It is at eye level, it is there in every season, and in most cases it will get you to a group of species in a few seconds. Australian field botany has leaned on this for a long time, and the common names of eucalypts — ironbark, stringybark, box, bloodwood — are almost all bark names rather than leaf or flower names. What follows is a primer on the seven types that carry most of the work.

1. Gums — smooth bark, shed annually

The default. Smooth-barked eucalypts shed their outer bark each year in strips, flakes, or plates, exposing a fresh surface underneath. The result is the pale, clean, faintly luminous trunk that most people picture when they picture a gum.

Look for a surface with no fibrous texture at all — the trunk feels cool and hard, closer to stone than to timber. Colour varies enormously by species and by how recently the tree shed: cream, olive, grey, pink, orange, sometimes several at once in mottled patches where shedding is uneven.

The snow gum, Eucalyptus pauciflora, is the most photographed example, its trunk streaked in cream and olive and rust. In Western Australia the salmon gum, E. salmonophloia, holds a smooth pinkish-copper trunk that reads as almost artificial in late light.

2. Scribbly gums — smooth bark, written on

A subset of the smooth-barked gums, and worth separating because the marking is unmistakable. The trunk carries dark, looping, meandering lines that appear to have been drawn on the surface with a fine pen.

They are feeding tunnels. The larvae of small moths in the genus Ogmograptis burrow in the layer between the old bark and the new. When the outer bark sheds, the tunnel is exposed as a scar on the fresh surface. The characteristic shape — a long wandering track that turns back and runs alongside itself — reflects the larva's habit of doubling back along its own tunnel to feed on the callus tissue that has formed there in the meantime. The scribble is a record of an animal eating its own scar.

Common on E. haemastoma, E. racemosa, and E. rossii, mostly in the sandstone country of New South Wales.

3. Ironbarks — dark, hard, deeply furrowed

Unmistakable once seen. Ironbarks retain their dead bark rather than shedding it. It accumulates year on year, becomes impregnated with kino — the dark red resin the genus exudes when injured — and hardens into a thick, deeply fissured armour, almost black, running the full height of the trunk and out along the larger branches.

The furrows are deep enough to put a finger into. The colour is the giveaway: no other bark type is this dark. The retained layer insulates the living tissue underneath, which is generally read as a fire adaptation.

E. sideroxylon, the mugga ironbark, is the widely planted ornamental, occasionally with pink flowers rather than cream.

4. Stringybarks — long fibres, pulls away in strips

Thick, rough, reddish-brown to grey bark composed of long fibres running vertically. It can be pulled away from the trunk in strips — this is the field test, and it is where the name comes from.

Where ironbark is hard and mineral, stringybark is soft and fibrous, and the surface has a shaggy, loose quality. The bark is persistent, held on trunk and branches alike.

E. obliqua, messmate stringybark, was the first eucalypt formally described, in 1788.

5. Boxes — short fibres, tessellated or flaky

The awkward middle category, and the one most often mistaken. Box bark is retained and rough like a stringybark, but the fibres are short rather than long, so it cannot be pulled off in strips. Instead it comes away in small flakes or short scales, and the surface often has a subtly tiled or fissured appearance.

Many box species are half-barked: rough bark on the lower trunk giving way abruptly to smooth bark on the upper limbs. That transition line is diagnostic in itself, and worth looking for on any tree that seems to be two species at once.

E. melliodora, yellow box, is the familiar one through the southeast, and one of the more significant nectar sources for Australian apiarists.

6. Peppermints — finely fibrous, densely matted

Similar to box at a glance, but finer. The bark is closely and densely fibrous, matted rather than flaky, and generally greyer. The confirming test is not the bark at all: crush a leaf. Peppermint eucalypts carry high concentrations of volatile oils that smell distinctly of peppermint rather than the sharper, more medicinal note of a typical eucalypt.

E. radiata is the common narrow-leaved peppermint of the southeast.

7. Bloodwoods — tessellated, like crocodile skin

Rough bark broken into small, more-or-less square segments across the whole trunk, giving a pattern closer to reptile skin than to bark. Once the tessellation is noticed it is hard to unsee.

The name comes from the copious dark red kino these species exude. Most bloodwoods have been moved out of Eucalyptus into the related genus Corymbia — including Corymbia ficifolia, the red flowering gum, which is a Western Australian species now planted as a street tree across much of the country.

A note on ribbons and minni ritchi

Two variations sit outside the main seven and are worth recognising.

Ribbon gums shed their bark in long strips that do not fall cleanly, instead catching in the branches and hanging there in streamers, sometimes for months. E. viminalis and E. rubida both do this, and a stand of them in summer has a distinctly dishevelled look.

Minni ritchi bark curls back on itself in small, hard, tightly rolled strips that stay attached at one edge, revealing a fresh reddish surface beneath. It is largely an arid-zone character — E. orbifolia is the clearest example, and E. caesia carries it at the base of the trunk.

A short key

Working from the trunk alone:

• Smooth and clean, no fibre — a gum. Check for pen-line scribbles: if present, a scribbly gum.
• Very dark, very hard, deep furrows — ironbark.
• Rough, long fibres, pulls off in strips — stringybark.
• Rough, short fibres, flakes rather than strips — box. Check whether the roughness stops partway up.
• Rough, fine and densely matted — peppermint. Crush a leaf to confirm.
• Rough, broken into small squares — bloodwood, and probably now a Corymbia.
• Long strips hanging in the branches — ribbon gum.
• Small tight curls over a red under-surface — minni ritchi.

None of this settles a species. Getting there means the buds, the fruit, and the arrangement of the leaves, and for some groups it means accepting that the boundaries are genuinely unresolved. But the bark will reliably narrow seven hundred species to a workable handful in the time it takes to walk past, which is a reasonable return on looking at something most of us have decided is scenery.

The first section of Eucalyptus Folio Volume I is given over to bark and trunk studies, drawn at a scale that holds the texture of each type. Each plate’s footer names the species in both common and botanical form.