A mallee is a shape rather than a species. Instead of one trunk it throws up a dozen thin ones from a single point at ground level, all leaning outward, so a mature plant looks less like a tree than like a bundle of saplings that agreed to share a base. Blue mallee gets to five or ten metres and stops. The interesting half is below your boots.
The lignotuber
Under that cluster of stems is a woody swelling called a lignotuber: a mass of stored starch studded with dormant buds, sitting at and just below the soil surface. It is the plant's insurance policy. Fire takes the stems, drought takes the stems, a grazing animal or a chainsaw takes the stems, and the lignotuber does not particularly distinguish between them. It simply pushes out a new set.
Mallee eucalypts evolved this in country that is hot, dry and burns often, where anything betting everything on a single trunk eventually loses. What it produces is a plant with no obvious lifespan. The stems above ground are young. The organism underneath them can be very old indeed.
Which is why it is cut, not felled
That biology decides how the oil is farmed, and it is the reason blue mallee harvesting looks nothing like forestry.
The stems are coppiced, meaning cut back near ground level, and the whole cut top goes to the still as leaf and fine stem. Nothing is replanted. The lignotuber sends up new growth and the same plants are cut again on a cycle of a year or two, then again, and again. Stands around West Wyalong and Bendigo have been worked this way since the early 1900s. The plants being cut today are, in a real sense, the same plants.
The cutting interval is not just about how much leaf has grown back. Oil concentration and oil quality move in different directions as regrowth ages: total oil is highest in the youngest regrowth, while the proportion of 1,8-cineole keeps climbing and is higher at twelve months than at three, six or nine. So the harvest date is a decision about chemistry, not only about volume. Cut too early and you get more oil that is worth less.
Why that number is the whole industry
1,8-cineole, also called eucalyptol, is the compound a pharmacopoeia actually measures, and a high, stable, reliable cineole figure is what makes a eucalypt commercially worth planting rather than merely pleasant. Blue gum will usually reach the bar. Blue mallee sits well above it, consistently, in a plant that regrows from its own base and asks very little of the ground it is on.
You can smell the difference straight away. Blue gum is broad and camphorous and slightly sweet around the edges. Blue mallee is narrower and colder, sharper on the first breath, cleaner in the finish, with less of the soft green undertone. Neither is better. Blue gum is the familiar one and blue mallee is the surgical one, and which you want depends on whether you are after comfort or clarity.
Two patches of scrub
Blue mallee is scattered thinly across southeastern Australia rather than confined to one district, which makes it sound more widely available than it is. Sporadic is the word the floras use. What matters commercially is that the harvested populations are just two disjunct pockets: the scrub around West Wyalong in New South Wales, and the mallee country running from Bendigo west and south toward Stawell, taking in Inglewood, Wedderburn and Wychitella. Those two have supplied the industry since the early 1900s.
A third natural population sits further west again, running north from near Quorn through the Flinders Ranges to the Gammon Ranges in South Australia. The plants were never lost, but their recognition as a distinct source region for the oil is recent, and they have never been harvested commercially or, as far as anyone has published, cultivated at all. A well-studied economic plant still holding an unworked reserve is a rare thing.
Almost all of Australia's eucalyptus oil traces back to two patches of inland scrub, cut on rotation, from root systems that were already there when the industry started.
The species was described in 1901 by R. T. Baker, working from material collected at West Wyalong. Baker is the same botanist who, with the chemist H. G. Smith, put a name to the northern cypress pine behind blue cypress. Two very different Australian oils, named by the same hand, within a decade of each other.
Usual cautions apply and they are the ones that apply to all the cineole-heavy eucalypts: dilute it, keep it away from the faces of babies and young children, and check with a practitioner if asthma is a factor. Treated with a bit of respect it is the most efficient thing in the cupboard for a blocked head and a room that needs clearing out.