In 1929, 2,200 sheep died in the Coonabarabran district of inland New South Wales after eating cycad seeds for an hour unmonitored. For decades, the blame fell on Macrozamia heteromera — the most conspicuous cycad in the Warrumbungle Ranges. It took a veterinary report in the June 1930 Agricultural Gazette of New South Wales to establish that the guilty species was actually Macrozamia diplomera, not M. heteromera. PACSOA authors Craig Thompson and Paul Kennedy have been emphatic in correcting this record: “M. heteromera, incidentally, is not the species associated by some writers with the mass poisoning of sheep in the Coonabarabran district in 1929.”
The confusion was understandable — both species have dichotomously divided pinnae, a character so rare among cycads that any forked-leaf Macrozamia in the Coonabarabran district would have looked like the same plant to a grazier. But to a botanist, the differences are clear, and they have become clearer with time: what Charles Moore described in 1883 as a single variable species with “different” leaf forms has, after David Jones’ 1998 revision, become an entire complex of five species with divided pinnae within the genus Macrozamia — Australia’s largest exclusively endemic cycad genus — each with its own ecology, its own morphology, and its own place in the harsh inland landscape of north-western New South Wales.”
Quick Facts
| Scientific name | Macrozamia heteromera C.Moore |
| Family | Zamiaceae |
| Origin | Warrumbungle Ranges and Coonabarabran district, NW New South Wales, Australia |
| Adult size | Subterranean stem, 15–25 cm diam.; 2–8 leaves, 50–90 cm long |
| Hardiness | −5 to −8 °C (23 to 18 °F) / USDA zone 8b–9a |
| IUCN | Least Concern (LC) |
| CITES | Appendix II (all cycads) |
| Cultivation difficulty | 3/5 |
Taxonomy and Nomenclature
Macrozamia heteromera was first discovered by Charles Moore during a collecting journey in 1858, and formally described by Moore in 1883 in volume 17 of the Journal and Proceedings of the Royal Society of New South Wales. Moore described the plant as having a small trunk 15–20 cm long, covered in reddish wool, with leaves about 60 cm long bearing pinnae “forked variously and simply.”
Etymology: the specific epithet heteromera (from Greek heteros, “different,” and meros, “part”) refers to the variable and inconsistent division of the pinnae — some pinnae fork once, others fork twice, others remain entire, sometimes all on the same frond.
The M. heteromera complex: Moore originally recognised two varieties of his species — var. glauca (longer, glaucous, lax leaves) and var. tenuifolia (rigid, dark green, doubly-forked pinnae with red bases). In 1998, David Jones revised the complex, segregating these variants into separate species:
- Var. glauca → Macrozamia glaucophylla D.L.Jones (the “blue form” of pre-1998 enthusiasts).
- Var. tenuifolia → Macrozamia polymorpha D.L.Jones.
The broader divided-pinnae group — the M. heteromera complex — includes five species: M. heteromera, M. diplomera, M. stenomera, M. glaucophylla, and M. polymorpha. These five share the extraordinary character of dichotomously divided pinnae — a trait almost unknown among cycads outside this complex. They grow in overlapping ranges across inland NSW, are often mistaken for one another, and hybridise where they meet.
Section: Macrozamia — despite its small size (2–8 leaves), M. heteromera has amphistomatic pinnae (stomata on both upper and lower surfaces), which places it in section Macrozamia alongside M. diplomera and M. glaucophylla. By contrast, the related M. stenomera has hypostomatic pinnae (stomata on the lower surface only) and belongs to section Parazamia. This cross-sectional distribution of the divided-pinnae character is one of the clearest demonstrations that the traditional Macrozamia/Parazamia division reflects convergent adaptation rather than a clean phylogenetic split.
Morphological Description
Macrozamia heteromera is a small, dioecious, evergreen cycad defined by its uniquely variable, forked leaflets.
Stem: subterranean, light green, 15–25 cm diameter. Moore described the original specimen as covered in reddish wool.
Leaves: 2–8 in the crown (typically 4–6), 50–90 cm long, moderately keeled. Petiole 15–20 cm long, 6–9 mm wide at the lowest pinna. Rachis not to moderately spirally twisted.
Pinnae — the diagnostic character: 60–100 per leaf, dull semi-glossy mid-green, with the defining feature of the species: dichotomous division. The typical pinna forks once into 2 sub-leaflets. However, it is common for some pinnae to bifurcate very close to the rachis and then divide again, producing 3, 4, or (very rarely) 5 sub-leaflets. Towards the tip of the frond, the last few pinnae often remain entire (undivided). The sub-leaflets overlap each other. Pinnae are angled forward and rise from the rachis in a characteristic “V” shape — the lower pinnae sweep sharply forward at an acute angle, while the upper pinnae extend more symmetrically. Creamy-yellow callouses mark the attachment point of each pinna to the rachis.
Amphistomatic: stomata occur on both upper and lower leaf surfaces — a section Macrozamia character unusual for a plant this small.
Male cones: fusiform (spindle-shaped), 14–21 cm long, 4.5–5.5 cm diameter; curved; up to 4 per plant.
Female cones: ovoid, 20–35 cm long, 8–12 cm diameter; green and pink sporophylls; usually solitary, occasionally 2.
Seeds: irregular prism-shaped, with red sarcotesta.
Coning: very irregular — a characteristic of inland Macrozamia species where rainfall and fire are unpredictable.
The Divided-Pinnae Complex: Comparison
| Character | M. heteromera | M. diplomera | M. stenomera | M. glaucophylla |
|---|---|---|---|---|
| Section | Macrozamia | Macrozamia | Parazamia | Macrozamia |
| Stomata | Amphistomatic | Amphistomatic | Hypostomatic | Amphistomatic |
| Pinnae division | 1–2× (rarely 3×) | 1× (often simple) | 1–4× (multi-divided) | 1–2× |
| Pinnae colour | Mid-green, dull | Blue-green | Green → blue | Glaucous blue |
| Leaf length | 50–90 cm | 30–90 cm | 40–80 cm | 60–120 cm |
| Leaves in crown | 2–8 | 2–10 | 2–14 | 2–8 |
| Distribution | Warrumbungle Ranges | Pilliga/Coonabarabran | Nandewar/Mt Kaputar | Gunnedah/Pilliga |
| Altitude | ~500 m | 200–500 m | 200–1,450 m | Low–moderate |
| IUCN | LC | LC | VU | LC |
Macrozamia heteromera is the “type” member of the complex — the green, moderately-sized species with typically once-divided pinnae. M. stenomera is the most extreme, with multi-divided pinnae giving a plumose, conifer-like appearance. M. glaucophylla is the blue-leaved, larger variant. M. diplomera often retains simple (undivided) pinnae and has blue-green colouring.
Distribution and Natural Habitat
Macrozamia heteromera is endemic to north-western New South Wales, centred on the Warrumbungle Ranges and extending to the Coonabarabran district. The Warrumbungle Range is composed of complex rocky formations — the remnants of a large, heavily eroded shield volcano active 13–17 million years ago. The species grows on sandy, stony, and infertile siliceous soils over acid volcanics, in dry sclerophyll woodland under a eucalypt canopy, on undulating foothills and stony hillsides.
Unlike the dense coastal populations of M. communis, M. heteromera grows as scattered individual plants, occasionally forming isolated, localised stands. Large dense populations are not typical.
Climate at Coonabarabran (elevation 509 m):
| Parameter | Coonabarabran |
|---|---|
| Annual rainfall | 735 mm (80 rain days) |
| Seasonal distribution | Summer 32%, Autumn 23%, Winter 22%, Spring 23% |
| Summer maximum (weekly) | 35.9 °C (January) |
| Winter minimum (weekly) | −3.6 °C (July) |
| Frost days per year | 76 |
Seventy-six frost days per year and weekly winter minima of −3.6 °C make the Coonabarabran climate one of the most severe experienced by any Australian cycad. Only M. stenomera (Mt Kaputar, up to 1,450 m, with snow) and M. montana (Blue Mountains, −9.3 °C records) endure comparably cold conditions. This natural frost hardening is the key horticultural asset of M. heteromera.
Conservation
Macrozamia heteromera is rated Least Concern (LC) on the IUCN Red List. It has a fairly widespread distribution across the Warrumbungle Ranges and is not considered immediately threatened. However, as with all inland Macrozamia:
- Livestock poisoning retaliation: the first reported poisoning from this species occurred in 1916 in the Coonabarabran district — cattle staggered and lost control of their hindquarters after eating leaves. Farmers have historically destroyed plants to protect stock.
- Fire: the dry sclerophyll habitat is fire-prone. Adults resprout from the subterranean stem, but seedlings and seeds are killed.
- Collecting: a lesser threat for this species than for the more spectacular M. stenomera.
Cultivation
| Hardiness | −5 to −8 °C (23 to 18 °F) / USDA zone 8b–9a |
| Light | Partial shade to full sun |
| Soil | Well-drained; sandy, stony, acid (volcanic-derived ideal) |
| Watering | Moderate; drought tolerant once established |
| Adult size | Compact: 50–90 cm fronds, 2–8 leaves |
| Growth rate | Very slow |
| Difficulty | 3/5 |
Cold hardiness: with 76 frost days per year and weekly winter minima of −3.6 °C in its core range, M. heteromera is among the most frost-hardy species in the genus. Applying the standard half-zone safety margin for slow-growing cycads: USDA zone 9a is the safe outdoor planting zone; zone 8b is achievable with winter protection (horticultural fleece, canopy shelter, dry root zone, 15–20 cm mulch). In the northern Mediterranean (Var, Hérault, Alpes-Maritimes), outdoor planting is feasible in well-drained, sheltered positions — but European cold events (February 1956 at −12 to −16 °C, January 1985, January 2012) can still destroy foliage and, in severe cases, the growing point. The subterranean caudex provides critical insurance: soil thermal inertia protects the growing point, and plants that lose all foliage can resprout from the stem if the frost does not penetrate to caudex depth.
Light: partial shade to full sun. In habitat, it grows under open eucalypt canopy — dappled light is natural.
Soil: well-drained, sandy to stony, acid. The species grows on volcanic-derived soils in the Warrumbungle Ranges — siliceous, infertile. It is well adapted to poor soils and should not be over-fertilised.
Watering: moderate. The 735 mm annual rainfall at Coonabarabran is reasonably evenly distributed across seasons. Drought tolerant once established.
Container culture: the compact size and slow growth make it suitable for containers. The divided pinnae are more intriguing at close range — a container on a bright terrace allows visitors to examine the forking pattern. Less visually spectacular than M. stenomera (with its multi-divided, plumose fronds and intense blue colour) but more robust and less demanding.
Buying Advice
Availability: rare in cultivation outside Australia. Occasionally available from specialist cycad nurseries. Be aware of the complex’s history of confusion: before 1998, plants sold as M. heteromera could have been any of the five species in the complex. The “blue form” of pre-1998 trade is now M. glaucophylla. True M. heteromera should have mid-green (not glaucous blue) pinnae, typically once-divided (not multi-divided like M. stenomera), and amphistomatic leaves.
Propagation
Seed: the only method. Clean the red sarcotesta (gloves — toxic). Sow in well-drained sandy mix at 20–28 °C. Delayed fertilisation: seeds not ready to germinate for approximately 12 months after ripening. Growth is very slow. Note: seedling pinnae are also divided — an unusual character that confirms identification from the earliest stage.
Pests and Diseases
Scale insects: occasional. Manageable with horticultural oil.
Root rot: in heavy, poorly drained soils — especially problematic for a species adapted to fast-draining volcanic substrates.
Toxicity: all parts are highly toxic. The species causes hindquarter paralysis (“rickets”) in cattle. Seeds caused fatalities in livestock as documented from 1916 onwards. Toxic to dogs, cats, livestock, and humans.
Landscape Use
Macrozamia heteromera is a botanical curiosity — a cycad whose pinnae fork like a fern’s, in a lineage where pinnae have been simple (undivided) for 280 million years. The “V”-shaped profile of the forked pinnae rising from the rachis, the creamy-yellow callouses, and the compact, scattered growth habit give it a distinctive presence quite unlike any other cycad. Use it:
- In a collection of divided-pinnae Macrozamia — alongside M. stenomera (multi-divided, blue, plumose), M. diplomera (blue-green, often simple), and M. glaucophylla (glaucous blue, large) to show the full range of dichotomous variation in the genus.
- As a frost-hardy cycad for zone 8b–9a — few cycads of comparable ornamental interest tolerate 76 frost days per year.
- On poor, stony, volcanic-derived soils — where other cycads would struggle, M. heteromera is at home.
Frequently Asked Questions
Why are the leaflets divided?
The dichotomous division of the pinnae is a character shared by five species in the M. heteromera complex. The evolutionary advantage, if any, is unknown, but all five species grow in the same inland NSW landscape — suggesting that divided pinnae may be a trait with a selective advantage in that specific environment, or a shared ancestral character retained in a lineage that has diverged relatively recently.
Is this the species that killed 2,200 sheep in 1929?
No. The mass sheep poisoning at Coonabarabran in 1929 was caused by Macrozamia diplomera, not M. heteromera. This was confirmed by a veterinary report in the June 1930 Agricultural Gazette of New South Wales. The confusion persists in some popular literature.
How does it differ from Macrozamia stenomera?
M. stenomera has narrower pinnae that divide 1–4 times (multi-divided, plumose appearance) and hypostomatic leaves (stomata on the lower surface only). M. heteromera has broader pinnae that typically divide once (sometimes twice) and amphistomatic leaves (stomata on both surfaces). M. stenomera is also more intensely blue and grows at higher altitudes (up to 1,450 m).
Can I grow it outdoors in Europe?
Yes — in the mildest Mediterranean areas (USDA zone 9a+) without special protection; in zone 8b with winter protection (fleece, mulch, canopy shelter). The 76 frost days/year at Coonabarabran make this one of the hardier Macrozamia species, comparable to M. communis and M. montana.
Authority Websites and Databases
POWO — Plants of the World Online (Kew)
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:297173-1
Accepted species. IUCN Least Concern (LC). EDGE score 1.16 (position 911). Evolutionary Distinctiveness 7.6.
PACSOA — Palm and Cycad Societies of Australia
https://www.pacsoa.org.au/wiki/index.php/Macrozamia_heteromera
Named by Moore 1884 after divided pinnae. Revised 1998 by Jones: M. glaucophylla and M. polymorpha segregated. Pinnae typically fork into 2 sub-leaflets (up to 4–5). “V”-shaped profile. Coonabarabran: 735 mm, −3.6 °C, 76 frost days. Not the species behind the 1929 sheep poisoning (that was M. diplomera).
Bibliography
Hill, K. D. (1998). Cycadophyta. Flora of Australia, 48, 597–661.
Hill, K. D., & Osborne, R. (2001). Cycads of Australia. Kangaroo Press, Sydney.
Jones, D. L. (1998). Revision of Macrozamia section Macrozamia (divided-pinnae complex). Flora of Australia, 48.
Jones, D. L. (2002). Cycads of the World (2nd ed.). New Holland Publishers, Sydney.
Moore, C. (1883). Macrozamia heteromera. Journal and Proceedings of the Royal Society of New South Wales, 17.
Whitelock, L. M. (2002). The Cycads. Timber Press, Portland.