In the Macrozamia heteromera article on succulentes.net, we cleared the name of M. heteromera from the mass sheep poisoning at Coonabarabran in 1929. Now it is time to name the guilty party. Macrozamia diplomera is the species whose seeds killed 2,200 sheep out of a flock of 6,000 after they grazed unmonitored for an hour in a paddock where the cycad grew. Five sheep died within 18–20 hours; deaths continued for three weeks. A veterinary report in the June 1930 Agricultural Gazette of New South Wales, complete with a photograph of the offending plant, confirmed the identification beyond doubt. It is a grim claim to fame for a species that is otherwise remarkable for very different reasons: Macrozamia diplomera is the smallest member of section Macrozamia — the group defined by basal pinnae reduced to spines, untwisted rachis, and amphistomatic leaves — yet it can produce up to 50 leaves in the crown, more than most section Parazamia species. Its pinnae are dichotomously divided, linking it to the M. heteromera complex of inland NSW. Its seedlings look more like section Parazamia than section Macrozamia. And deep below its subterranean caudex, a 1.2–1.5 m whip-cord tap root plunges straight down into the sandy Pilliga soils — a feature so unusual that PACSOA authors describe it as “peculiar.” Dave’s Garden calls it “the peculiar member of this group of Macrozamias.” The genus Macrozamia, Australia’s largest endemic cycad genus, has no other species quite like it.
Quick Facts
| Scientific name | Macrozamia diplomera (F.Muell.) L.A.S.Johnson |
| Family | Zamiaceae |
| Origin | Pilliga district and Coonabarabran area, NW New South Wales, Australia |
| Adult size | Stem mostly subterranean, 20–40 cm diam.; up to 50 leaves, 60–120 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 diplomera was originally described by Ferdinand von Mueller as Encephalartos spiralis var. diplomera — the use of the African genus Encephalartos for Australian cycads was common in early colonial botany. L. A. S. Johnson transferred it to Macrozamia in 1959 during his landmark revision of Australian Zamiaceae, raising it to species rank.
Etymology: from Greek diplos (“double”) and meros (“part”) — referring to the dichotomously divided pinnae, which typically fork once into two segments.
Synonyms: Encephalartos spiralis var. diplomera F.Muell.; Macrozamia tridentata f. diplomera (F.Muell.) J.Schust.
The divided-pinnae complex: M. diplomera is one of five species with dichotomously divided pinnae in inland NSW: M. heteromera, M. diplomera, M. stenomera, M. glaucophylla, and M. polymorpha. Of these, M. diplomera is the only one placed in section Macrozamia by all authorities — it has basal pinnae reduced to spines, a character that unambiguously separates it from the Parazamia members of the complex (M. heteromera and M. glaucophylla are amphistomatic but assigned variably; M. stenomera is hypostomatic/Parazamia).
Section: Macrozamia — despite being the smallest species in the section. The key characters: rachis not twisted, basal pinnae reduced to spines, up to 50 leaves in the crown. Carpenter (1991) noted shared cuticular characters between M. diplomera and species of both sections, suggesting possible neotenic events in its ancestry — the plant may have retained a compact, subterranean habit from a juvenile stage of a larger-bodied ancestor.
Morphological Description
Macrozamia diplomera is a small to medium, dioecious, evergreen cycad — compact in habit but remarkably leafy for its size.
Stem: mostly subterranean, 20–40 cm diameter. Normally unbranched, though PACSOA documents one multi-headed plant damaged by road grading that produced three separate heads and two underground suckers — suggesting a capacity for vegetative regeneration under trauma.
Root system: some plants produce a 1.2–1.5 m whip-cord-like tap root extending straight down from the caudex base into the sandy soil — “quite possibly as a means of drawing extra moisture from the sub-layers of the soil” (PACSOA). This extraordinary root is unique in the genus.
Leaves: up to 50 in the crown (PlantNET), 60–120 cm long. Rachis not twisted. The crown is dense — Dave’s Garden describes the habit as either “upright and shuttlecock” or “lax and droopy.” The leaves are a lighter pale green compared to the dark green of the co-occurring M. reducta — a colour difference immediately visible in the field. Petiole 10–20 cm long.
Pinnae — the three-type puzzle: 70–120 per leaf, 15–20 cm long, 5–10 mm wide, with 6–13 raised veins beneath. Most pinnae are dichotomously once-divided (into two segments of 2.5–5 mm wide with 3–7 veins each). However — and this is the “inconsistency” that PACSOA emphasises — a detailed examination of plants reveals three distinct types of pinnae on the same plant:
- Entirely divided fronds: most pinnae forked into 2 sub-leaflets.
- Entirely entire fronds: all pinnae simple (undivided) — these plants look like M. reducta.
- Mixed fronds: some pinnae divided, others entire, on the same rachis.
Very rarely, pinnae division produces 5 sub-leaflets. The division usually ceases towards the frond apex, with the last few pinnae remaining entire. Basal pinnae are progressively reduced and spine-like — the diagnostic section Macrozamia character.
Male cones: fusiform; coning very erratic. Solitary or multiple. Seeds can have red or yellow coloured flesh — the yellow-seeded variant is notable.
Female cones: ovoid; usually solitary, occasionally 2 or more.
Seedlings: small and secund — PACSOA notes they “appear to resemble Section Parazamia seedlings, more so than a Section Macrozamia seedling.” This Parazamia-like juvenile morphology supports Carpenter’s (1991) neoteny hypothesis.
Comparison with Macrozamia reducta
PACSOA draws a direct comparison: “If any enthusiasts familiar with Macrozamia reducta were to see a stand of M. diplomera for the first time their initial impression would probably be that they were viewing a stand of M. reducta plants.” The key differences:
| Character | M. diplomera | M. reducta |
|---|---|---|
| Pinnae | Mostly dichotomously divided | Entire (never divided) |
| Colour | Lighter pale green | Dark green |
| Leaves in crown | Up to 50 | 12–40 |
| Stem diameter | 20–40 cm | 20–40 cm |
| Leaf length | 60–120 cm | 70–150 cm |
| Distribution | Pilliga / Coonabarabran | Newcastle–Mudgee–Glen Davis |
| Seeds | Red or yellow | Red |
Distribution and Natural Habitat
Macrozamia diplomera grows in the Pilliga district and Coonabarabran area of north-western New South Wales — the same broad landscape occupied by M. heteromera, M. glaucophylla, and M. polymorpha. It grows in dry sclerophyll woodland on deep sandy soils, in a pattern of scattered individual plants or localised stands.
The climate is comparable to Coonabarabran: ~735 mm annual rainfall (evenly distributed), summer maxima to 36 °C, winter minima to −3.6 °C, and 76 frost days per year. This is a harsh, inland, frost-prone climate.
Hybridisation: M. diplomera hybridises naturally with Macrozamia polymorpha in areas of sympatry. PACSOA hypothesises that emu seed dispersal is responsible for the mingling of species: emus are attracted to the brightly coloured seed flesh, swallow the entire fruit, and excrete the seeds intact during their daily travels — potentially depositing M. diplomera seeds in M. polymorpha territory and vice versa. This is supported by observations of fleshless cycad seeds in probable emu droppings near M. diplomera stands, and by anecdotal evidence of emus dispersing the native peach (Santalum acuminatum) in the same manner.
Conservation
Macrozamia diplomera is rated Least Concern (LC) on the IUCN Red List. It has a reasonably broad distribution in the Pilliga district and is not immediately threatened. Threats are comparable to other inland NSW Macrozamia: fire, livestock poisoning retaliation, forestry disturbance, and feral animal impacts.
Cultivation
| Hardiness | −5 to −8 °C (23 to 18 °F) / USDA zone 8b–9a |
| Light | Full sun preferred |
| Soil | Well-drained; deep sandy soils ideal; excellent drainage essential |
| Watering | Low to moderate; drought tolerant once established |
| Adult size | 60–120 cm fronds, up to 50 leaves; dense crown |
| Growth rate | Very slow (but “one of the faster growing” small Macrozamia per Dave’s Garden) |
| Difficulty | 3/5 |
Dave’s Garden describes it as “an excellent, though very slow-growing, container plant; or a good landscape plant for smaller areas of the garden. This species, perhaps more than any other in this subgenus, really needs good drainage or it is prone to rot.” The emphasis on drainage is critical — the Pilliga’s deep sandy soils provide natural fast drainage that must be replicated in cultivation.
Cold hardiness: “quite cold hardy and can handle many degrees of frost with little trouble” (Dave’s Garden). With 76 frost days/year at Coonabarabran, M. diplomera has the same frost-hardened provenance as M. heteromera and M. glaucophylla. Applying the standard half-zone safety margin: USDA zone 9a safe; zone 8b achievable with winter protection. The subterranean caudex and the extraordinary deep tap root both provide thermal insurance.
Light: full sun preferred. Dave’s Garden notes that the bluish-green foliage colour is best in bright positions.
Container culture: excellent. The dense crown (up to 50 leaves in the shuttlecock form), the glaucous green to blue-green colour, the pink-coloured leaflet bases, and the compact size make it one of the most ornamental small Macrozamia for containers. Ensure very fast drainage to prevent rot.
Buying Advice
Availability: rare outside Australia. Dave’s Garden lists it as “one of the more attractive of the small Macrozamias, having glaucous green to blue-green upright, closely spaced blunt-tipped leaflets.” Plants with entirely entire (undivided) pinnae may be confused with M. reducta — check for the lighter pale green colour, the basal spines (absent in section Parazamia species), and provenance (Pilliga = M. diplomera; Newcastle–Mudgee = M. reducta). Plants from the southern Pilliga showing intermediate characters may be hybrids with M. polymorpha.
Propagation
Seed: the only method. Seeds have either red or yellow sarcotesta. Clean thoroughly (gloves — toxic). Sow in well-drained sandy mix at 20–28 °C. Delayed fertilisation: 12 months before germination readiness. Note: seedlings are small, secund, and Parazamia-like in appearance.
Pests and Diseases
Root rot: the primary risk in cultivation. Dave’s Garden emphasises that this species “really needs good drainage or it is prone to rot.” Use fast-draining, sandy substrates. Avoid heavy clay or waterlogging at all costs.
Scale insects: occasional. Manageable with horticultural oil.
Toxicity: extremely toxic. This is the species confirmed as the cause of the 1929 Coonabarabran sheep disaster (2,200 deaths from a flock of 6,000). All parts contain cycasin and macrozamin. Toxic to dogs, cats, livestock, and humans. The first reported cattle poisoning (hindquarter paralysis) from *Macrozamia* in the Coonabarabran district (1916) was attributed to M. heteromera, but given the taxonomic confusion of the era, M. diplomera may have been involved.
Landscape Use
Macrozamia diplomera is the oddball of the genus — section Macrozamia by its spines and leaf count, section Parazamia by its seedlings and stature, with divided pinnae that link it to four other inland NSW species, a 1.5 m tap root that links it to the sandy Pilliga earth, and a history that links it to 2,200 dead sheep. Use it:
- As the section Macrozamia representative in a divided-pinnae collection — the only member of the complex with basal spines, alongside *M. heteromera* (green, section Macrozamia by stomata), *M. stenomera* (blue, Parazamia), and *M. glaucophylla* (blue, ambiguous).
- As a dense, bushy container specimen — up to 50 blue-green leaves in a shuttlecock form, with pink leaflet bases. Few Parazamia-sized cycads produce this many fronds.
- In a frost-tested, drought-tolerant planting — zone 8b–9a, full sun, sandy soil, minimal irrigation. The Pilliga provenance means this species is pre-adapted to the kind of neglect that kills tropical cycads.
Frequently Asked Questions
Is this really the species that killed 2,200 sheep?
Yes. Confirmed by a veterinary report with photographs in the June 1930 Agricultural Gazette of New South Wales. The confusion with M. heteromera persists in some popular literature but has been emphatically corrected by PACSOA.
Why is it in section Macrozamia if it’s so small?
Because it has the diagnostic characters: basal pinnae reduced to spines, rachis not twisted, up to 50 leaves. The small stature may reflect neoteny (retention of juvenile characters in a reproductively mature plant) — Carpenter (1991) noted shared cuticular characters with species from both sections.
What about the tap root?
Some plants produce a 1.2–1.5 m whip-cord-like tap root extending straight down into the sandy soil. This is not documented in any other Macrozamia and may be an adaptation to the deep, fast-draining Pilliga sands.
Authority Websites and Databases
POWO — Plants of the World Online (Kew)
https://powo.science.kew.org/taxon/…
Accepted species.
The World List of Cycads
https://cycadlist.org/scientific_name/384
Basionym: Encephalartos spiralis var. diplomera F.Muell. Synonym: Macrozamia tridentata f. diplomera. Distribution: New South Wales.
PlantNET — New South Wales Flora Online
https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/…
Stem mostly subterranean, 20–40 cm diam. Leaves to 50, 60–120 cm, rachis not twisted. Pinnae 70–120, most dichotomously once-divided, 15–20 cm × 5–10 mm. Lowest pinnae reduced to spines. Petiole 10–20 cm.
PACSOA — Palm and Cycad Societies of Australia
https://www.pacsoa.org.au/wiki/index.php/Macrozamia_diplomera
Resembles M. reducta but with divided pinnae and lighter pale green. Three types of pinnae: entire, divided, mixed. 1.2–1.5 m whip-cord tap root. Hybridises with M. polymorpha (emu dispersal hypothesis). Seeds red or yellow. Confirmed species behind 1929 Coonabarabran sheep poisoning. Seedlings resemble section Parazamia.
Bibliography
Carpenter, R. J. (1991). Cuticular morphology and aspects of the ecology and fossil history of North Queensland rainforest Proteaceae. Australian Systematic Botany, 4, 559–588. [Neoteny hypothesis for Macrozamia.]
Hill, K. D. (1998). Cycadophyta. Flora of Australia, 48, 597–661.
Johnson, L. A. S. (1959). The families of cycads and the Zamiaceae of Australia. Proceedings of the Linnean Society of New South Wales, 84, 64–117.
Jones, D. L. (2002). Cycads of the World (2nd ed.). New Holland Publishers, Sydney.
Whitelock, L. M. (2002). The Cycads. Timber Press, Portland.