The leaves are (very unusually for a cycad) deciduous in the dry season (though persistent if grown in moister situations), 55–90 cm long, slightly keeled or flat, pinnate with 100-220 leaflets; the leaflets densely orange-pubescent at first, then glossy bright green above, light green below, 5.5–14 cm long and 4.5–8 mm wide, angled forward at 40 degrees.

[3] The species is named after the plant collector John Armstrong (d. 1847), a Kew Gardens plant collector, who was based at Port Essington on the Cobourg Peninsula.

[4] This cycad is common in open forest and woodland around the Top End and can be easily confused with Cycas maconochiei.

[3] While locally abundant, less than 1% of these living fossils are located within conservation reserves.

It is one of the most abundant cycads in the world, with a population estimate of over ten million.