The Australian tree fern, one of the most beautiful and elegant tree ferns for mild winter climate regions, is appropriate for outdoor landscapes in U.S. Department of Agriculture hardiness zones 9 through 12. It remains the most widely cultivated tree fern species according to tropical plant and palm expert Robert Riffle. The Australian tree fern hails from the cloudy, humid rainforests and tropical lowlands of extreme southeastern Queensland to central coastal New South Wales in Australia.
Indeed a fern, the Australian tree fern belongs to its own family, Cyatheaceae, distinguishing it from other other terrestrial and tree ferns. Its botanical name is Cyathea cooperi, named after Sir Daniel Cooper, a mid-19th century legislator from New South Wales. Older botanical names that remain synonyms include Alsophila cooperi and Sphaeropteris cooperi. Additional common names for the Australian tree fern are lacy tree fern, Cooper's tree fern and Queensland tree fern.
Quickly growing 20 to 40 feet tall, Australian tree fern develops a trunk-like stem, an upright rhizome, or fleshy storage stem. This stem is clad in numerous leaf scars and dark fibers and drops away only on very old specimens. A broad rosette of huge fronds radiates at the growing tip at the top of the stem. Overall, each frond is a tapering oval but comprises two to three-ranked branchlets lined in thousands of segments. Each frond can reach up to 10 to 18 feet in length. Spores occur on the undersides of fertile fronds in summer, and uncoiling new fronds appear most heavily in spring and summer.
Grow Australian tree fern in fertile, moist, well-draining soils that are not alkaline in pH (over 7.2). A sandy or loamy soil rich in organic matter is ideal. In cool summer areas with high humidity, the fronds tolerate nearly all-day sunlight exposure, but in general this plant fares best in spots with shifting dappled shade and bright indirect light. Water it freely from spring to fall, and allow soil to become slightly drier in winter. Apply well-rotted manures as a root zone mulch. Excessive wind, low humidity and cold or hot, dry winds harm the fronds, but it is more tolerant of these sub-prime conditions than all other species of tree ferns. Frosts will nip the fronds, and temperatures below 25 to 28 degrees F. completely kill fronds. If the growing tips and emerging fronds become fully frozen, the plant will die. Tall-stemmed plants survive to 20 degrees F. without death to the growing tip, according to Robert Riffle in "The Tropical Look."
In the United States, the Australian tree fern can be mistakenly marketed and sold as Cyathea australis, a rarer, endangered tree fern species. This misidentification causes confusion, as Cyathea australis is more slow growing, more cold hardy, and grown mainly by rare plant enthusiasts, or as part of protected collections in botanical gardens.
The Australian tree fern is an invasive plant in Hawaii, displacing habitat for native tree ferns on several islands. Also avoid getting hairy fibers of the stem and fronds on your skin when you work around this plant, as they create an incessant need to scratch.