Cibotium glaucum
Hapuu tree fern
Overview
Cibotium glaucum is a tree fern endemic to Hawaii, carrying a stout fibrous trunk 10-25 feet (3-7.5 m) tall and 8-16 inches (20-40 cm) thick topped by a spreading crown of fronds 6-13 feet (2-4 m) long. Fronds are tripinnate, mid green above and coated beneath with a waxy bloom that reads blue-white, the character behind the epithet glaucum spp.. Emerging croziers are wrapped in soft golden-brown hairs known as pulu, dense enough that a single unfurling frond carries a visible tuft. Spore-bearing sori sit along the pinnule margins in two-valved cup-shaped indusia. The trunk is a mass of adventitious roots rather than woody tissue, so it holds water and supports mosses, seedlings, and epiphytes on its surface, and fallen trunks act as nurse logs for Metrosideros polymorpha and other wet-forest natives. Trunk growth runs 1-2 inches (2.5-5 cm) a year, so a 20 foot (6 m) specimen represents many decades. Feral pigs open the trunk base and eat the starchy core, killing plants outright, and populations fell further during the 19th-century pulu trade when trunks were stripped for pillow stuffing. The species tolerates neither drying, direct midday sun, nor frost.
Native Range
Cibotium glaucum is endemic to the Hawaiian Islands and is most abundant on Hawaii island, with scattered populations on Maui, Molokai, Oahu, and Kauai. It occupies wet and mesic montane forest between 1,000 and 6,000 feet (300-1,800 m), forming the understorey layer beneath Metrosideros polymorpha canopy. Annual rainfall across that range is 80-250 inches (2,000-6,400 mm).Suggested Uses
Planted in shaded tropical and subtropical gardens, fern houses, and conservatories at spacings of 8-12 feet (2.4-3.7 m). Used in Hawaiian wet-forest restoration as a nurse plant, where the trunk surface and fallen logs supply germination sites for native seedlings. Trunk sections were formerly cut as orchid-growing substrate, a use now restricted in Hawaii under state conservation rules.How to Identify
Appearance
Size & Dimensions
Height10' - 25'
Width/Spread10' - 20'
Reaches mature size in approximately 30 years
Colors
Foliage Colors
Fall Foliage Colors
Bloom Information
Cibotium glaucum reproduces by spores and produces no flowers. Sori ripen on mature fronds through the warm months, chiefly May to October, and release spores that germinate on constantly damp substrate within 2-6 weeks. Gametophytes develop over 3-6 months before the first small frond appears. A plant starts producing spores once it has formed a trunk of 12 inches (30 cm) or more, generally at 8-15 years.
Detailed Descriptions
Foliage Description
Green above, waxy blue-white beneathGrowing Conditions
Sun Requirements
Tolerates up to 4 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight
Care & Maintenance
Care Guide
Cibotium glaucum grows in dappled to deep shade with 0-4 hours of filtered sun, in acidic humus, cinder, or leaf mould at pH 4.5-6.0 held constantly moist. Air humidity above 70 percent and 80-250 inches (2,000-6,400 mm) of annual rainfall, or equivalent overhead misting, support normal frond size. The trunk absorbs water directly, so irrigation is directed over the crown and trunk as well as the root zone, and a trunk that dries through kills the crown even where soil stays wet. It withstands brief drops to 28F (-2C) in shade and is damaged below that, holding it to USDA zones 10-11. Transplanting large trunks frequently fails, since the fibrous root mass regenerates slowly. Slugs graze emerging croziers, and feral pigs excavate the trunk base at unfenced sites.Pruning
Dead fronds are cut back to 4-6 inches (10-15 cm) from the trunk in late spring, and the stubs are left in place because they armour the trunk surface and hold moisture. Green fronds are retained, since each crown carries only 10-20 at a time and refoliates slowly. Fronds broken by wind are cut at the base of the damaged stipe. The trunk top is left uncut, as the single growing point neither branches nor resprouts.Pruning Schedule
J
F
M
A
M
J
J
A
S
O
N
D
late spring
Container Growing
✓ Suitable for container growing
Minimum container size: 25 gallons