Sphaeropteris medullaris aka Tree Fern Black

Sphaeropteris medullaris is an evergreen tree fern in the Cyatheaceae family. Its names include black tree fern, mamaku, and sago fern. It also bears the name Cyathea medullaris. Its native range spans New Zealand and the South Pacific, including Fiji. In New Zealand, it grows in lowland forests. These occur on North, South, and Stewart Islands.

This upright fern has a slim, black trunk. RHS gives a height range of 23–52 feet [7–16 metres]. Its size guide lists heights above 39 feet [12 metres]. The spread can exceed 26 feet [8 metres]. Long fronds rise from the crown, then arch outward. Each can reach 16 feet [5 metres] in length. Small, glossy, oblong leaflets make up each frond. Their undersides bear scales, and their edges have spines. Plants take 20–50 years to reach full size.

Grow it in partial shade, with shelter from harsh exposure. It needs moist soil that drains well and contains humus. Sandy or loamy soils suit it, with acid or neutral pH. RHS rates its UK hardiness as H3. Only mild areas allow outdoor growth without winter protection. In frost-prone places, keep it in a pot under glass. Maintain winter temperatures of at least 10°C there. Move pots to a sheltered, partly shaded site for summer.

During hot, dry summers, water both the trunk and soil. Keep water out of the crown in winter. Give liquid feed each month during the growing season. Remove dead or damaged fronds when needed. Sow spores when ripe to raise new plants. The tall trunk and broad crown give gardens a strong shape. RHS suggests courtyards, city gardens, coastal sites, and planted borders. Dappled woodland edges also suit its light needs. RHS reports few usual problems with pests or disease.

The stem pith has served as a starchy food. People have eaten it raw or cooked as a sago substitute. Harvesting that tissue kills the fern. Cooked frond bases are another recorded food use. PFAF reports no known medicinal uses. It found no reports of toxicity in this species. It still urges caution because some ferns contain harmful compounds. That warning does not establish this species as safe to eat.