Hawaiian Begonia

A botanical rarity

Hillebrandia sandwicensis sits just outside the true begonias, as their only living cousin. It is the single species in its own genus, the one member of the begonia family that sits outside the vast genus Begonia, and it grows wild only in the Hawaiian islands. Botanists treat it as a living relic, the only surviving member of a lineage that split from the begonias many millions of years ago. For a grower, that means a plant with the familiar begonia look, lopsided leaves and winged seed capsules, but with its own habits. It is scarce in cultivation and valued by collectors of unusual plants.

How it looks

The leaves are large, rounded and shallowly lobed with toothed edges, pale green and softly hairy, held on long fleshy stalks that rise from a creeping underground stem. The whole plant is soft and herbaceous, lacking the woody canes of many begonias. In its season it produces airy sprays of pale pink to white flowers, delicate and five-petalled, over the mound of foliage. The flowers keep the family pattern of separate males and females on one plant, the female bloom sitting on a small winged seed capsule, though this genus differs from true begonias in the fine detail of the ovary that first set it apart.

Its island home

In the wild it grows on shaded, dripping cliffs, stream banks and the walls of steep ravines in the wet mountain forests of Hawaii, rooted in the film of soil over wet rock. It lives in cool, misty, constantly humid air and light softened by the forest canopy, and it follows the islands’ wet and dry rhythm by dying back to its rootstock through the drier months and returning with the rains. That habitat points to warmth without heat, high humidity and shade.

Getting the light right

Bright but indirect light matches the shaded ravines it comes from. An east or north window, or a well-lit spot out of direct sun, keeps the soft leaves in good order. Direct sun, even for an hour or two through summer glass, scorches and bleaches them, while heavy shade draws the stalks up thin and pale. It grows best in a terrarium or a bright, humid corner.

Watering and its resting phase

While in leaf, keep the compost consistently moist, watering as soon as the surface begins to dry, since this is a plant of wet places that does badly with drying out hard. As the leaves fade towards the end of its season, cut watering right back and let the rootstock rest cooler and much drier, or it will rot in cold, saturated compost. Bring the watering back gradually as new growth appears. Rainwater or filtered water suits it, as the plant is adapted to the soft water of mountain streams.

Warmth and humidity

Steady warmth without extremes suits it, and it should stay above about 13 deg C in growth and out of any draught. Humidity is where this plant is least forgiving, having evolved in permanently misty forest, so a dry heated room browns and crisps the leaves fast. A humid bathroom, a large pebble tray, a plant grouping, or a glass case keeps the moisture around it high. Keep some air movement, so the damp air does not become stagnant and rot the soft foliage.

Feeding

Feed sparingly through the growing season, using a balanced liquid fertiliser at a quarter to half strength every few weeks while the plant is in active leaf. Its soft tissues and modest root system are easily scorched by strong feed. Give nothing once it begins to die back, and none at all while it rests.

Propagation

Division of the rhizome in spring, as new growth begins, is the surest way to increase it, potting each piece into open, moisture-holding compost and keeping it shaded and humid until it takes hold. Fresh seed, sown on the surface of damp compost and kept humid and warm, will germinate, though seedlings grow slowly and need constant moisture. Because the plant is rare and slow, handle every division and seedling with care and grow the young plants on in a humid, shaded place. Wild populations are limited and localised, so cultivated plants are worth sharing among growers who can meet their needs.

Keeping it healthy

Given the moist, shaded, frost-free conditions it needs, the plant is easy enough, though it suffers in dry air and cold wet roots. Brown, papery leaf edges point to a room that is too dry. Grey mould can spread on soft leaves in still, damp conditions, so keep air gently moving. Spider mites arrive when the air dries out and stipple the foliage; higher humidity helps prevent them. A rootstock that fails to return has almost always rotted in cold, wet compost during its rest.

The year in outline

Spring starts growth from the rootstock; divide, sow seed and begin light feeding as the shoots rise.

Summer is full leaf and flower; keep the compost moist, the air humid and the light indirect.

Autumn brings the plant towards dormancy; reduce water and feed as the leaves fade.

Winter is a cool, dry rest for the rootstock; keep it barely moist and wait for the new season’s growth.

Common questions

Is it really a begonia? It is the begonia family’s only other genus, a close relative instead of a true begonia, and unique to the Hawaiian islands.

Why have the leaves gone brown and crisp at the edges? The air is too dry. Raise the humidity sharply and move it away from any heat source.

How much light does it want? Bright, indirect light; keep it out of direct sun, which scorches the soft leaves.

How do I propagate it? Divide the rhizome in spring as growth restarts, or sow fresh seed on damp compost kept humid.

Is it safe around pets? Treat it with caution. It has not been tested, and true begonias contain oxalates that burn the mouth, so keep it away from pets that chew.