Begonia ‘Magma’ (Rex Cultorum Group)

How horticulture produced ‘Magma’

‘Magma’ is a distinct Rex Cultorum clone identified by broad silver-pink blades. The wider lineage begins with Begonia rex from humid rock faces, cave margins and shaded valleys in the eastern Himalaya, forming the principal wild background of the group.

Later breeders crossed that lineage with tropical rhizomatous begonias from additional regions, then selected seedlings that retained broad silver-pink blades with burgundy backs. ‘Magma’ exists only in cultivation and is propagated vegetatively to preserve broad silver-pink blades with burgundy backs and its established growth habit.

How to care for Begonia ‘Magma’ (Rex Cultorum Group)

Light

Place Begonia ‘Magma’ (Rex Cultorum Group) at an east window. A south or west window also works from 0.5–1.5 m back, provided hot sun does not reach the leaves. Direct sun tolerance is limited: metallic and pale zones bleach first, followed by dry brown margins under a strong unacclimated beam. If the room is too dim, the petioles elongate, the crown leans and new leaves are smaller with flatter colour contrast. Natural leaf pigments produce the silver-pink wash among the green zones. Growth made in dim light is less distinct, but hot direct sun fades or scars the coloured tissue.

Watering

Water Begonia ‘Magma’ (Rex Cultorum Group) when roughly the upper 2–3 cm has dried and the lower shallow root zone still holds a trace of moisture. Pour slowly enough that dry channels rewet, stopping only after water exits freely below. One hard dry-out may leave the rhizome alive but unable to support its full canopy. Wet compact mix around the rhizome can soften living tissue before the leaves show obvious stress. Water ‘Magma’ according to the active root zone, which may stay moist after the pot edge has dried. Drying depends on light, temperature, airflow, how fully roots fill the pot and the container material.

Substrate and roots

Begonia ‘Magma’ (Rex Cultorum Group) has a shallow root mat directly below a creeping rhizome. A practical potting medium is a fine, open foliage mix containing peat-free substrate or coir plus small bark and pumice. The feeder roots need close contact with moisture without being sealed inside dense compost. Use a shallow, wide pot with drainage so the rhizome can travel across the surface. Give the rhizome of ‘Magma’ a clear path across the top of the pot without covering it.

Temperature

Keep Begonia ‘Magma’ (Rex Cultorum Group) at 18–26 °C, with 13 °C as a cautious indoor minimum. The lower limit assumes active drainage. A saturated pot should be kept warmer. Moderate day-to-night change is useful. Sharp temperature swings are not.

Humidity and airflow

Most Rex begonias manage ordinary room air, but 55–70% humidity improves expansion of soft new leaves. Give ‘Magma’ open space among other plants so moderate humidity does not turn into a wet, still pocket. Use mild continuous air exchange and keep occasional strong blasts away from the foliage.

Fertilising

Feed Begonia ‘Magma’ (Rex Cultorum Group) with a balanced complete fertiliser at quarter strength every four to six weeks while it is actively making leaves or shoots. Do not follow a rigid summer-versus-winter rule. Check how actively the plant is growing and how quickly the substrate dries. Alternate feeding with plain water so fertiliser residue does not build up around delicate feeder roots. Regular weak feeding suits ‘Magma’ better than an occasional strong application.

Repotting

Repot Begonia ‘Magma’ (Rex Cultorum Group) when the rhizome reaches or circles the rim, roots fill the shallow pot, watering becomes uneven or the mix loses its open structure. Select the next modest size, generally 2–3 cm wider. Preserve the fine root mat around each rhizome section and divide only actively growing rooted pieces. Choose a warm, well-lit period of active growth, when disturbed roots can recover promptly. Keep the root mat of ‘Magma’ together and lay the rhizome across the surface of the new mix.

Support, pruning, handling and leaf care

Let Begonia ‘Magma’ (Rex Cultorum Group) form a broad foliage crown over a surface-running rhizome. Turn the growing end into free pot space, and divide only a rooted piece that retains a bud and healthy leaf. Rotation is useful for balance, but wait until soft foliage has expanded before changing its direction. The metallic surface of Begonia ‘Magma’ can show fingerprints and pressure marks. Support mature blades underneath and clean with a soft lightly damp cloth without polishing. Remove a dead leaf with clean scissors, leaving healthy stem and growing points untouched.

Common problems with Begonia ‘Magma’ (Rex Cultorum Group)

Silver-pink centres turn beige

Dry beige tissue facing a lamp or window is likely scorch, especially where it stops at a shaded fold. Increase distance or filter the light. If the area starts translucent, grows despite the move or develops a yellow halo, isolate the plant and remove the leaf because the mark may be leaf spot.

Dark serrated margins crisp

Magma’s undulating edge is naturally dark; dead tissue is lighter brown, dry and brittle. Check for erratic root moisture, a draught or fertiliser residue. Keep root moisture and airflow steady. Flush salts only through freely draining mix and let the rhizome drain well.

Burgundy undersides carry pale insects

Check the marks closely. Mealybugs form cottony clusters around veins and petiole bases; adult whiteflies lift from the foliage when disturbed. Isolate either pest, remove heavily occupied leaves and use a treatment labelled for the insect, covering the underside without leaving the crown wet.

Fiery rosette collapses inward

Dense petioles can conceal a decaying rhizome. If several leaves loosen together, expose the centre immediately and cut out every dark soft part. Root a healthy outer section shallowly in fresh, airy mix. If only one healthy leaf remains, use it for a leaf cutting immediately.

Pet precautions for ‘Magma’

‘Magma’, including its broad silver-pink blades, contains soluble calcium oxalates and is toxic to cats and dogs. A pet that chews the leaves or a stem may drool or vomit; the highest concentration occurs in the roots. Keep the pot beyond a pet’s reach. Collect removed leaves and block access to exposed roots or crown pieces during repotting. Recheck access after moving furniture or placing the pot on a lower surface. If drooling, vomiting or swallowing follows contact with the leaves or a stem, contact a veterinarian. Bag discarded roots, spent substrate and cuttings after repotting, then wash hands.

Molten colour in the name ‘Magma’

Magma is molten or partly molten rock beneath Earth’s surface. In this cultivar name it evokes hot reds, dark crust-like edges or flowing colour.

The plant is a Rex Cultorum begonia. That horticultural group arose through repeated hybridisation and selection after Begonia rex entered European cultivation, producing leaves far beyond the pattern of the original species.