Begonia Rex Cultorum Group
Rex begonia · Painted-leaf begonia · King begonia
- Family
- Begoniaceae
- Native to
- Tropics and subtropics to central China
- Habit
- Rhizomatous perennial
Where it comes from
Nowhere — and that is the point, so this page has no native range where the others do.
A Group is a category of cultivated plants under the Cultivated Plant Code. It has members, not a distribution. What has a wild range is the species one of those members descends from: Begonia rex Putz. ex K.Koch, which Kew gives as native to Assam, the East Himalaya and Myanmar, a rhizomatous geophyte of the subtropical biome.1 Its altitude band is roughly 200-1,250 m.2 Missouri Botanical Garden gives the range as north-east India, southern China and Vietnam; Kew has no China and no Vietnam, and NC State independently matches Kew.34
That is worth holding onto because it explains the care. This is a subtropical montanemontaneOf the cool, humid forest belt on a mountainside, above the lowland tropics. Plants from it dislike hot dry rooms.Glossary plant, not a lowland tropical one — which is why the prescription is warm days, distinctly cooler nights and high humidity, rather than uniform warmth.
The received origin story is that the whole group descends from one B. rex that reached Europe in 1856 hidden in a shipment of orchids. Very little of that is documented. The society’s own account says 1857 and never mentions 1856, and the chain from there runs hobby page to encyclopedia to everyone.5 What is documented: the plant is native to Assam and adjoining regions, it was named from cultivated material in Belgium and validated in Berlin in 1857, and it was in the European trade fast enough to accumulate eight synonymssynonymA botanical name that is no longer the accepted one for a plant, usually because the same species was described twice. Old books and plant labels are full of them.Glossary — several of them nurserymen’s names — within a few years.16 Not documented anywhere I could open: the year 1856, the orchid, England, or who found it. A useful control: UF/IFAS has a publication on begonia hybridization history that names year, breeder and parents for the events it does record, and it does not mention any of this.7
Naming and description
“Begonia rex-cultorum” is not a name. It is not in IPNIIPNIThe International Plant Names Index. It records where and when a name was published, without ruling on whether it is the accepted one — that is POWO's job.Glossary and it is not in Kew — a search of each returns nothing at all.89 It is a horticultural shorthand that acquired a species-shaped spelling, and this page does not use it.
The correct form comes from the Cultivated Plant Code itself. Article 15.1: formal Group status is shown by the word “Group” as the first or final word of the epithetepithetThe second word of a botanical name — the deliciosa in Monstera deliciosa. It belongs to the species, and it never stands on its own.Glossary — and the Code’s own worked example is literally Begonia Elatior Group.10 So the plant is Begonia Rex Cultorum Group. Article 15.3 covers the case where a cultivarcultivarA plant selected and propagated by people rather than described from the wild. Written in single quotes after the species — Monstera deliciosa 'Thai Constellation' — and never italicised.Glossary is known: the Group epithetepithetThe second word of a botanical name — the deliciosa in Monstera deliciosa. It belongs to the species, and it never stands on its own.Glossary goes in round brackets immediately before it — Begonia (Rex Cultorum Group) ‘Escargot’, on the pattern of the Code’s Fagus sylvatica (Atropunicea Group) ‘Riversii’.10 The common form without brackets is wrong.
The American Begonia Society has been the international registration authority for Begonia since 1955,11 and holds about a thousand registered cultivarscultivarA plant selected and propagated by people rather than described from the wild. Written in single quotes after the species — Monstera deliciosa 'Thai Constellation' — and never italicised.Glossary across the whole genusgenusThe rank above species: a group of species close enough to share a name. Monstera and Philodendron are genera; the first word of a botanical name is the genus.Glossary — so most rex begonias in the trade are unregistered.
This page cannot tell you which cultivarcultivarA plant selected and propagated by people rather than described from the wild. Written in single quotes after the species — Monstera deliciosa 'Thai Constellation' — and never italicised.Glossary you have, and neither can a photo of a leaf: estimates of the number in circulation run from several hundred to several thousand.12 If yours came with a name, keep the label.
One thing worth knowing about the look: the silver ring is not a hybrid invention. The wild species is described with a pale ring marking on a dark green lamina.2
Fun facts
- A bald rex in midwinter is usually asleep, and there is a timer that prevents it. UF/IFAS states that rex begonias require 14 hours of day length to maintain growth, and that in autumn and winter many cultivarscultivarA plant selected and propagated by people rather than described from the wild. Written in single quotes after the species — Monstera deliciosa 'Thai Constellation' — and never italicised.Glossary stop growing and may drop every leaf.13 Three other sources describe the same rest.3145 So it is photoperiodphotoperiodDay length, as a plant measures it. Several plants here take their cue from it rather than from temperature, which is why a warm windowsill can defeat a dormancy or a flowering they would manage outdoors.Glossary, not cold and not neglect — and a grow light on a fourteen-hour timer stops it happening.
- Which means the most useful thing on this page is how to tell sleep from rot. Firm rhizomerhizomeA thickened stem that creeps along or under the surface, storing food and putting up leaves as it goes. It is a stem, not a root.Glossary, no leaves, midwinter: resting — water sparingly and wait for spring, and do not feed it.5 Soft, blackened, water-soaked stem base: Pythium, which is favoured by waterlogged and cooler media.15 The first is fixed by doing less. The second is not fixed at all.
- “Low light plant” is wrong. UF/IFAS puts the ideal at 1,500-2,200 foot-candlesfoot-candleA unit of light on a surface, one lumen per square foot, still the everyday unit in horticulture. About 10.8 lux. Bright indirect light starts around 1,000 of them.Glossary — though that is a Florida production figure under shade, and Clemson notes few homes reach even 1,000.1316 The practical reading is: as bright as you can manage indirect, and more than you think.
- The toxic part is the one on top. The ASPCA’s entry gives soluble calcium oxalates and says the most toxic part is underground.1718 For most plants that is reassuring. A rex is grown with its rhizomerhizomeA thickened stem that creeps along or under the surface, storing food and putting up leaves as it goes. It is a stem, not a root.Glossary sitting on the surface of a shallow pot, so the part carrying most of it is exposed — at every division, and to anything that chews.
- The parentage is tradition, not a pedigree. The group is said to descend from B. rex crossed with around thirty other species, but I could find no molecular study resolving it. Real work on the species does exist and places it in section Platycentrum.219
Care
- Light
- Bright indirect, no direct midday sun. UF/IFAS's 1,500-2,200 foot-candles for rex begonias is a Florida production figure under shade, not a windowsill target — Clemson classes anything over 1,000 as very high and notes few homes reach it. The useful indoor floor is 150-300 foot-candles, which kept trial plants in good condition for nine weeks where 75 did not.
- Water
- Water through once the top third of the mix has dried, and err dry — these rot from below far more easily than they wilt. Keep water off the leaves and out of the crown: that is a disease instruction, not a cosmetic one, and five separate extension services give it.
- Humidity
- Above 50% for a rex. Cane types are not fussy and do not need a humidifier. But humidity without air movement is the trap here — three different pathogens want exactly the conditions that raising humidity creates, so ventilation and clearing dead leaves matter more than the number.
- Temperature
- 65-80 °F (18-27 °C), floor 55 °F (13 °C). A rex prefers 72-80 °F (22-27 °C) by day and 60-65 °F (16-18 °C) at night — the cool-night preference of a montane plant — while cane types are content a little cooler.
- Substrate
- A light, free-draining, peat- or coir-based mix at pH 5.7-6.4, in a shallow pot. The shallow pot is the same physics as the watering advice: less wet mix sitting under a shallow root system.
- Fertilizer
- Dilute and frequent beats strong and occasional — a balanced feed at 100-150 ppm nitrogen monthly, or quarter strength every fortnight. Feed cane types higher in potassium and rhizomatous types higher in nitrogen. Never feed a resting plant.
- Propagation
- Leaf cuttings for a rex — triangular wedges each containing a main vein, set about 1 cm into the mix under a dome, roots in three to five weeks with a shoot shortly after. Cane types do not root from a leaf: take stem cuttings of 8-10 cm with two or three nodes. Rooting hormone speeds it up but is not necessary.
- Dormancy
- Depends on the type, and getting this wrong kills plants. Cane-stemmed begonias are evergreen and simply slow down. Rex begonias often do rest — they stop, and may drop every leaf — and the trigger is photoperiod: UF/IFAS records that they need a 14-hour day to keep growing, so a timer prevents it. A resting rhizome wants barely moist mix and no feed at all until spring.
- Common problems
-
- Powdery mildew is the defining disease. It does not need water on the leaf to infect, and it is favoured by 60-80 °F (16-27 °C), shade and humid nights after dry days — which describes an indoor windowsill exactly. Above 85 °F (29 °C) it stops
- Botrytis wants humidity above 85% and a film of moisture lasting eight to twelve hours, and it turns into crown rot on an established plant or base rot on a cutting
- Bacterial leaf spot (Xanthomonas campestris pv. begoniae) splashes from leaf to leaf, has no cure, and survives a year in a dried fallen leaf — so clearing dead foliage off the mix is disease control, not tidiness
- A bald rex in midwinter is usually resting, not dying. Firm rhizome and no leaves in January is dormancy; a soft, blackened, water-soaked stem base is Pythium, and the two call for opposite responses
- Misting is bad advice for this genus, and the RHS and Clemson both say so outright — spray the air around the plant instead, or raise humidity by other means
- Every part carries soluble calcium oxalates and the ASPCA says the most toxic part is underground. That matters more for a rex than most: the rhizome sits on the surface of a shallow pot, within reach