Begonia

Species pages start from this and record only where they differ.

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

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