Rex Begonia (ht 20 cm) are generally regarded as indoor plants rather than permanent garden plants, as they don't like cold weather - even the relatively mild Sydney winters. However, they can be used as temporary plantings in shady garden areas through the warmer months to provide fabulous effects and it is worth leaving them in as they often do survive through winter - mine do, at least for a year or so. They can also be grown in decorative pots, which can be placed in a garden bed in the warmer months then brought under the cover of a verandah or shade house in the cooler weather. One cultivar that seems to be the most long-lived in Sydney gardens is known as 'Grey Ghost', shown in the photo below. Rex Begonia - more properly called Begonia Rex-cultorum Group - are rhizomatous in habit and are hybrids developed from crossing the species Begonia rex with rhizomatous Begonia with distinctive foliage. The allure of these plants is their stunning foliage, with colours rarely found in other plants, such as pinks, red, silver, mauve, purple and black, and interesting spots and stripes. Many have a lovely metallic lustre and an interesting texture.
Rex Begonia like well-drained soil and high humidity. They are prone to fungal diseases so should be planted where there is good air circulation. They are best planted out in spring, and they should grow well through summer and autumn. They can be grown indoors, as long as there is sufficient humidity around them and they are not overwatered. Add perlite to the potting mix to keep the mixture open and well drained. The plants like bright light but not direct sun. Propagation is via leaf cuttings in a pot of perlite; keep the pot covered with plastic or in a plastic box until roots develop.
Some pairings I have made using them in my garden include a silver variety with very dark purple markings placed nearby the sultry leaves of Colocasia 'Black Magic'; and a silver and pink one with pink Plectranthus ecklonii, some shock-headed pink Aechmea fasciata, pink Justicia carnea and the dainty pink Justicia brasiliana. The echo of the leaf colour with the hue of the flowers seems to lift the composition to a better level.
For more information, visit the very informative website of the NSW Begonia Society.