Dry, inhospitable shade ... we all have such spots in our gardens, usually under trees or shrubs where few plants will thrive. But by massing groups of the plants known as rhizomatous Begonia, we can make an effective evergreen carpet in these adverse conditions. Sometimes thought of as rather old-fashioned plants, they deserve another look for all their obliging qualities. They are tough, evergreen groundcover plants with thick creeping rhizomes that have shallow roots and low water needs. What's more, they actually love shade! There are two main types of rhizomatous Begonia: the Asian ones, which include Begonia rex and which are hard to grow successfully in Sydney as long-term plants; and the South American ones, which do brilliantly in our climate, and which are discussed here. Semi-tropical in origin, they are ideal for life outdoors in warm, humid climates such as Sydney. A tree canopy will protect them from light frosts, but in cold climates they are best grown in pots, so that they can be sheltered in winter.
Like most Begonia, the rhizomatous sorts have an incredible diversity of leaf forms, which vary in shape, size, surface texture, colour and patterning. These range from rounded forms which look like lily pads, to lop-sided hearts, stars and slim-fingered hands. Others have leaves with their centres coiled into three-dimensional spirals. Some have leaves with a highly lacquered sheen while others have velvet, hairy or pimpled surfaces. Leaf edges may be smooth, ruffled, twisted or saw-toothed. Leaf colours vary from bright to dark green, brown, gold, lime, silver, pewter, pink, purplish brown to near black; with or without contrasting coloured veins, edges, splashes, streaks or coloured undersides.
A planting of a few different types of these Begonia will soon knit together to form a rich patchwork quilt in a dry shady situation. Plain-leaved ones can be alternated with patterned forms, and colour can be played with: juxtaposing silvery leaves with darker foliage for example, to create a dramatic effect. They also contrast well with upright, strap-leaved plants that tolerate dry shade, such as Liriope, renga-renga lilies (Arthropodium cirratum) and bromeliads; as well as with ferns. They of course make a wonderful underplanting beneath their taller cousins, the cane-stemmed and shrub-like Begonia.
As an added bonus, in mid-spring, rhizomatous Begonia are smothered in a haze of dainty, rounded flowers, usually pink or white. These hover above the leaves for several weeks and can act as a substitute in warmer areas for the airy spring blooms of Heuchera, Tiarella and Tellima that do not thrive so well out of cooler climates, and they can provide a pretty display under spring-flowering shrubs.
There are many cultivars of rhizomatous Begonia, and collecting them can easily become an obsession. Some reliable favourites include B. bowerae (ht 15cm, dark 'eyelash' edging on bright green foliage; pale pink flowers); 'Cleopatra' (ht 30cm, lime green, star-shaped leaf with brown markings; pale pink flowers); 'Erythrophylla (ht 30 cm, the so-called 'beefsteak begonia', with leathery, olive green leaves, red underneath; pink flowers); 'Blackie' (ht 30 cm, pointed, near-black leaves; pink flowers); 'Kara' (ht 60 cm, large, crinkled, dark green and chocolate foliage; pink flowers); and 'Silver Jewel' (ht 15 cm, pebbled, green leaves marked with silver; white flowers). Some of the very fancy, unusual cultivars can be difficult to grow in garden settings, and are better suited to life in a pot.
In fact, where space doesn't permit the plants to be grown in the ground, any of the rhizomatous forms can live very happily in shallow pots or hanging baskets on a shady veranda. They should be kept on the dry side, as overwatering them can be fatal, whether they are in a pot or garden bed. Add perlite to the potting mix to keep the medium well drained and open. Alternatively, use an African violet potting mix. They can be re-potted after several years when they become congested, although the effect of the plants as their rhizomes clamber over the sides of a pot can be quite charming!
The only pest these plants seem to be affected by is the snail, so it's advisable to use snail bait around them. Although the plants are very forgiving of neglect, an occasional drink of water, a scattering of slow-release fertiliser in spring, and a blanket of compost tucked around them at the same time will encourage them in their ground-covering ways. Tip-pruning the plants after flowering will help to produce thicker growth.
Propagation is simplicity itself. Just snap off a piece in spring or summer, stick it into the ground or a pot of propagating mix and keep it watered for a while. They can also be grown from leaf cuttings placed into a pot of perlite or vermiculite (which can be mixed with hydrated coco peat): stick the stem vertically into the perlite and push the leaf down so it is making contact with the surface of the mixture. Keep in a sheltered, humid environment (such as a large, clear plastic box with a lid and drainage holes drilled at the bottom); it should take root within eight weeks. Obtaining pieces from other gardeners is a good way to build up a collection of rhizomatous Begonia. The plants get exhausted after about five years, when it is time to replace them with a freshly propagated specimen. Cuttings strike easily; the best months of the year to do this seem to be March, April and November.