Semi tuberous begonias are easily distinguished by the thickened base of their stems, a characteristic that sets them apart. Typically, these stems develop a tuber either at or just below the soil surface, though unlike tuberous begonias, they do not undergo complete dormancy. Most semi-tuberous varieties exhibit small leaves and fall into distinct types, such as Weltonensis, dregei, and partita. Known for their prolific blooming, these begonias benefit from spring pruning, with subsequent pinching of new growth to encourage lateral branching until the desired size and shape are achieved. Once shaped to satisfaction, they can be left to bloom. After 2 to 3 months, a trim may be necessary to prevent them from becoming straggly, prompting a swift regrowth and resumption of blooming.
Originating primarily from South Africa, with a few varieties hailing from Bolivia and Brazil, semi-tuberous begonias include favorites like Begonia dregei, also called ‘The Maple Leaf Begonia.’ Discovered by J.F. Drege near the coast of South Africa on shaded slopes in the Cape of Good Hope, this variety showcases a unique thickening of the stem at the base, tapering off into slender branches. With their distinctive appearance, resembling small trees, they are well-suited for bonsai-like cultivation, enhanced by their preference for under-potting. Responding positively to pruning for shaping purposes, these begonias produce small white flowers in spring and summer.
Thriving under shade cloth with good air circulation, particularly in frost-free zones, or within controlled glasshouse conditions, semi-tuberous begonias can be grown successfully. In cooler conditions, growth may slow, and some leaf loss may occur, but they generally avoid complete dormancy. Applying an anti-fungal spray in variable conditions helps prevent mildew.
Propagation is achieved through stem cuttings during the active growth phase in spring. Begonia dregei, in particular, grows well from seed, with seedlings quickly developing the characteristic swollen base. Although semi-tuberous hybrids are not abundant, the enduring Begonia ‘Weltonensis’ remains in cultivation today. Varieties of the maple leaf begonia have also played a role in hybridizing miniature Rex and Tuberous hybrids. Overall, cultivating semi-tuberous begonias is a delightful and rewarding endeavor.
Begonia Society of Western Australia 