Tropical hibiscus (Hibiscus rosa-sinensis L.), also commonly known as the shoe flower or chinese hibiscus, is a widely planted tropical flowering shrub throughout the world. This cultivated species is generally a highly heterozygous polyploid of complex ancestry (Singh and Khoshoo, 1970). At least 27 chromosome numbers have been reported for this species (Singh and Khoshoo, 1989) making determination of the basic number very difficult with a solitary report of 18 by Thombe (1959) being the lowest and widely accepted count. The Linnaean plant type had red double flowers and was believed to be native to China where it was cultivated for its showy flowers (Kimbrough, 1997), although no wild forms of the species are known to exist there. An alternate view is that the species is native to the south Indian Ocean islands with Madagascar as the center of origin. This view is supported by the fact that species from the region are genetically compatible with H. rosa-sinensis and that the Polynesian migration parallels the distribution of the species to China and eventually throughout the entire Pacific region. Species indigenous to the region thought to play a major role in the development of the modern H. rosa-sinensis of commerce include the south Indian Ocean island species H. schizopetalus Hook., H. liliiflorus Cav., H. fragilis DC., and H. boryanus Hook and Arn. in combination with the Pacific Island species H. kokio Hillebrand, H. arnottianus Gray, H. wimeae Heller, H. denisonii auct., and H. storckii Seeman (Singh and Khoshoo, 1989).
Seed set in H. rosa-sinensis is extremely rare under normal conditions in the tropics (Sharma and Sharma, 1962), but the species has moderately high pollen fertility of ≈60% (Singh and Khoshoo, 1989). Research in Italy verified that setting seed on H. rosa-sinensis can be very difficult but can be improved with proper attention to environmental conditions during pollination. Highest seed set was achieved in the spring and fall in moderate shade when the temperature ranged from 16 to 27 °C and dew point was between 10 and 16 °C (Mercuri et al., 2009). As measured by seed set, some H. rosa-sinensis cultivars make better female parents, whereas others make superior male parents (Lawton, 2004).
Tropical hibiscus plants display great diversity in flower color, size, and shape as well as plant habit. As compared with older hibiscus cultivars, modern tropical hibiscuses generally bloom more profusely, have more diverse intense flower colors, and are cutting-propagated instead of grafted (Dickey, 1950; Lawton, 2004). Breeders have traditionally focused on developing cultivars adapted to either of two basic market segments, florist grade and landscape types. Florist-grade hibiscus cultivars are developed for the flowering pot plant market and generally need to be compact, responsive to growth regulators, have high bud counts under greenhouse production environments, and have greener denser foliage. Outdoor patio hibiscus cultivars are developed for the landscape trade and can be planted in the ground in warmer climates such as Florida or California or used as summer potted accents on patios and around pools. Faster growth to fill larger pots, climatic tolerance, and extended bloom cycle are traits valued in this group (Lawton, 2004). Both ‘USS Arizona’ and ‘USS California’ were selected for the outdoor patio market.
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