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Hibiscus ovalifolius
Hibiscus ovalifolius | LemoHibiscus ovalifoliusn-yellow Rosemallow, Kaua‘i Yellow Hibiscus, Ma‘o Hau Hele, Rock’s Kaua‘i Hibiscus

This is a Hibiscus with a history! Formerly and variously known as Hibiscus calyphyllus, Hibiscus calycinus, Hibiscus chrysanthus, Hibiscus rockii or Hibiscus brackenridgei var. kauaiana. The name ovalifolius currently has priority over calyphyllus Cav. (1788), since ovalifolius is based on the older Urena ovalifolia Forssk. (1775). This perennial shrub grows 1 to 2m tall, with a spreading growth habit. It's 13cm flowers are yellow with a deep brownish or crimson eye. Hibiscus ovalifolius blooms from late spring through the summer with flowers that last a few hours. It requires full sun and well-drained soil.

Rock’s Kaua‘i Hibiscus —Hawaiian populations of Hibiscus ovalifolius were originally described as an endemic taxon, formerly known as Hibiscus rockii. There is continuing debate over their status, with some horticulturalists asserting that the Hawaiian population is sufficiently different from African and Madagascan ones that specific rank is warranted (Bornhorst, 1996). I wholeheartedly agree with this view. The most noticeable differences are found in the stems and growth habit: A side by side comparison of the plants I grow shows the Hawaiian Hibiscus ovalifolius to be a more relaxed, sprawling shrub with longer reddish stems as compared with the African variety. Furthermore, the Hawaiian version is almost thornless whereas the African version is not. There is also a slight difference in color —the Hawaiian having lighter yellow flowers, the African with a slightly more vibrant yellow hue.

Reference: Hibiscus rockii is a low sprawling shrub with large yellow blossoms, and is a variety native to Kaua‘i. It's natural growth habit is shrublike: from 2 to 4 feet tall. It is sometimes found in old kamaaina gardens on Kaua‘i and the other islands. For some scientists, this is evidence that it is a naturalized species. However, we grew the Kaua‘i plant next to the African Hibiscus calyphyllus in the Honolulu Botanical Gardens, and we found them to be distinct and different. The Kaua‘i variety is much prettier, with larger flowers, a more manageable growth habit, an fewer prickles on the base of the flower and along the leaves.

Growing Native Hawaiian Plants: A How-to Guide for the Gardener
By Heidi Leianuenue Bornhorst
Published by Bess Press, 2005

Historical Reference: Hibiscus calycinus.—This Hibiscus, also known as H. chrysanthus, is one of the shrubby members of the genus, and in the temperature of a warm greenhouse it grows well and flowers freely even in a small state. It is of a good branching habit of growth, clothed with rather pale green leaves, which, as well as the young shoots, are covered with hairs. The flowers, which are freely borne towards the ends of the shoots, are each nearly 3 inches in diameter and of a sulphur-yellow colour, with a large purplish brown blotch at the base of each petal, thus forming a dark coloured eye. The flowers, though somewhat campanulate in shape, as in many of the different kinds of Hibiscus, are produced in nearly an upright manner. This Hibiscus is very readily propagated from cuttings of the young growing shoots put in at any time during the spring and early summer months. Plants so obtained will flower in the autumn, but older specimens yield the best display of blooms. The individual blooms, as in most of the others, do not last long, but a succession is kept up for some time. As with many thin hairy-leaved plants, the greatest enemy to contend with is red spider, especially during a hot, dry summer.—T.

The Garden: an illustrated weekly journal of gardening, Volume 50
By William Robinson
Published 1896