Plant Story--Hibiscus!

Hibiscus rosa-sinesis, Hawaii

I really like hibiscus. They have big showy flowers. They are also easy to recognize.

There are probably 300 species of Hibiscus, it is the biggest genus in a big family. The plant family they are in, the mallow family, Malvaceae, boasts 243 genera and at least 4,225 species. Hibiscus, sometimes but less often called rose mallows, are found from the warmer temperate regions into the tropics.

A quick search found hibiscus species that are native to the eastern United States and across the southern U.S. to California, central and northern South America, Africa, Yemen which suggests warmer parts Eurasia, India, southeast Asia, Australia, New Zealand, and Hawaii. As far as I can tell, none are native to Europe. With 300 species it is pretty easy to have natives all over the world. Many species are small and relatively inconspicuous.

The big cultivated tropical flowers that you see in vacation holiday pictures and behind the ears of beautiful island girls are hybrids between species native to Hawaii, southeast Asia and India, created in the last 200 years. Often these are primarily Hibiscus rosa-sinensis. The species epithet rosa-sinensis means "Chinese rose." It is the national flower of Malaysia and very commonly grown in the tropical Pacific. It is probably native to southeast Asia; the Chinese wrote about growing it in 295 BCE. Originally red, it now comes in many colors and patterns.

Hibiscus rosa-sinensis

Hibiscus rosa-sinensis does not survive more than a touch of frost. The common hibiscus grown in cooler regions, the Confederate rose (Hibiscus mutabilis) and rose-of-Sharon (Hibiscus syriacus) were both domesticated in southeast Asia--the Chinese wrote of them in the 12th century and 1st century CE respectively. The Confederate rose gets its common name because it was widely planted in the U.S. South in the 1800s. Rose-of-Sharon is a relatively modern common name for a plant that is the national flower of Korea, where it has been cultivated for several thousand years. The rose-of-Sharon name almost certainly refers to the Bible verse (Song of Solomon, 2:1) but the reference in the Bible, though debated, is probably to a tulip (see Wikipedia summary link).  The name is catchy and has been applied to a variety of flowers, but currently, especially Hibiscus syriacus. 

Rose-of-Sharon's scientific name, Hibiscus syriacus, literally "Syrian hibiscus" is the result of an error. We forget how recently the world became interconnected. Rose-of-Sharon was named by Linnaeus, a Swede who had never traveled farther than Paris and who died in 1778. In the 1300s the Chinese emperor forbade Europeans to leave selected port cities or travel the interior, an edict which remained in place until after the Second Opium War in 1860. In 1639 Japan similarly restricted European trade to Dutch ships landing at the tiny island of Hirado and rarely permitted a Dutchman off that little island, a policy that lasted until 1853. The hibiscus Linnaeus was shown must have traveled the Silk Road through central Asia, to be grown in Syria and there discovered by Europeans, so as far as he knew, it was from Syria.

Hibiscus syriacus, rose of Sharon--correction Hybiscus xmoscheutos
see note and comment at end

Elsewhere in the world hibiscus became a vegetable, the roselle, Hibiscus sabdariffa. Native to southern Africa, it is grown all over the tropics, to the point that one common name is Jamaican sorrel. The calyces (fleshy red sepals of the flower, see photo below) are gathered and dried. Called hibiscus flowers, the calyces are used as an important sweet-sour flavoring across Africa and Asia and as a tea or tea ingredient ("hibiscus flowers"), beautiful red in color.

roselle, Hibiscus sabdariffa, seen in Bali

All the species of hibiscus are edible, as are most, if not all, closely related species in the mallow family (Yes, marshmallow was originally a sweet made from the roots of a European mallow, Altheaea officinalis.) More use could certainly be made of hibicus as food, for example, garnishing salads with the glorious flowers. Right now, though, my two plants are struggling so I appreciate their flowers but don't pick them. 

Hibiscus rosa-sinensis

The pigments in hybiscus flowers are strong enough to dye cloth. Generally flower colors are not good dyes, either failing to transmit the color to the fiber or fading very rapidly.  Sheron Buchele Rowland and I pulled the dark red hibiscus flowers from the plant in the picture below and got a rich stable reddish shade, much to my surprise. (On wool, mordanted with alum, for the record.)

Flowers of this hybiscus made a nice dye (H. xmoscheutos not H. syriaca)

Below is the lemonyellow rose mallow Hibiscus calyphyllus. It is found all over Hawaii and was thought to be endemic when I took the photo years ago, but it is now believed to have been brought from Madagascar after which it naturalized. 

Hibiscus calyphyllus formerly called Hibiscus rockii

Definitely endemic to Hawaii, koki'o, Hibiscus kokio.

Hibiscus kokio, Hawaii

Tropical hibiscus have extrafloral nectaries on the bracts, outside the flower where these sugar-water producing glands attract ants but not pollinators. Ants have been repeatedly shown to reduce the damage to plants by defending the sugar source (nectary) or preying on small herbivorous insects. Hawaii is the largest, most complex landmass in the world with no native ants. In the 1980s I hypothesiized that native Hawaiian plants would lack extrafloral nectaries since they lacked ants and spent most of a year gathering data on the topic. You can see the insect at the base of the green floral bracts in the photo of the endemic koki'o above. Most but not all Hawaiian species in genera with extrafloral nectaries elsewhere in the world had extrafloral nectaries like their relatives. Perhaps 10 million years (the age of the oldest island bits) was not enough for plants to lose a trait not in use, perhaps other insects, just not ants, provided defense for the plant. Elsewhere in the world wasps, beetles and a variety of other insects feed at extrafloral nectaries. Moreover, where Hawaii has an endemic genus--plants that had evolved so much on Hawaii as to be given their own genus found nowhere else on earth--extrafloral nectaries were lacking, in particular, Hibiscadelphus, derived from Hibiscus. My conclusion was a qualified yes, without ants extrafloral nectaries tend to be lost.

Hibiscadelphus flower
endemic Hawaiian genus derived from Hibiscus

The photo below shows ants on the extrafloral nectaries of the bracts of a hibiscus, lemonyellow rose mallow, Hibiscus calyphyllus. There's a shiny drop of nectar about as long as one of those ants if you look carefully. Hawaii has more than a few species of ants today, they've stowed away with human transport over the last several 100 years. In the 1980s I would have called this hibiscus an endemic Hawaiian species but today it is considered a naturalized alien, so the extrafloral nectaries came with it centuries not millennia ago. That is one fewer counter-example to the idea that extrafloral nectaries evolved and are maintained because ants act to protect the plant from damage. Scientific data is forever shifting as new information comes in, and as the data changes, conclusions are shored up or undermined.

Ants drinking nectar at hibiscus extrafloral nectary

Hibiscus are so gorgeous.  I started this post to share their handsome photos.

But in checking my facts I matched plants I saw in Asia with roselle and learned that lemonyellow rose mallow is no longer considered an Hawaiian endemic. I'm a plant geek--learning new things about plants is great. And then I get to share them in this blog. Happy dance!

Hibiscus rosa-sinensis, in Hawaii

Appreciate hibiscus!

Comments and corrections welcome.

Note (3/6/18): as BNM commented, the two pictures described as rose-of-Sharon are both Hybiscus moscheutos hybrids.  (I have shown the correction the caption of the white-flowered plant. The other H. moschuetos hybrid is the plant with the red flowers used for dye). The text above, about rose-of-Sharon is good, its just that I included no photos of it (those plants being from my yard.) See rose-of-Sharon, Hybiscus syriacus, on Wikipedia pictures. The one I showed, H. xmoscheutos is a hybrid cultivar based on a southeastern U.S. species, is a handsome plant and deserves correct identication.
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References
Keeler, K. H. 1985. Extrafloral nectaries on plants in communities without ants: Hawaii. OIKOS. 44: 407-414. link
Keeler, K. H. Plants with extrafloral nectaries. A Wandering Botanist (This blog) link
Marazzi, B., J. L. Bronstein and S. Koptur. 2013. The diversity, ecology and evolution of extrafloral nectaries: current perspectives and future challenges. Annals of Botany. 111 (6):1243-1250 link
Valder, P. 1999. The Garden Plants of China. Timber Press, Portland, Oregon.
van Wyck, B-E. 2005. Food Plants of the World. Timber Press, Portland, Oregon.
Weber, M.G., Porturas, L.D., and K.H. Keeler, 2015. World list of plants with extrafloral nectaries. www.extrafloralnectaries.org. Accessed 1/6/18.   link

Kathy Keeler, A Wandering Botanist

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