After five years of poking around the private lives of Hawaii’s green inhabitants, I’ve come to the inescapable conclusion that most of the species that make up the lush vegetation I am so fond of writing about and photographing are a bunch of thugs and bullies who arrived in the Islands with the sole purpose of crowding-out or eliminating our mostly defenseless native species. With their flashy foliage, aggressive root systems, and wildly colorful blossoms, these invasive exotics have trashed our once-pristine ecosystem. One of the few indigenous plant species that more than holds its own against the alien horde is a deceptively pleasant-looking yellow hibiscus I was introduced to last year while visiting Kaua’i, the northernmost of the six main islands.
A typical blossom of Hibiscus tiliaceus, Lawai, Kaua'i.
DESCRIPTION
Hibiscus tiliaceus is known by many common names throughout its range, including Beach Hibiscus and Sea Hibiscus. It is called Hau in Hawaiian. This is a highly variable species in the taxonomic sense: it includes several subspecies that are worthy of being classified as distinct species in their own right. There is also a considerable degree of morphological variation as well, particularly in flower coloration and leaf morphology; this is probably due to environmental factors, since it is such an adaptable species. H. tiliaceus is an evergreen tree that can grow to heights exceeding 10 m with a trunk as much as 15 cm in diameter. Usually the canopy of a mature tree is much wider than the height of the tree. It has a short trunk with many crooked and intertwined branches that form an impenetrable thicket; however, it may also have a shrub-like form with prostrate, ground-hugging branches. The outer bark is gray to light brown and is smooth to slightly fissured when the tree is young, becoming thicker and rougher with age. The large (8-20 cm long, 6-15 cm wide), simple heart-shaped leaves are bright green on the upper surface and grayish-green and hairy on the under surface. The flowers are bright yellow with a dark red center when they first open. Individual flowers are roughly cup-shaped with a corolla consisting of five radiating oblong petals 4-6 cm long. They are typical of the genus Hibiscus sp.: brightly colored, fragile, and short-lived (typically just one day). During the course of the day, the color of the petals deepens to a dark orange and finally pinkish-red when the flower drops off. The tree produces new leaves, flowers, and seeds all year long.
The flower and foliage of Hibiscus tiliaceus.
PROPAGATION & GROWTH
Seeds are borne in light-brown ovoid capsules (2 cm long) that take about seven weeks to ripen and then split open into five segments, each one containing 15 to 25 dark-brown hairy seeds (3-5 mm) that germinate in 2-4 weeks. The seeds remain viable after floating in seawater for several months, which probably accounts for this species’ wide distribution. H. tilaceus also propagates by aggressive vegetative growth from broken twigs and branches, as well as producing numerous “suckers” from its shallow lateral root system. The long side branches can take root where they touch the ground. Plants that are produced from vegetative propagation can set flowers in less than one year, while seedlings require 2-3 years before they are mature enough to produce flowers. This species is well known for its rapid and aggressive branching and spreading habit. Its growth rate is quite fast, producing as much as 1.5 m of new growth annually. As a tree ages, the increase in height gradually slows. After about 5-7 years, most of the plant’s energy is diverted to increasing trunk and branch diameter (as much as 50 cm annually) and lateral branch growth. The life span of a tree usually exceeds several decades.
DISTRIBUTION & HABITAT
H. tiliaceus is a common coastal plant native to Australia, Oceania, and Southeast Asia. It was probably brought to the Hawaiian Islands by the first Polynesian colonists (as one of the “Canoe Plants”) and quickly became naturalized. Normally found at elevations ranging from sea level to 800 m, H. tiliaceus prefers areas that receive 35 to 95 inches of annual rainfall. It is well adapted to living in coastal environments and is common along beaches and river banks, and the inner margins of mangrove swamps. It is able to tolerate salt spray and waterlogged and salt-saturated soils. Once established, it is very drought tolerant. It grows well in the coral and basalt sands that are common in the Hawaiian Islands. It does best in soils that are slightly acidic (pH of 5) to alkaline (8.5). H. tiliaceus is able to tolerate a fair amount of shade (as much as 75 percent) because it can produce larger leaves; however, its growth rate slows considerably under these conditions. It prefers full sunlight. Although it is able to withstand high winds, this species does suffer a good deal of branch breakage during violent tropical storms. However, it recovers quickly after such damage, particularly because of the ability of its branches to bend to ground level and take root. In fact, modest stands of H. tiliaceus that have been virtually flattened by hurricanes have regenerated quickly, growing into impenetrable thickets.
Closeup of a Hau blossom.
TRADITIONAL USES IN HAWAII
H. tiliaceus has been and is culturally significant throughout the tropical Pacific. In Pre-Contact Hawaii, this plant was so valuable that one had to obtain permission from the local village chief to harvest it. Rubbing the pointed end of a hardwood stick in a groove cut into a piece of Hau wood was once the preferred method for starting a fire. The white timber is very boyant due to its low specific gravity (0.6), and it is light weight but extremely tough. Because it is so durable and boyant in seawater, Hawaiian canoe builders once used the wood of the Hau tree for the spars (‘iako) and pontoons (‘ama) of their outrigger canoes (wa’a), and for fishing-net floats (mouo ). They also used the inner bark (or “bast”) fibers to make a general-purpose cordage (kaula ‘ilihau) with very high tensile strength (which actually increased when wet). The rope was made by braiding together several long strips of the fiber; the number of fiber strips used depended upon the strength required by the user. The bast fiber was also used to make durable sandals. The sap from the inner bark was used as a laxative. The juicy sap found in the dome of the flowerbud and in the inner bark was used as a mild laxative. The flowers were also eaten to aid in digestion and relive mild throat irritations caused by bronchitis and dry-throat. The flowers were (and still are) used in making flower leis because of their bright yellow color and the fact that they are available year round.
The characteristic heart-shaped leaf of Hibiscus tiliaceus.
Although it is not recognized as a commercially important species, H. tiliaceus has some traits that are beneficial to its environment. Its dense branches and foliage make it an ideal windbreak, hedge, or boundary plant. Well-established stands of this tree actually improve soil fertility by adding valuable organic matter by means of leaf drop, which occurs in large quantities year round. The long spreading branches are able to take root where they touch the ground, thus helping to stabilize soil on slopes, along river banks, swamps and mangroves, and exposed sandy shorelines. It some regions, this species has been used to stabilize eroding sand dunes.
INVASIVE CHARACTERISTICS
Although it is not classified as a nuisance species, H. tiliaceus does produce a complex tangle of highly invasive, laterally-spreading roots. In addition to producing a large number of suckers from the roots, the root system has a well-deserved reputation for severely damaging irrigation pipes and sewer lines, concrete walks, fences, and building foundations. The major drawback of this species is that it is extremely difficult to eradicate once it has become established. In addition, its ability to become naturalized in well-watered environments and reproduce rapidly by both vegetative growth and seed propagation enables it to crowd out crop plants and native plant species by creating a shade-producing tangle of surface roots, branches, and foliage.
Tags: canoe plants, hau, hawaiian conoes, hawaiian hibiscus, hibiscus, hibiscus tiliaceus
