Kiawe (Prosopis pallida): Hawaii’s Tropical Mesquite

A couple of years ago I was photographing some volunteers at a Beach Cleanup Day at Kanaha Beach park near Kahului on Maui’s windward side. Part of the cleanup involved piling up some large tree branches that had been cut the day before by a Maui County tree-trimming crew. As I and the volunteers began stacking the cut limbs by the side of the access road for the County people to dispose of, a pickup truck full of locals hurriedly pulled up behind us. The driver asked if he and his boys could have all of the cuttings. The crew leader agreed as long as they took the entire pile with them. “No problem, bruddah! Dat’s Kiawe and it’s great for makin’ barbecue fire!” In an instant, the slash pile was thrown into the bed of the truck by its enthusiastic passengers. The truck then sped off down the dirt road, its occupants all waving happily as if it was double-pay day. In this way I was introduced to the Kiawe, Maui’s tropical mesquite tree.

A healthy mature Kiawe tree helping to stabilize the berm on Kealia Beach, Ma’alaea Bay, Maui.
(To view a larger version, click on image.)

Prosopis pallida is a perennial that belongs to the Mimosoideae subfamily (mesquite & mimosa) of the Leguminosae/Fabaceae family. It is commonly referred to by its Hawaiian name “kiawe” (pronounced “kee-AH-vay”). As with all other members of this family, it produces its seeds in pods referred to as “legumes”, hence the common name for the family.

The outer bark and foliage of the Kiawe tree.
(To view a larger version, click on the image.)

Physical Characteristics — Under ideal conditions, Kiawe trees can grow to be more than 30 m tall and form dense, continuous forest canopies (one of the few species of dry coastal trees to do so). However, they may also develop into gnarled, stunted bush-like trees in dry, wind-swept areas with poor soil such as those found in stabilized sand dunes and beach berms.

Leaves emerge from a common node on the twig, which changes direction often as it grows, giving it a corkscrew or zig-zag appearance when mature. The small, delicate-looking leaves (usually less than 2 cm long by a half a centimeter wide) are doubly compound with 3 to 4 pairs of stemlets branching from the main leaf stem. The twigs and branches include a fair number of very formidable thorns (see image below) notorious for drawing blood from careless beach-goers who seek out the Kiawe’s ample shade.

In the spring, small yellowish-green flowers are borne on long (8 to 15 cm) cylindrical spikes that give way to dense clusters of long (10 to 20 cm) yellowish-brown seed pods. The 10 to 20 seeds per pod (see image below) are encased in a sticky, sugary pulp.  The heartwood is dense-grained and hard, making it a favorite fuel for cooking fires on Maui and throughout the world. Generally, the root system is shallow, spreading out laterally; but in arid soils it easily develops a long taproot.

A typical Kiawe twig displaying its characteristically delicate foliage and very nasty thorns.

Kiawe seed pods fall from a single tree’s branches by the thousands, ensuring that many seeds survive & germinate.
(To view a larger version, click on the image.)

Introduction to Hawaii — Prosopis pallida is native to the arid coast of northwestern South America. In 1828 it was introduced to the Hawaiian Islands by Father Alexis Bachelot, the head of the first catholic mission to Hawaii. He planted a tree on the grounds of the Catholic Mission on Fort Street in Honolulu that he had raised from the seed of a Peruvian tree growing in the royal gardens of Paris. By 1840, the progeny of that single tree became the principal shade trees of Honolulu and were already spreading to the dry leeward plains on all of the neighbor islands, including Maui.

Growth Habits — A Kiawe seedling’s survival depends on receiving sufficient rainfall and sunlight (the seedlings do not tolerate constant shade) during the first few weeks after germination. The seedling can grow more than one meter in its first year under the right conditions. It will produce a strong and rapidly growing taproot system that can penetrate deep into even the hardest soils. Many old trees have been saved as garden and park trees during land development and have grown to large sizes with constant irrigation. Trees that grow on coastal plains where groundwater is shallow and abundant grow to be quite large, but they develop shallow root systems and windstorms can topple them easily. Once mature, growth is very slow compared to that of other trees under similar conditions. On windy or dry sites, Kiawe grows as a shrub or a small twisted tree only a few meters tall. Where it grows in strong trade winds, it is sculpted and shaped by the prevailing winds and lies along the slopes as a rounded bush. Although it is a coastal species, Kiawe is easily defoliated by the windblown salt spray of winter storms. Kiawe trees grows in areas where fire hazard is often extreme. Trees rarely survive slow-burning fires; they are usually killed outright by fire.

Invasive Habits — Once Kiawe was introduced to Hawaii it quickly became a pest species, invading, out-competing, and overwhelming native grass species and woody plants. It is a successful invasive species due to its ability to reproduce in two ways: production of large numbers of easily-dispersed seeds, and vegetative growth (by suckering) to create thick monotypic stands that shade out all other nearby plant species. It requires less than four inches of annual rainfall to establish itself and survive. It survives well in dry environments due to its extremely long taproot. It is so efficient at withdrawing moisture from soil that it can kill nearby plants by depriving them of water. It is often found growing in areas where other plants do not grow, such as sandy, dry, degraded slopes; salty soils; disturbed areas; and rocky cliffs.

A dense, continuous canopy of Kiawe trees growing along a beach front in the Kawalilipoa neighborhood of Kihei, south Maui.
(To view a larger version, click on image.)

Ironically, it is these very traits that have made Prosopis species so valuable in efforts to control soil erosion due to desertification in Africa and South Asia. Kiawe serves a similar purpose on islands such as Maui where beach erosion is a serious problem.

These well-established Kiawe trees rooted in the unstable sands of Kealia Beach protect a delicate intertidal marine habitat from the ravages of wind-blown sand and storm-driven waves.
(To view a larger version, click on the image.)

Maui’s Blue Morning Glory: Ipomoea indica

Before I moved here I never gave the Morning Glory a second thought: to me it was just a colorful but mostly innocuous ground cover on the beaches of California where I grew up. However, on a small island like Maui they are kind hard to ignore because there are so many of them and they are seemingly everywhere you look. From the high-tide line on most of our white-sand beaches to almost 4,000 feet up Haleakala’s volcanic slopes, their cookie-cutter-perfect flowers and massive amounts of foliage and choking vines spill out over the landscape, engulfing everything from derelict automobiles to the headstones in cemeteries.

Flowers & foliage, the Blue Morning Glory, Ipomoea indica.
(Click on image to see a larger version.)

The Morning Glories are plentiful in the Hawaiian Islands for two reasons: (1) they are able to adapt to almost any environment and (2) they propagate like oversexed rabbits. Their roots can absorb large quantities of seawater, they tolerate almost any kind of soil, and though they may be torn to shreds and small pieces they are still capable of taking root and flourishing. They choke off the saplings of competing species and shade-out established trees and bushes. On the other hand, Morning Glories can be important pioneer species that stabilize sand dunes and prevent beach erosion.

A solitary blossom of Ipomoea indica.
(Click on image to see a larger version.)

The Blue Morning Glory (Ipomoea indica)

Ipomoea indica is a perennial vine native (indigenous) to Hawaii (“koali awa” in Hawaiian). It is variously known as Ocean Blue Morning Glory, Blue Morning Glory, and Blue Dawn Flower. It’s also called a “Dunny Creeper” in Australia (a “dunny” is an outhouse toilet), referring to its habit of covering everything in sight with its vines, including small out-buildings. Its geographic origin is uncertain: it may be native to the West Indies. It is now naturalized throughout the tropics and is found on all the main Hawaiian Islands. It is usually found at lower elevations (below 200 m) but has been known to grow at altitudes above 1,200 m.

Close-up of a mature Ipomoea indica plant. Note the characteristic 3-lobed leaves.
(Click on the image to see a larger version.)

Ipomoea indica prefers sunny open ground in a moist environment, but will tolerate seasonal dry spells once it is well established. It is a perennial that grows as a sprawling (ground-hugging) vine that produces a dense ground cover, but it is often seen climbing and twining over other plants. The vine itself (called a “stem”) is softly hairy and may grow to more than seven meters in length. The distinctively shaped leaves are large (as much as 15 cm wide and 17 cm long) and numerous. They may be heart-shaped when immature; mature leaves are 3-lobed to 5-lobed with pointed tips. The upper and lower surfaces of the leaf are covered by silky, short, closely-spaced hairs.

The flower is large (as much as 80 mm in diameter) It usually ranges in color from sky blue to purple-blue; less commonly it may also be light magenta and reddish-pink. It has paler mid-petal bands and a pink to whitish tube inside. The petals (3 to 15 per flower) are joined, which accounts for the distinctive trumpet-like shape of the flower. Ipomoea indica sets flowers all year, continuously flowering during warmer months. The short-lived flowers are replaced as they fall from the vine.

The fruit is a small (about 10 mm in diameter) 3-chambered spherical capsule. These are rarely produced by the plant. It has almost no self-fertilization and so grows vegetatively: its preferred method of propagation is by the production of numerous stolons, stems that grow above ground and are able to produce both roots and foliage. Individual stolons are readily detached from the parent plant and dispersed by mechanical means such as flooding, stream flow, landslides, or human activity (ground clearing and weed removal). Even when badly damaged, a plant can easily recover 4-6 m of its length in a single growth season. One theory about why Ipomoea indica evolved this unique reproductive behavior is that the species is a sterile hybrid and its genes are self-incompatible.

This species contains toxic alkaloids (including Ergonovine, which bears a strong molecular resemblance to Lysergic Acid Diethylamide, or LSD) that discourage ingestion by grazing animals. This defense mechanism contributes to its prevalence as a weed species.

Photo shows the tendency of the Blue Morning Glory to choke off sunlight from other plants by spreading its tendrils and vines in all directions, wrapping itself around anything from tree trunks to derelict automobiles.
(Click on image to see larger version.)

The Blue Morning Glory is a vigorous climbing vine that not only forms a dense ground cover but can easily extend itself high into the forest canopy. The twining stems can quickly choke off the growth of seedlings of other species and smother even older well-established plants. It also poses a significant shading hazard to other species.

Hawaiian Ginger: Not just for eating

I live on a part of the Island of Maui’s leeward shore that is surrounded by and engulfed in many small private and larger public gardens and parks. The enclosed courtyard of my own housing compound is a modest botanical garden that includes tall Coconut Palms, Strangler Figs, banana trees, and many of the flowering bushes and shrubs that grow on Maui. Despite competition from the usual hordes of gaudy hibiscus and orchid tree blossoms, the plants whose flowers stand alone in that crowd of colorful blossoms and foliage are the different species of the Zingiberaceae Family, or ginger.

Now that I’ve been photographing and blogging about Hawaii’s plant life, I’ve grown to appreciate the members of this plant family because of the great variety of their size, form, and color. It should be no surprise, then, that along with all of the photographs of palm trees and hibiscus, the online photo galleries of recent visitors to our islands are crowded with images of Hawaii’s photogenic ginger species.

Pink variety of Red Ginger (Alpinia purpurata). Flower head is about 20 cm long. Hana, east Maui.
(Click on image to see larger version.)

The English word “ginger” comes to us from the French word “gingimbre” by way of the ancient south Asian words “inji ver” (literally, “the root of Inji” in Dravidian) and “singivera” (Pali), from which the Greek word “zingiberis” and the medieval Latin word “gingiber” probably were derived. As you might expect, the Hawaiian names for individual species of ginger evolved along a different linguistic line.

Aside from being the ideal subjects for amateur photographers (unlike birds and whales, they tend not to move around much), the various species of ginger have long been known for their aromatic properties and as an ingredient in prepared ethnic foods. More recently, many “root ginger” species that belong to the taxonomic family Zingiberaceae have been found to possess chemical compounds sought after by the pharmaceutical industry. Zingerone, gingerols, and shogaols are the volatile oils (about three percent by weight) responsible for ginger’s fragrance, distinctive taste, and numerous beneficial medicinal properties.

Several extracts and distillations of ginger plants possess antibacterial, analgesic, sedative, and antipyretic (fever-reducing) medicinal properties. In addition, several species of ginger are currently being researched as candidates for the treatment and cure of skin and ovarian cancer. There is also on-going research into the blood-thinning and cholesterol-reducing properties of ginger. Powdered ginger root capsules have been shown to be effective in treating and preventing the nausea associated with morning sickness and motion sickness; however, clinical studies in this area have proven inconclusive.

The following are my photographs of a few of the most common species of ginger plants growing in Maui’s gardens and rain forests. These species also grow in the garden of my own housing compound in Kihei (leeward Maui).

Close-up of the flower head of a Torch Ginger (Etlingera elatior), Hana, east Maui.(Click on image to see larger version.) 

Torch Ginger (Etlingera elatior), Hawaiian = ‘awapuhi ko‘oko‘o – A herbaceous perennial, individual plants grow in large clumps whose stalks can be as much as 6 m high with leaves as long as 85 cm. Although it is cultivated throughout the tropics and has escaped to become naturalized in some localities, its original native range is thought to be a few isolated islands in Indonesia.  Typically it prefers a moist climate with somewhat acidic soil (pH = 5.6 to 7.5). It prefers full sun but does well in the partial shade of the rain forest. It is found at altitudes up to 9,000 feet. The various plant tissues and parts of this species of ginger are rich in volatile aromatic compounds. The majority of the essential oils extracted from the leaves, stems, flowers, and rhizomes of Torch Ginger are monoterpene hydrocarbons. Terpenes are the primary constituents of the essential oils of many types of plants and flowers. Essential oils are used widely as natural flavor additives for food, as fragrances in perfume, and in traditional and alternative medicines. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery and flavors used in food additives. Vitamin A is an example of a terpene. Currently, this and other species of ginger are the subjects of extensive research into the various antioxidant compounds present in their leaves and rhizomes.

Crepe Ginger (Costus speciosus), Keanae Peninsula. The fluted “petal” is actually the male part of the flower. (Click on image to see larger version.)

Crepe Ginger (Costus speciosus) – This species is a native of the Greater Sunda Islands of Indonesia and the Malay Peninsula. Although it has become naturalized in the Hawaiian Islands, its invasive tendencies enable it to crowd out native species. Its preferred habitat is moist fertile soils and partial sun, but will grow well in full sun if it gets plenty of moisture. One of the most cold-hardy of the gingers, it has been shown to withstand freezing temperatures as low as 17 degrees Celsius. Crepe Ginger is a tall and substantial-looking plant with large (15-30 cm long by 5-7 cm wide) dark-green leaves arranged on the stalk in a spiral. This species can grow to more than 3 m tall under ideal soil and climatic conditions. The unusual-looking flowers bloom throughout the year in tropical climates, but only in late summer to early fall in colder conditions. The flower’s single ,petal (actually the male part of the flower) looks like crepe paper, hence the common name “Crepe Ginger”. This and other ginger species propagate most effectively by producing a large network of thick fleshy rhizomes that are similar in appearance to the “ginger root” sold in grocery stores and at farmer’s markets. A single rhizome will produce new shoots and grow into a clump more than one meter in diameter in less than two years. Traditionally, the  rhizome has been used to treat fever, rash, asthma, bronchitis, and intestinal worms. In India and Southeast Asia, this species is still used to treat boils, constipation, diarrhea, dizziness, headache, and vertigo (“seasickness”). This species is also a source of diosgenin, a compound used for the commercial production of clinically important steroids such as progesterone.

White Ginger (Heydychium coronarium), Hana, east Maui. (Click on image to see larger version.)

White Ginger, Ginger Lily (Hedychium coronarium), Hawaiian = `awapuhi ke`oke`o – A native of India, it was exported throughout the tropics because of its popularity as an ornamental species. It soon escaped the confines of private gardens and became naturalized throughout the Hawaiian Islands. It is considered an invasive weed because of its ability to propagate quickly by sending out many shallow roots (rhizomes) in all directions, creating dense thickets of individual plants. This creeping growth quickly overwhelms and crowds out other low-growing plant species. Hedychium coronarium is a perennial herb that can grow as two meters in height. It prefers moist habitats, such as rain forests, the banks of streams, moist forest, roadsides, and open fields and pastures. It has large (30 cm long by 10 cm wide at maturity) simple elliptical or lance-shaped leaves.  It produces large numbers of flowers throughout the year. The fragrant white flowers are borne on long (10-20 cm) elliptical spikes in groups of two to six flowers atop a leafy stem.  The fruit is borne in an oblong many-seeded capsule, but is not usually present or easily seen since propagation of the species is usually accomplished by spreading rhizomes.

Indonesian Wax Ginger (Tapeinochilos ananassae). Flower head is about 25 cm long. Well adapted to the moisture and shade of Maui’s rain forests.
(Click on image to see larger version.)

Indonesian Wax Ginger or Pineapple Ginger (Tapeinochilos ananassae) – Like other ginger species growing in Hawaii, this plant was introduced for its ornamental attributes: a large, showy inflorescence and beautiful foliage. Its foliage consists of thick, jointed bamboo-like stems that grow directly from numerous rhizomes. The long, smooth, lance-shaped leaves are arranged on the stem in an inward-curving spiral typical of this species; the leaves radiate outward from the spiral.  The small yellow flowers (which function as attractors to pollinating insects and birds) are aggregated in a large (10-30 cm long), bright-red pineapple-shaped terminal spike consisting of what appear to be waxy bright red flower petals, but are actually bracts (modified leaves that enclose the actual flower). To reach its full height (about 3 m), this species requires very high humidity. It prefers the full shade and moist, fertile, well-drained soils of the rain forest. Unlike many other tropical ginger species, this plant possesses no aromatic essential oils.

Scarlet Spiral Flag Ginger (Costus woodsonii), Hana Maui Botanical Gardens, east Maui.
(Click on image to see larger version.)

Scarlet Spiral Flag Ginger, Indian Head Ginger, or Red Button Ginger (Costus woodsonii Maas), Hawaiian: ‘awapuhi ‘inikini po‘o – This species has gently spiraled stems and grows to about a meter in height. The leaves are deep green. The bracts of the inflorescence are bright waxy red, forming a pine cone-like spike (10-15 cm long) from which the yellowish-orange flowers protrude outward one at a time. The numerous small black seeds (1-2 mm long, the smallest of the true gingers) are usually dispersed by birds, but it is possible that they are also dispersed by water if the plant is growing in a wetland environment. C. woodsonii is an aggressive and invasive species. Its rhizomes spread rapidly and, once established, they are difficult to remove.  This species grows well in full sun, flourishing even in the sandy soils and dune sands of tropical coasts, but it prefers the shade, ample moisture, and deep, fertile soil of the rain forest. Although the genus Costus sp. is found throughout the tropics on all continents, this species was first collected and described in Panama in 1941. It was an introduced ornamental garden plant that became a naturalized escapee in some parts of Hawaii. It is considered to be an invasive weed on Oahu, Maui, and Kauai islands. It is present in large but widely scattered populations at low elevations on Maui, particularly along the Hana Highway. This species also displays a classic behavior of species mutualism: it exudes what is known as “extrafloral nectar” from the bracts of the flower spike, attracting ant species that harvest the nectar. In turn, the ants protect the plant from the larvae of flies and other flying insects that lay their eggs in the flowers.

The pink variant of Red Ginger (Alpinia purpurata), a garden in Kihei, south Maui. (Click on image to see larger version.)

Red Ginger (Alpinia purpurata), Hawaiian = ‘awapuhi ‘ula‘ula – Originally a native of Southeast Asia, A. purpurata was introduced to Hawaii as an ornamental species in 1928; it is now naturalized. Grows to about 3 m tall in the wild (can grow to more than 9 m tall under cultivation) and forms large clumps of foliage bearing large oblong leaves (30-80 cm long by more than 20 cm wide).  As with other ginger species, the large inflorescence (15-30 cm long, getting longer as the plant ages) consists of brightly colored (usually pink or red, occasionally white) bracts that look like a bloom, but the true flower is a small white blossom (corolla) at the outermost tip of the inflorescence. The fruit is a globular seed capsule about 3 cm in diameter that contains numerous small seeds (2-3 mm long). Although this species of ginger will tolerate direct sunlight, it requires very high humidity and soil moisture, as well as warmer air temperatures (more than 50 degrees F) to grow to its full size. Research has revealed that this species is a possible natural source for bioactive compounds useful for the treatment of hypertension.

Maui’s Beautiful Weeds: The African Tulip Tree

Atop the itinerary of most first-time visitors to Maui is a day trip to view the rain forests on the road to Hana… the Hana Highway. One of the first trees to grab your attention as you enter the rainforest canopy is a tall fellow with wide-spreading branches overflowing with impossibly bright-red flowers, the African Tulip Tree. But curb your enthusiasm, malahini… it’s a WEED!

The African Tulip Tree (Spathodea campanulata) is a shade-tolerant, evergreen tree native to equatorial Africa. It is a member of the Bignoniaceae Family, which includes the Jacaranda (also growing on Maui). It is also known as Flame Tree, Fountain Tree, Indian Cedar, and Santo Domingo Mahogany.

African Tulip Tree blossoms.
(Click on image for a larger version.)

Although it was originally introduced to The Islands as a domesticated ornamental tree, the African Tulip Tree has escaped cultivation and invaded agricultural land, forest plantations, and natural forests; it is now one of the dominant canopy trees in all of Hawaii’s rain forests and has become a serious threat to the biodiversity of that ecosystem.

An African Tulip Tree in full bloom along the Hana Highway.
(Click on image for a larger version.)

The African Tulip Tree is a tall tree, growing to more than 75 feet (30 m) in some habitats. It favors moist and wet areas from sea level to 1,000 m throughout Hawaii. The flower’s calyx is a leathery sack filled with watery sap (which attracts many ants, though the flower’s aroma is quite foul) from which blooms a bright scarlet-orange flower that grows in large terminal clusters. It sets flowers year-round, but the most prolific flowering occurs in Winter through Spring. The fruit consists of clusters of upright, canoe-shaped capsules about 10 inches long and 2.5 inches in diameter; these contain hundreds of small flat winged seeds that are easily disbursed by the wind. The seed pods are buoyant and so are easily carried off by streams and surf action to germinate far from the parent tree. The tree also propagates readily from root suckers, broken root pieces, and fallen branches.

Rain forests and drier mesic forests are very susceptible to invasion by this tree. Its high reproductive rate and capacity allow it to colonize disturbed areas (either created by human activity or by storms) at the expense of native plant species. Once established in an area, the tree grows rapidly and it can easily exceed the height of the native flora and shade it. Furthermore, this tree has no natural enemies in the Pacific region.

If the African Tulip Tree replaces native tree species, the effect on the biodiversity of Hawaii’s forests would be disastrous because so many of the native species support numerous tree-dependent flora, such as vines and epiphytes.

Banana Poka: Look out for the vines!

There’s an old saying in the world of popular nutrition: “If it tastes good and looks good, it must be bad for you.” This old chestnut certainly holds true in the case of the Banana Poka (Passiflora tarminiana). Although it has large, beautiful flowers that attract pollinating insects and photographers in large numbers, and bears large, tasty banana-like edible fruit, this woody vine is one of the most aggressive and destructive of Hawaii’s many invasive plant species. The Banana Poka isn’t really common on Maui yet, but it is well established on Hawai’i, the Big Island, and also on Kaua’i, where I first encountered it (see photos below).

Flowers and foliage of the Banana Poka (Passiflora tarminiana), near Waimea Canyon, west Kaua’i.
(Click on image for a larger version.)

Passiflora tarminiana is known as “Banana Poka” in the Hawaiian Islands; it is also called Banana Passionfruit and Passionflower. In most of South America (where it originated) it is known as Curuba.  In Hawaii there has been some confusion about the taxonomic designation of this species. It was originally thought to be two distinct species of the genus Passiflora: P. tripartita and P. mollissima. In 2009, the Banana Poka that is found in the Islands was officially given the taxon Passiflora tarminiana; however, many popular books, blogs, and websites about Hawaii’s plant life still use one of the old taxonomic designations.

Description

Three-lobed leaf of the Banana Poka.
(Click on image for a larger version.)

Under normal conditions, P. tarminiana is a perennial climbing vine that can attain lengths in excess of 20 m and often lives more than 20 years. In the absence of any vertical support, it can assume either a bush-like or trailing habit.

Leaves are three-lobed and about 15 cm long by 20 cm wide with very serrated margins. The upper surfaces are smooth, dark green, and moderately lustrous, while the under surfaces are light green and covered with fine hairs. The veins are prominent on both surfaces.

The distinctive flower of the Banana Poka (Passiflora tarminiana).
(Click on image for a larger version.)

Flowers are large (5 to 10 cm in diameter) and are borne singly at irregular intervals along the length of active vines. The 10 petals (3 to 6 cm long) are generally light pink to pinkish white, often displaying a whiter “sport” (or centered stripe). The base of the flower has pale green bracts (modified leaves) enclosing a swollen nectary chamber.  The internal structure of the flower is simple but distinctively colored: it has five stamens with bright yellow anthers, and three pistils with light green stigmas.

Fruit of the Banana Poka.
(Click on image for a larger version.)

Fruit and Seeds – The fruit is an elongated berry (8-15 cm long and about 4 cm in diameter ) that resembles a banana’s fruit. Light green when immature, it ripens to a yellowish-orange color, at which point it is edible. The berry contains approximately 180 seeds, each embedded in an edible aril (a sweet, fleshy capsule enclosing the seed, as with a pomegranate seed). The seeds are small (about 5 mm in diameter), asymmetrical, and dark reddish-brown when dry.

A ripening curuba (fruit) of a Banana Poka, sliced in half to show the edible arils (seed coat).

Reproduction and Propagation

Pollination – After one year’s growth, the Banana Poka produces its large and conspicuous flowers throughout the year, so there are many opportunities for spontaneous self-pollination and cross-pollination (or “outcrossing”) by the numerous flying insect species that inhabit the Islands (mostly bees and syrphid flies). A newly opened flower displays its many prominent exposed stamens that increase its chances of being cross-pollinated by flying insects. In flowers where cross-pollination does not occur, each one can pollinate itself by moving its stigmas to touch the stamens.

Dispersal – The fruits ripen in about three months and are produced throughout the year in Hawaii. The fruit (and, consequently, its numerous seeds) is widely dispersed locally by birds, as well as domestic and feral mammals, particularly feral pigs (Sus scrofa). Fruit- and seed-eating birds, as well as humans, also spread the seeds over longer distances. Seeds can remain dormant yet viable for more than 60 weeks when buried under as little as 10 cm of topsoil. Banana Poka is also very capable of vegetative reproduction, where the interconnected portions of a mature plant decay or are otherwise separated from the parent plant and take root on their own.

Seeds of the Banana Poka (Passiflora tarminiana).

Range and Distribution

P. tarminiana is a native of the tropical regions (specifically Bolivia, Venezuela, and Colombia) of the Andes Mountains of South America. Populations of this species are sparse (two or three plants per hectare) in this part of the world and its fruit and flowers are aggressively consumed by many species of herbivorous insects, thus preventing it from becoming a pernicious weed as it has in Hawaii. Here it inhabits mesic forests of Koa (Acacia koa), ‘Ohia (Metrosideros polymorpha), Black Wattle (Acacia mearnsii), and other native species of trees. It’s altitude range extends from 850 to more than 2,200 m. Recently Banana Poka has been invading subalpine shrublands on Maui, as well as agricultural and residential areas.

Environmental Tolerances and Preferences

Banana Poka can tolerate a wide range of climate variation. It is relatively frost-tolerant and will grow in both full shade and full sunlight, but seedlings tend to be less tolerant of full shade, and optimal growth only occurs in full sunlight. P. tarminiana prefers a seasonal climate where annual mean temperature is 13°C and mean annual rainfall is about 130 cm.

Introduction History

Banana Poka was first introduced to the island of Hawai’i in about 1920, and shortly thereafter to the other neighbor islands. By 1926, botanists were finding naturalized specimens in the wild on the Big Island, as well as on Maui and Kaua’i, which attests to its ability to spread quickly in a favorable climate. Currently, P. tarminiana occupies large areas of Kaua’i and Hawai’i, in some places forming a continuous cover.

A pair of vines of the Banana Poka (Passiflora tarminiana). These were part of a plant that extended from the tops of some Koa trees to where I was standing, more than 30 feet below!

Ecological Impacts

The Banana Poka’s very effective methods of reproduction, propagation, and competition are directly responsible for its successful colonization of the Hawaiian Islands and its well-deserved epithet as a serious pest species outside its native range.
•    The tendency of P. tarminiana to rapidly increase its growth rate as the intensity of sunlight increases enables it to invade openings in an established forest canopy caused by storms, logging, and feral pig damage. It also forms dense colonies along forest margins where the shade of the canopy is sparsest.
•    Once the active vines of the Banana Poka reach the uppermost heights of a forest’s canopy, they rapidly spread out laterally, smothering or shading-out less aggressive native species of trees and shrubs.
•    Disturbance of topsoil by the rooting activities of feral pigs increases the rate of invasion of this species. The increasing numbers of feral pigs on all of the Hawaiian Islands has contributed immensely to P. tarminiana’s invasion of native forests, scrublands, pastures, forestry plantations, and even relatively barren lava flows. Wherever pig damage is the worst you’ll find large patches of Banana Poka.
•    There are almost no species of herbivorous insects that naturally prey on P. tarminiana in Hawaii, and so it is able to set more flowers and fruits, which results in greater plant densities than in its native range.
•    P. tarminiana’s ability to employ either cross-fertilization or self-fertilization, coupled with its adaptations that enable long-distance dispersal of its seeds, allow it to invade isolated areas far from the center of its main infestation.
•    In addition to all of the other negative ecological impacts of the Banana Poka, a very dense population of this species can literally smother even the tallest native trees in a shroud of vines. Native species can be shaded out; prevented from regenerating damaged limbs, trunks, and roots; and can even be toppled by the sheer weight of the dense curtain of vines. Where forest canopies have been opened in this or other ways, dense mats of Banana Poka vines soon drape themselves over the understory trees and shrubs. In the long term, this competitive activity alters the composition and physical structure of a forest, which adversely affects already endangered species of native birds and other animals.

Control and Management

Attempts at eradication and controlling the spread of this aggressively invasive species have, for the most part, failed completely; use of modern methods of biological control have been shown to be totally ineffective. The very labor-intensive application of physical and (to a lesser extent) chemical control methods have been more successful when employed along the leading edge of infestations to prevent the infestation from advancing further.

Chemical Control – On a large scale, application of chemical control agents such as herbicides is impractical because the current infestation of Banana Poka is spread over large, often inaccessible areas and the main biomass of the infestation overlies many non-target plant species. However, frequently repeated physical applications of contact herbicides (such as Roundup) over small areas, particularly at the leading edges of an infestation, have been successful at controlling its spread.

Physical Control – A few purely mechanical means of controlling Banana Poka have been successful in small selected areas; these include manually pulling the entire plant out of the ground by its roots. Don’t laugh: a lot of property owners swear by this method and so far it seems to be the only effective means of controlling this monster. Natural areas and other valuable parcels of land not yet infested can be fenced-off to prevent feral pigs from spreading the seeds and disturbing topsoil.

Biological Control has not been very successful in controlling the spread of P. tarminiana. Species of mold and fungi have been introduced throughout the infested areas on Maui, but the species known to prey on Banana Poka elsewhere in the world have had a great deal of difficulty establishing themselves in our climate. Introduced predatory species often fail to survive the dry season, perishing before they can become naturalized. The same fate has befallen herbivorous insect species known to consume the flowers and foliage of P. tarminiana.