Banana Poka: Look out for the vines!

23 Feb

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

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

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

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.

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

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.

Banana Poca seeds

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.

Banana Poka vines

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.

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