Asimina triloba

Names

This plant's scientific name is Asimina triloba. The genus name Asimina is adapted from the Native American (probably Miami-Illinois) name assimin or rassimin through the French colonial asiminier. The epithet triloba in the species' scientific name refers to the flowers' three-lobed calices and doubly three-lobed corollas, the shape not unlike a tricorne hat.

The common name of this species is variously spelled pawpaw, paw paw, paw-paw, and papaw. It probably derives from the Spanish papaya, an American tropical fruit (Carica papaya) sometimes also called "papaw", perhaps because of the superficial similarity of their fruits. (In some parts of the world, such as Australia and New Zealand, the name Paw Paw is commonly used for a variety of Papaya.) Asimina triloba has had numerous local common names including: wild banana, prairie banana, Indiana banana, Hoosier banana, West Virginia banana, Kansas banana, Kentucky banana, Michigan banana, Missouri banana, the poor man’s banana, Ozark banana, and banango.

Description

Asimina triloba is a large shrub or small tree growing to a height of 35 feet (11 m) (rarely to 45 feet or 14 m) with a trunks 8-12 inches (20–30 cm) or more in diameter. The large leaves of pawpaw trees are clustered symmetrically at the ends of the branches, giving a distinctive imbricated appearance to the tree's foliage.

The leaves of the species are simple, alternate and spirally arranged, entire, deciduous, obovate-lanceolate, 10-12 inches (25–30 cm) long, 4-5 inches (10–13 cm) broad, and wedge-shaped at the base, with an acute apex and an entire margin, with the midrib and primary veins prominent. The petioles are short and stout, with a prominent adaxial groove. Stipules are lacking. The expanding leaves are conduplicate, green, covered with rusty tomentum beneath, and hairy above; when fully grown they are smooth, dark green above, and paler beneath. When bruised, the leaves have a disagreeable odor similar to a green bell pepper. In autumn the leaves are a rusty yellow, which make spotting pawpaw groves possible from a long distance.

Pawpaw flowers are perfect, about 1-2 inches (3–5 cm) across, rich red-purple or maroon when mature, with three sepals and six petals. They are borne singly on stout, hairy, axillary peduncles. The flowers are produced in early spring at the same time as or slightly before the new leaves appear, and have a faint fetid or yeasty smell.

The fruit of the pawpaw is a large, yellowish-green to brown berry, 2–6 in (5–16 cm) long and 1–3 in (3–7 cm) broad, weighing from 0.7–18 oz (20–500 g), containing several brown/black seeds 1/2 to 1 in (15–25 mm) in diameter embedded in the soft, edible fruit pulp. The conspicuous fruits begin developing after the plants flower; they are initially green, maturing by September or October to yellow or brown. When mature, the heavy fruits bend the weak branches down.

Other characteristics:

Range and ecology

Asimina triloba, the pawpaw, is native to the Eastern, Southern, and Midwestern United States and adjacent southernmost Ontario, Canada, from New York west to southeastern Nebraska, and south to northern Florida and eastern Texas.

The tree commonly grows in floodplains and shady, rich bottomlands, where it often forms a dense, clonally spreading undergrowth in the forest, often appearing as a patch or thicket of individual small slender trees. Pawpaws are not the first to colonize a disturbed site (arriving roughly four years after a clearcut), but may become dominant and slow the establishment of oaks and hickories. Although shade-tolerant, pawpaws do not persist in undisturbed old growth forest. Pawpaws spread locally primarily by root suckers; sexual reproduction by seed does also occur, but at a fairly low rate.

Pawpaw flowers are insect-pollinated, but fruit production is sometimes limited as few if any pollinators are attracted to the flower's faint, or sometimes non-existent scent. The flowers produce an odor similar to that of rotting meat to attract blowflies or carrion beetles for cross pollination. Other insects that are attracted to pawpaw flowers include scavenging fruit flies, carrion flies and beetles. Because of irregular fruit production, some believe pawpaw plants are self-incompatible, requiring cross-pollination between trees of different clones (patches).

The fruits of the pawpaw are eaten by a variety of mammals, including raccoons, gray foxes, opossums, squirrels, and black bears.

The disagreeable-smelling leaves, twigs, and bark of pawpaws contain natural insecticides known as acetogenins. Pawpaw leaves and twigs are seldom consumed by rabbits, deer, or goats, or by many insects. However, mules have been seen eating pawpaw leaves in Maryland.

Larvae of the zebra swallowtail (Protographium marcellus), a butterfly, feed exclusively on young leaves of Asimina triloba and various other pawpaw (Asimina) species, but never occur in great numbers on the plants. Chemicals in the pawpaw leaves confer protection from predation throughout the butterfly's life, as trace amounts of acetogenins remain present, making them unpalatable to birds and other predators.

Conservation status

On a global (range-wide) scale, the common pawpaw (Asimina triloba) has a NatureServe global conservation rank of G5 (very common).

In the United States, the species has a NatureServe national conservation rank of N5 (very common), but is considered a threatened species in New York, and an endangered species in New Jersey.

In Canada, where the species is found only in portions of southern Ontario, it has a NatureServe national conservation rank of N3 (vulnerable), and a NatureServe subnational conservation rank of S3 (vulnerable) in Ontario. The Ontario Ministry of Natural Resources has given the species a general status of "Sensitive", and its populations there are monitored.

In areas in which deer populations are dense, pawpaws appear to be becoming more abundant locally, since the deer avoid them but consume seedlings of most other woody plants.

History

A candidate for the natural distribution of the common pawpaw (Asimina triloba) in North America, prior to the Ice Ages and lasting until roughly 10,000 years ago, were extant but now extinct megafauna. Such animals became extinct during the Quaternary extinction event, and with the arrival of humans and subsequent extinction of such megafauna for distributing Asimina triloba, the probable distribution of these large fruit bearing plants was by humans.

The earliest documented mention of pawpaws is in the 1541 report of the Spanish de Soto expedition, who found Native Americans cultivating it east of the Mississippi River. The Lewis and Clark Expedition consumed pawpaws during their travels. Chilled pawpaw fruit was a favorite dessert of George Washington, and Thomas Jefferson planted it at Monticello, his home in Virginia.

Cultivation

In cultivation, lack of successful pollination is the most common cause of poor fruiting. Cross-pollination of at least two different genetic varieties of the plant is recommended, and growers often resort to hand pollination or to use of pollinator attractants such as spraying fish emulsion or hanging chicken necks or other meat near the open flowers to attract pollinators. While pawpaws are larval hosts for the zebra swallowtail butterfly, these caterpillars are usually present only at low density, and not detrimental to the foliage of the trees.

Pawpaws have never been cultivated for their fruits on the scale of apples (Malus domestica) or peaches (Prunus persica), primarily because pawpaw fruits ripen to the point of fermentation soon after they are picked, and only frozen fruit will store or ship well. Other methods of preservation include dehydration, production of jams or jellies, and pressure canning (using the numerical values for bananas).

In recent years, cultivation of pawpaws for fruit production has attracted renewed interest, particularly among organic growers, as a native fruit with few to no pests, successfully grown without pesticides. The commercial cultivation and harvesting of pawpaws is strong in southeastern Ohio and also being explored in Kentucky and Maryland, as well as various areas outside the species' native range, including California, the Pacific Northwest., and Massachusetts

The pawpaw is also gaining in popularity among landscapers and backyard gardeners because of the tree's distinctive growth habit, the appeal of its fresh fruit, and its relatively low maintenance needs once established. However, only container-grown pawpaws should be transplanted; use of bare-rooted pawpaws is not recommended, since their fragile root hairs tend to break off unless a cluster of moist soil is retained on the root mass.

Propagation

Trees are easily grown from seed. Germination is hypogeal and cotyledons remain within the seed coat. Strictly speaking, hypogeal means the cotyledons stay in the soil, acting as a food store for the seedling until the plumule emerges from the soil on the epicotyl or true stem. However, pawpaw seeds have occasionally been observed to emerge from the ground and form the true stem and plumule above ground. Desirable kinds (cultivars) of pawpaw are propagated by chip budding or whip grafting.

Seeds will lose viability if they dehydrate to 5% moisture. The seeds need to be stratified, achieved by storage for 9 weeks at 5 degrees C, losing their viability if stored for 3 years or more. Some seeds survive if stored for 2 years.

In one study, propagation using cuttings was not successful.

Habitat restoration

Pawpaws are sometimes included in ecological restoration plantings since this tree grows well in wet soil and has a strong tendency to form well-rooted colonial thickets. The pawpaw is particularly valued for establishing fast-growing vegetation in areas where frequent flooding might produce erosion, since their root systems help hold streambanks steady.

Fruits

As described by horticulturist Barbara Damrosch, the fruit of the pawpaw "looks a bit like mango, but with pale yellow, custardy, spoonable flesh and black, easy-to-remove seeds." Wild-collected pawpaw fruits, ripe in late August to mid-September, have long been a favorite treat throughout the tree's extensive native range in eastern North America, and on occasion are sold locally at farmers' markets. Pawpaw fruits have a sweet, custardish flavor somewhat similar to banana, mango, and cantaloupe, varying significantly by source or cultivar, with more protein than most fruits. Nineteenth-century American agronomist E. Lewis Sturtevant described pawpaws as

... a natural custard, too luscious for the relish of most people

Ohio botanist William B. Werthner noted that

The fruit ... has a tangy wild-wood flavor peculiarly its own. It is sweet, yet rather cloying to the taste and a wee bit puckery – only a boy can eat more than one at a time.

Fresh fruits of the pawpaw are commonly eaten raw, either chilled or at room temperature. However, they can be kept only 2–3 days at room temperature, or about a week if refrigerated. The easily bruised pawpaw fruits do not ship well unless frozen. Where pawpaws grow, the fruit pulp is also often used locally in baked dessert recipes, with pawpaw often substituted with volumetric equivalency in many banana-based recipes. Pawpaws may also be blended into ice cream or included in pancakes.

Due to its potential for allergic reaction causing contact dermatitis and possible presence of pesticides, pawpaw consumption may be harmful to humans.

Nutrition

According to a report from the Kentucky State University Pawpaw Program (right table), raw pawpaw (with skin) in a 100 gram amount provides 80 Calories and is a rich source (20% or more of the Daily Value, DV) of vitamin C (22% DV), magnesium (32% DV), iron (54% DV) and manganese (124% DV). The fruit also contains a moderate amount (10–19% DV) of vitamin A (11% DV).

Phytochemicals

Phytochemical extracts of the leaves and fruit contain acetogenins, including the neurotoxin, annonacin. The seeds and bark contain the chemical asimitrin and other acetogenins, including asimin, asiminacin and asiminecin.

Effect on insects

Due to the presence of acetogenins, the leaves, twigs, and bark of pawpaw trees can be used to make an organic insecticide. The one notable exception is the zebra swallowtail butterfly (Eurytides marcellus), whose larvae feed on the leaves of various species of Asimina, conferring protection from predation throughout the butterfly's life, as trace amounts of acetogenins remain present, making them unpalatable to birds and other predators.

Historical uses

The tough, fibrous inner bark of the pawpaw was used by Native Americans and settlers in the Midwest for making ropes, fishing nets, mats, and for stringing fish.

Pawpaw logs have been used for split-rail fences in Arkansas.

The hard, brown, shiny lima-bean-sized seeds were sometimes carried as pocket pieces in Ohio.

Old song

A traditional American folk music portrays wild harvesting of pawpaws; Arty Schronce of the Georgia Department of Agriculture gives these lyrics:

Where, oh where is dear little Nellie?
Where, oh where is dear little Nellie?
Where, oh where is dear little Nellie?
Way down yonder in the pawpaw patch

Pickin' up pawpaws, puttin' 'em in your pocket
Pickin' up pawpaws, puttin' 'em in your pocket
Pickin' up pawpaws, puttin' 'em in your pocket

He notes that "picking up pawpaws" refers to gathering the ripe, fallen fruit from beneath the trees, and that the "pocket" in the song is that of an apron or similar tie-on pocket, not a modern pants or blue jeans pocket, into which pawpaws would hardly fit. A "pawpaw patch" refers to the plant's characteristic patch-forming clonal growth habit.

Place names

The pawpaw (Asimina triloba) is the basis for various place and school names in the United States, almost all using the older spelling variant "paw paw".

Lakes

Streams and river segments

Towns

Townships

Other places

Schools

Three US high schools have names based on the pawpaw, due to their location in or near towns named Paw Paw:

Art

Nineteenth-century naturalist and painter John James Audubon included pawpaw foliage and fruits in the background of his illustration of the yellow-billed cuckoo (Coccyzus americanus) in his classic work, The Birds of America (1827–1838).

Pawpaw fruits and a pawpaw leaf are featured in the painting Still Life with Pawpaws (c. 1870-1875) by Edward Edmondson, Jr. (1830–1884), at the Dayton Art Institute in Dayton, Ohio.

Other