Asimina triloba
Asimina triloba, the American papaw, pawpaw, paw paw, or paw-paw, among many regional names, is a species of small deciduous tree. It has large leaves and produces a large, yellowish-green to brown fruit.
The species is native to eastern North America, in a more temperate range than its tropical relatives. It is a patch-forming understory tree of hardwood forests, being found in well-drained, deep, fertile bottomland and also hilly upland habitat.
Although much of the plant contains the neurotoxin annonacin, the ripe fruits are edible. They are sweet, with a custard-like texture and a flavor somewhat similar to banana or pineapple. They are commonly eaten raw, but are also used to make ice cream and baked desserts.
Description
A. triloba is a large shrub or small tree growing to a height of, rarely as tall as, with trunks 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 large leaves with drip tips are more characteristic of plants in tropical rainforests than within this species' temperate range.The leaves of the species are simple, alternate and spirally arranged, entire, deciduous, obovate-lanceolate, long, 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, allowing pawpaw groves to be spotted from a long distance.
Pawpaw flowers are perfect and protogynous, about 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 to attract pollinators.
The fruit of the pawpaw is a large, yellowish-green to brown berry, long and broad, weighing from, containing several brown or black seeds 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 green, yellowish green, or brown. When mature, the heavy fruits bend the weak branches down. Full ripening often happens only after the fruit falls naturally, thus signifying a seed dispersal strategy aimed at ground-based, rather than arboreal, mammals.
Other characteristics:
- Calyx: Sepals three, valvate in bud, ovate, acuminate, pale green, downy
- Corolla: Petals six, in two rows, imbricate in the bud; inner row acute, erect, nectariferous; outer row broadly ovate, reflexed at maturity; petals at first are green, then brown, and finally become dull purple or maroon and conspicuously veiny
- Stamens: Indefinite, densely packed on the globular receptacle; filaments short; anthers extrorse, two-celled, opening longitudinally
- Pollen: Shed as permanent tetrads
- Pistils: Several, on the summit of the receptacle, projecting from the mass of stamens; ovary one-celled; stigma sessile; ovules many
- Branchlets: Light brown, tinged with red, marked by shallow grooves
- Winter buds: Small, of two kinds, the leaf buds pointed and closely appressed to the twigs, and the flower buds round, brown, and fuzzy
- Bark: Light gray, sometimes blotched with lighter gray spots, sometimes covered with small excrescences, divided by shallow fissures; inner bark tough, fibrous; bark with a very disagreeable odor when bruised
- Wood: Pale, greenish yellow, sapwood lighter; light, soft, coarse-grained and spongy with a specific gravity of 0.3969 and a density of
- Longevity of fruit production: Undetermined
Taxonomy
Etymology
This plant's scientific name is Asimina triloba. The genus name Asimina is adapted from the Native American name assimin or rassimin combining the root terms rassi = "divided lengthwise into equal parts" and min = "seed, fruit, nut, berry, etc." through the French colonial asiminier. The specific epithet triloba in the species' scientific name refers to its lobed fruits.Distribution and habitat
Asimina is the only genus native to temperate regions within the otherwise tropical and subtropical flowering plant family Annonaceae. A. triloba has the northernmost range of any species in the family, extending north into southern Ontario, Canada. Within the United States, pawpaw is native to the eastern, southern, and midwestern states, ranging from New York westward to southeastern Nebraska, southward to eastern Texas and the panhandle of Florida. Pawpaw has even been be successfully cultivated as far north as Nova Scotia, Canada and in several regions of Europe and Asia. Pawpaws typically grow in USDA hardiness zones 5 to 8, though some growers have had success cultivating them in USDA hardiness zone 4b with careful site selection and protection to compensate for the shorter growing season.Within its natural habitat, pawpaw grows in slightly acidic, well-drained soils. These trees typically establish as part of the understory in Eastern Temperate Forest region. Pawpaws are also found along floodplains, stream banks, and shaded, nutrient-rich bottomlands, but they prefer gently elevated slopes because it has a deep-reaching taproot. While pawpaws are shade tolerant, maximum fruit yields occur under full sun conditions with some wind protection. However, germinating seedlings are extremely sensitive to full sun and require partial shading during their first one or two years.
Ecology
Hybridization with other ''Asimina'' species
The common pawpaw is the largest and most well known of the 13 species of the Asimina genus in North America. Of those 13, 11 prefer very warm weather and have ranges rarely extending northward of Florida or coastal Alabama. Their ranges do not overlap with Asimina triloba.One southern U.S. species, Asimina parviflora, does overlap in range with pawpaw. This species is smaller than pawpaw in both its flower and its woody growth. A. parviflora is more shrublike, rarely growing even a third as tall as pawpaw. Genomically verified hybrids of A. triloba and A. parviflora have been classified as Asimina piedmontana.
Pollination
Pawpaw are self-incompatible, meaning pollen cannot fertilize flowers on the same plant. This, coupled with the pawpaw's tendency to form clonal patches, can reduce fertilization success. A single patch consisting of many stems may therefore produce no fruit if all stems are genetically identical. Fruitless pawpaw patches have been documented in Ohio.The floral scent of Asimina triloba has been described as "yeasty", which is one of several features that signify a "beetle pollination syndrome". Other floral features of pawpaw indicative of beetle pollination include petals that curve over the downward-pointing flower center, along with food-rich fleshy bases of the inner whorl of petals. A "pollination chamber" is thereby created at a depth that only small beetles can access during the initial female-receptive stage of floral bloom. As with other well-studied species of Annonaceae, the delay in the shift from female to male floral stage offers beetles a secure, and possibly thermogenic, residence in which not only to feed but also to mate. Receptive stigmas at their arrival, followed by pollen-shedding stamens during pollinator departure, are regarded as an early form of mutualism evolved between plants and insects that is still dominant in the most ancient lineages of flowering plants, including the Magnoliids.
Beetles are the dominant form of pollinator ascribed for genera and species within the Annonaceae family. However, two species of genus Asimina bear a floral character that has given rise to an alternative hypothesis that carrion or dung flies are their effective pollinators. That floral characteristic is the dark maroon color of the petals. Hence, while no scholarly papers have documented carrion or dung flies as effective pollinators in field observations, the strength of this hypothesis has led some horticultural growers to place carrion in pawpaw orchards during the bloom time.
Professional papers on genus Asimina and its species have warned of the difficulties in discerning whether insects observed on or collected from flowers are effective pollinators or merely casual and thus opportunistic visitors.
A citizen science project in southern Michigan utilized natural history forms of observation, along with video and photo documentation, during a "pawpaw pollinator watch" in May 2021. Two species of tiny sap beetle were reported as the most abundant and the most consistently present insect types at depth within the flowers, and thus as the most likely effective pollinators. The two species are Glischrochilus quadrisignatus and Stelidota geminata. Both are in the taxonomic family Nitidulidae. Nitidulid beetles are described by Clemson University as likely "night flying" pollinators of pawpaw.
Larvae and adult beetle stages of Glischrochilus quadrisignatus were also documented by the citizen project on the ground-level side of rotting fruit in a pawpaw orchard in Michigan following the fruit harvest. This aligns with scientific reports of well-studied relatives of pawpaw: the cherimoya and other valued fruits of the subtropical genus Annona. Because the beetle pollinators are not strong fliers, their entire lifecycle must be facilitated onsite in both orchard and forest restoration settings. Abundant leaves, rotting fruit, and other organic material on the ground beneath or near the flowering trees are crucial for the larval stage of the beetle. There, eggs are laid and the larvae hatch and feed on decaying matter, then dig downward into the soil layer to pupate through the winter. Beetles emerge when flowering begins the following year.