From NewCROPTM, Center for New Crops & Plant Products, Purdue University
by Desmond R. Layne
Pawpaw
1. Common Names
2. Scientific Names
3. Uses
4. Origin and Botany
1. Description of the Plant
5. Crop Status
6. Crop Culture
1. Seedling Production
2. Field Planting
3. Vegetative Propagation
7. Germplasm
1. Collections
2. Commercial Seed Sources
3. Commercial Seedling Sources
4. Commercial Named Variety Sources
8. Key References
9. Selected Experts
Common Names
Indiana Banana
Hoosier Banana
Poor Man's Banana
Scientific Names
Species: Asimina triloba (L.) Dunal
Family: Annonaceae (Custard Apple Family)
Uses
Eaten
in-hand as fresh fruit or processed into desserts. Twigs are source of
annonaceous acetogenins which are being used in the development of
anti-cancer drugs and botanical pesticides.
Origin and Botany
The
pawpaw is the only temperate member of the tropical Annonaceae family
and is the largest tree fruit native to the United States. Pawpaws grow
wild in the rich, mesic hardwood forests of 25 states in the eastern
United States ranging from northern Florida to southern Ontario
(Canada) and as far west as eastern Nebraska. Pawpaws flourish in the
deep, rich fertile soils of river-bottom lands where they grow as
understory trees or thicket-shrubs.
In addition to the tropical
Annona relatives, there are eight members of the Asimina genus that are
native to the extreme southeastern states of Florida and Georgia. These
include A. incarna (flag pawpaw), A. longifolia, A. obovata, A.
parviflora (dwarf pawpaw), A. pygmaea, A. reticulata, A. tetramera
(opossum pawpaw), and A. X nashii.
Description of the Plant
Pawpaw
is a small, deciduous tree that may attain 5 to 10 m in height. In the
forest understory, trees often exist in clumps or thickets. This may
result from root suckering or seedlings developing from fruits that
dropped to the ground from an original seedling tree. In sunny
locations, trees typically assume a pyramidal habit, straight trunk and
lush, dark green, long, drooping leaves that turn gold and brown in
color during the fall.
Flowers emerge before leaves in mid
spring. The blossoms occur singly on previous year's wood and may reach
up to 5 cm in diameter. Flowers are strongly protogynous,
self-incompatible and require cross pollination although some trees may
be self-compatible. Pollination may be by flies and beetles which is
consistent with the presentation appearance of the flower: dark,
meat-colored petals and a fetid aroma.
Fruit set in the wild is
usually low and may be pollinator or resource-limited but under
cultivation, tremendous fruit loads have been observed. Fruits are
oblong-cylindric berries that are typically 3 to 15 cm long, 3 to 10 cm
wide and weigh from 200 to 400 g. They may be borne singly or in
clusters which resemble the "hands" of a banana plant (Musa spp.). This
highly aromatic, climacteric fruit has a ripe taste that resembles a
creamy mixture of banana, mango, and pineapple. Shelf-life of a
tree-ripened fruit stored at room temperature is 2 to 3 days. With
refrigeration, fruit can be held up to 3 weeks while maintaining good
eating quality. Within the fruit, there are two rows of large, brown,
bean shaped, laterally compressed seeds that may be up to 3 cm long.
Seeds contain alkaloids in the endosperm that are emetic. If chewed,
seed poisons may impair mammalian digestion but if swallowed whole,
seeds may pass through the digestive tract intact.
Crop Status
Pawpaws
are not yet a commercially important crop in the U.S. but they have
tremendous potential based on the following reasons: 1) adaptation of
trees to existing climatic and edaphic conditions; 2)
nutritional/cosmetic value of fruit; 3) valuable natural compounds in
plant; 4) nursery wholesale and retail tree production; and 5) as a
component in residential 'edible' landscapes. Pawpaw is well adapted to
the 25 states to which it is native and where it already grows in the
wild. It is hardy to zone 5 (-25°C) and requires a minimum of 400
hrs annual chill units, 160 frost-free days, and 80 cm of annual
precipitation with most falling during spring and summer.
Pawpaw
is an excellent food source. It exceeds apple, peach, and grape in most
vitamins, minerals, amino acids, and food energy value. Pawpaw fruits
are best eaten fresh when fully ripe. The intense tropical flavor and
aroma may also be useful for developing processed food products
(blended fruit drinks, baby food, ice creams, etc.). The flesh purees
easily and freezes nicely. Pawpaws easily substitute in equal part for
banana in most recipes. Aromas may be used commercially in cosmetics
and skin products.
Pawpaw plants produce natural compounds
(annonaceous acetogenins) in leaf, bark and twig tissues, that possess
both highly anti-tumor and pesticidal properties. Current research by
Dr. Jerry McLaughlin at Purdue University (personal communication)
suggests that a potentially lucrative industry, based simply on
production of plant biomass, could develop for production of
anti-cancer drugs (pending F.D.A. approval) and natural (botanical)
pesticides. The high level of natural defense compounds in the tree
make it highly resistant to insect/disease infestation (R.N. Peterson,
The PawPaw Foundation, personal observation). With proper management,
organic commercial fruit production may be possible.
Currently in
the U.S., there are more than 40 commercial nurseries selling pawpaw
trees. Seedling and grafted trees in the retail nursery trade are
currently selling briskly for as much as $18.50 and $26.50 apiece,
respectively, versus $3-4 for a 2-year old, grafted apple tree.
Standing orders are currently in excess of 40,000 trees in the
wholesale market (Jim Gilbert, manager, Northwoods Wholesale Nursery,
Molalla, OR, personal communication).
Pawpaws are ideally suited
for the residential 'edible' landscape due to their lush, tropical
appearance, attractive growth form, size, fall color, and delicious
fruit. In addition, Asimina spp. are suitable for butterfly gardens as
they attract the zebra swallowtail (Eurytides marcellus) for whom they
are the exclusive larval host plant.
CROP CULTURE
Seedling Production
As
soon as flesh is soft, fruit should be collected for seed. Seeds are
easily extracted following maceration of fruit in water and floating
off of pulp. The seed can be sterilized by shaking seeds with a 10-20%
Chlorox solution for 1-2 minutes followed by several rinses of
distilled water. This aids in reducing fungal and bacterial
contamination during storage.
Seeds should not be allowed to dry
out. Once cleaned, they should be stored refrigerated in Ziplock-type
polyethylene bags with slightly moist sphagnum (or peat) moss. Seeds
have a dormant immature embryo and require stratification. Storage
under refrigerated conditions (5°C) for 100 days is recommended to
overcome embryo dormancy. Provided that desiccation and microbial
contamination do not occur, seeds may be stored for several years under
refrigerated conditions with little loss in viability.
Stratified
or after ripened seeds are planted either in a prepared seedbed or in
containers. Scarification of the seed coat is not necessary. Pawpaw
seed germination is hypogeal. Seeds should be sown to a 3 cm depth in a
moist, well drained soil or other medium that has good aeration in
containers that are 20-25 cm deep. Following radicle emergence, tap
root growth proceeds to the bottom of the container and lateral roots
begin to develop. Typically, under soil temperatures of
24°-29°C, the shoot does not emerge from the soil until 9 weeks
following sowing. Shoot emergence is hastened 10 days by elevating soil
temperature to 29°-32°C from sowing date thereafter. The
optimal conditions for greenhouse production of robust, container-grown
seedlings include the following: average day and night temperatures of
27° and 24°C, respectively, max. light intensity 1000
µmol•m-2 s-1 photosynthetic photon flux density (greenhouse
whitewashing recommended during summer months, especially), 16 h
photoperiod (extended by high pressure sodium lamps), fertilization
2x/wk to runoff during shoot growth phase, and soil temperature of
29°-32°C. For seedlings with 2-12 unfolded leaves grown in 740
cu. cm containers, 250 ppm 20N-20P-20K water soluble fertilizer plus
trace elements is recommended. After 12 or more unfolded leaves are
attained, seedlings require transplanting to a larger container.
Seedlings
will continue actively growing once 'potted-on' if transplanted before
they exhaust the existing soil volume; otherwise, they will set a
terminal bud and stop shoot growth. We transplant into 40 cm deep 2
gallon pots and then boost the fertilization rate to 500 ppm
20N-20P-20K water soluble fertilizer plus trace elements. If root
spiraling in containers is a problem, containers can be coated with a
latex paint mixture containing copper compounds of low solubility. By
utilizing photoperiod extension, light intensities not exceeding 50% of
full sunlight, temperature regulation, soil warming and fertilization,
we have produced pawpaw seedlings with up to 1.5 m of top growth in one
season in the greenhouse. Trees of this size are ideal for field
transplanting and have sufficient caliper for chip-budding.
Field Planting
Field
planting should be done when trees are not actively growing. Trees can
be planted in fall or spring. Spring-planted trees should have had
their chilling requirement met at or before planting. Ideally, a
dormant tree is planted in early Spring, although similar
considerations as noted below should be adhered to for Fall planting of
a dormant tree. Planting holes should match the existing containerized
root system. For 2 gal. containers, a hand-held power auger works
nicely for hole drilling. Care should be taken not to plant trees when
soil is too dry or wet. Hole drilling in wet, clayish soils will result
in glazing of the hole walls which may impede root penetration into the
soil.
Soil should be well drained, deep, fertile and slightly
acid (ie. pH 5.5-7). Preplant soil tests are desirable in order to make
necessary amendments. Spring planting should be done during the local
pawpaw budbreak period (April in Kentucky). Trees should be planted
such that the soil line of the pot is even with the soil line of the
field.
It is essential that seedlings receive adequate water in
the year of establishment. Pawpaw trees establish and grow best when
they are given shelter the first year in the field. This is reliably
accomplished by utilizing tree shelters that are used in reforestation.
Weed control is necessary especially in the establishment year.
Fertilizer
application can be accomplished by broadcasting granular fertilizer in
spring. Fertigation with liquid fertilizer can be useful if drip
irrigation is installed in the orchard. We do not currently have
recommendations for feeding field planted trees but we fertigate with
500 ppm 20N-20P-20K water soluble fertilizer plus trace elements once a
month in May, June, and July during the active growth phase. Fertilizer
recommendations based on foliar and soil analyses will be developed in
the future. Recommended tree spacing at present is 5.5 m between rows
and 2 m apart in the row. Row orientation should be North-South.
Transplanting
trees from the wild is usually unsuccessful. Young trees dug from a
thicket or grove are often root suckers with few, brittle roots that
have very few root hairs. Due to the poorly developed root system and
frequent absence of shelter following transplanting, transplanting
shock is usually severe resulting in the death of the root sucker.
Transplanting
of seedlings from the wild is most successful when done in the Spring
during budbreak. If many roots are lost in the digging process, it is
desirable to prune the shoot to bring it into balance with the existing
root system. Unlike transplanting seedlings collected from the wild,
containerized seedlings transplant with high success if the guidelines
described above are adhered to.
Vegetative Propagation
Pawpaws
are easily propagated by several grafting and budding techniques. These
include whip-and-tongue, cleft, bark inlay, and chip budding. Chip
budding is most successful when the seedling rootstock is pencil thick
or greater in diameter and actively growing. Winter collected, dormant
scion budwood should have had its chilling requirement fulfilled. When
performing chip budding, it is desirable to try to match the diameter
of the budwood with that of the stock plant. It is recommended to wrap
the graft with parafilm laboratory film using strips cut to 2 cm x 15
cm. Parafilm is flexible, moldable, self sealing and moisture
resistant. When stretched, it applies adequate pressure to stimulate
callus production and it maintains good humidity for union formation.
Within
2 weeks, buds will begin expanding and may penetrate the parafilm or
just enlarge under it. In the latter case, parafilm is easily removed
by using a sharp budding knife and making a shallow incision along the
length of the wrap on the side opposite the bud. Care must be taken not
to damage the scion bud in the process of removing parafilm. Once scion
growth commences, we recommend cutting back the top of the stock to
30-60 cm in height leaving 6 or more functional lower leaves and
rubbing off all competing buds on the rootstock. We have found that
cutting the stock back to just above the bud reduces scion growth. Once
the scion is 30 cm or more in length, the lower leaves on the stock can
be removed and the stock can be cut back to a height of 20-25 cm above
the union. Under greenhouse conditions, we have also found that leaving
the stock as described above is extremely helpful because the stub of
the stock projecting above the union then provides a `stake' to secure
the growing scion.
Some scion varieties appear to grow more
horizontally from the graft union than others and using this technique
ensures uniform habit and development of a strong union and vertically
oriented scion. We use masking tape to tie up the scion as it grows.
Once the scion becomes woody, this tape is no longer needed. Starting
with a large, healthy rootstock, up to 1.5 m of scion growth can be
attained in as few as 3 months under optimal greenhouse conditions
described above. Whip grafting is successful on seedling rootstock
material that is as small as 3-4 mm in diameter, provided the scion is
of similar diameter (Jim Gilbert, Northwoods Wholesale Nursery,
Molalla, OR, personal communication).
Other vegetative
propagation techniques such as root cuttings, hard and softwood
cuttings, and tissue culture have met with poor or marginal success.
Propagation by hardwood cutting techniques has never been attempted for
pawpaw indicating a need for research to evaluate the commercial
potential thereof. However, pawpaw propagation by softwood cuttings has
been successful although at a low percentage. Micropropagation
techniques have been developed for many Annonaceous relatives of pawpaw
indicating potential promise of this technology for pawpaw.
GERMPLASM
Collections
Kentucky
State University, Frankfort, KY (USDA National Clonal Germplasm
Repository for Asimina spp., satellite site of Corvallis, OR
repository).
The PawPaw Foundation (University of Maryland property).
Commercial Seed Sources
F.W. Schumacher Seed Co., 36 Spring Hill Rd., Sandwich, MA 02563.
Sheffield Seed Co., 273 Auburn Rd., Locke, NY 13092.
Commercial Seedling Sources
Edible Landscaping, P.O. Box 77, Afton, VA 22920.
Oikos Tree Crops, P.O. Box 19425, Kalamazoo, MI 49019.
Commercial Named Variety Sources
Sherwood Greenhouses, P.O. Box 6, Sibley, LA 71073.
Northwoods Retail Nursery, 27635 S. Oglesby Rd., Canby, OR 97013.
Hidden Springs Nursery, Route 14, Box 159, Cookville, TN 38501.
Key References
Kral, R. 1960. A revision of Asimina and Deeringothamnus (Annonaceae). Brittonia 12:233-278.
Layne, D.R. 1996. The pawpaw [Asimina triloba (L.) Dunal]: A new fruit crop for Kentucky and the United States. HortScience (in press).
McGrath, M.J. and C. Karahadian. 1994. Evaluation of physical, chemical, and sensory properties of pawpaw fruit (Asimina triloba) as indicators of ripeness. J. Agric. Food. Chem. 42:968-974.
McLaughlin, J.L. and Y.-H. Hui. 1993. Chemotherapeutically active acetogenins. U.S. Patent No. 5,229,419.
Peterson, R.N., J.P. Cherry, and J.G. Simmons. 1982. Composition of pawpaw (Asimina triloba) fruit. Ann. Rpt. N. Nut Growers Assoc. 77:97-106.
Peterson, R.N. 1991. Pawpaw (Asimina). In: J.N. Moore and J.R. Ballington (eds.) Genetic resources of temperate fruit and nut trees. Acta Hort. 290:567-600.
Ratanyake, S., J.K. Rupprecht, W.M. Potter, and J.L. McLaughlin. 1992. Evaluation of various parts of the paw paw tree, Asimina triloba (Annonaceae), as commercial sources of the pesticidal annonaceous acetogenins. J. Econ. Entomol. 85:2353-2356.
Selected Experts
Desmond
R. Layne, Assistant Professor of Horticulture, Department of
Horticulture, Poole Agriculture Center, Box 340375, Clemson University,
Clemson, SC 29634-0375
FAX: (864)-656-4960, phone: (864)-656-4964
Contributor: Desmond R. Layne, Community Research Service, Kentucky State University (8/25/1995)
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Copyright © 1995
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