Pawpaw Planting Guide
Pawpaw Research Project, Community Research Service, Atwood Research Facility, Frankfort, KY 40601-2355
This
bulletin was created to meet the increasing demand for information on
cultivation of the native American pawpaw, Asimina triloba. Very little
scientific research has been done on pawpaw. The advice given here is
based partly on research and partly on the experiences of many pawpaw
growers. These guidelines should help you to become a successful pawpaw
grower.
Climate
The
pawpaw is a tree of temperate humid growing zones, requiring warm to
hot summers, mild to cold winters, and a minimum of 32 inches (81 cm)
of rainfall spread rather evenly throughout the year, with the majority
falling in spring and summer. It can be grown successfully in USDA
plant hardiness zones 5 (-15o F/-26o C) through 8 (15o F/-9o C).
Pawpaws grow wild over a wide range of latitude, from the Gulf Coastal
plain to southern Michigan. However, the trees may not receive adequate
chilling hours if planted too close to the Gulf Coast. Most named
cultivars originated in the Midwest, which is the northern portion of
the pawpaw's range. A national regional variety trial is underway to
determine which varieties perform best in different parts of the
country, and results should become available in the next several years.
In the meantime, for best results, choose cultivars that were selected
in a climatic zone and latitude similar to the area where they will be
planted.
Site, soils, and habitat
Although
the pawpaw is capable of fruiting in the shade, optimum yields are
obtained in open exposure, with some protection from wind (on account
of the large leaves). Germinating seedlings, however, will not survive
under those conditions because they are extremely sensitive to full
sunlight, which can kill them. (Containerized seedlings may be grown
without shade in a greenhouse.) Shading for the first year, and
sometimes the second, is normally required outside, and it is for this
reason that pawpaws are almost always found in nature as an understory
tree. The soil should be slightly acid (pH 5.5-7), deep, fertile, and
well-drained. Good drainage is essential to success. Pawpaws will not
thrive in heavy soil or waterlogged soil. In habit it is a small tree,
seldom taller than 25 feet. Grown in full sun, the pawpaw tree develops
a narrowly pyramidal shape with dense, drooping foliage down to the
ground level. In the shade it has a more open branching habit with few
lower limbs and horizontally held leaves.
Seed Propagation
Pawpaw
seed is slow to germinate, but it is not difficult to grow seedlings if
certain procedures are followed. Do not allow the seed to freeze or dry
out, because this can destroy the immature, dormant embryo. If seeds
are dried for 3 days at room temperature, the germination percentage
can drop to less than 20%. To break dormancy, the seed must receive a
period of cold, moist stratification for 70-100 days. This may be
accomplished by sowing the seed late in the fall and letting it
overwinter; the seed will germinate the following year in late July to
late August.
Another way is to stratify the seed in the refrigerator (32o- 40o F/0o- 4o
C). In this case the cleaned seed should be stored in a plastic ziplock
bag with a little moist sphagnum moss to keep the seed moist and
suppress fungal and bacterial growth. After stratification the seed
should be sown 1 inch (2.5 cm) deep in a well-aerated soil mix, pH
5.5-7, with an optimum temperature of 75o- 85oF (24o- 29o C).
Use
tall containers, such as tree pots (ht. 14"-18"/35-45 cm) or root
trainers (ht. 10"/25 cm), to accommodate the long taproot. The seed
will normally germinate in 2-3 weeks, and the shoot will emerge in
about 2 months. Germination is hypogeal: the shoot emerges without any
cotyledons. For the first two years, growth is slow as the root system
establishes itself, but thereafter it accelerates. Trees normally begin
to bear fruit when the saplings reach 6 feet, which usually requires
five to eight years.
Vegetative or clonal propagation
Pawpaw
clones are easily propagated by a number of grafting and budding
techniques, such as whip-and-tongue, cleft, bark inlay, and chip
budding. The only method that does not produce good results is
T-budding. Softwood and hardwood cuttings have proved virtually
impossible to root. Although it is common for a pawpaw to sucker from
the roots, propagating a clone by transplanting root suckers is often
not successful. Pawpaws are ordinarily quite difficult to transplant.
They have fleshy, brittle roots with very few fine hairs.
Experimentation has shown that, to be successful, transplantation
should be done in the spring at the time that new growth commences, or
soon after. (This is basically the same as for magnolia.) If many roots
are lost, it may be desirable to prune the top to bring it into balance
with the remaining roots. Grafted trees may bear fruit in as few as 3
years.
Purchasing trees
Pawpaw
trees available from nurseries are generally either seedlings or
grafted named cultivars. Seedling trees are typically one year old at
time of purchase, and they are less expensive than grafted trees. Since
seedlings are not identical to their parents, fruit quality cannot be
guaranteed. Trees that have been grafted to named cultivars are usually
2 years old at time of purchase, and they are more expensive than
seedling trees. Since they retain the clonal identity of the parent,
fruit quality is assured, given adequate cultural conditions. If fruit
production is desired, purchase at least two genetically different
trees (i.e. two different cultivars, or at least two seedlings), to
ensure that cross pollination can occur. Container-grown trees may have
a higher establishment (survival) rate than field-dug trees; pawpaws
have a long tap root and delicate root system that can be damaged by
digging.
Caring for young trees
When
planting trees, allow 8 feet (2.5m) between them. Water newly planted
trees immediately after planting, and as needed throughout the growing
season. Pawpaw trees require adequate soil moisture, especially in the
first two years after planting. Transplant shock may be reduced by
providing temporary partial shade to newly planted trees. Application
of a balanced fertilizer (20N-20P-20K) every few weeks during the first
half of the growing season may also improve growth and establishment of
young trees.
Pollination: natural and artificial
Pollination
can be a limiting factor in pawpaw fruit set. The flowers are
protogynous, meaning that the stigma (the female receptive organ)
ripens before the pollen, and is no longer receptive when the pollen is
shed. Thus the flower is designed not to be self-pollinated. In
addition, pawpaw trees are self-incompatible, usually requiring pollen
from a genetically different tree in order to be fertilized. Finally,
the natural pollinators of the pawpaw--various species of flies and
beetles--are not efficient or dependable. Although it requires a little
extra labor, hand pollination to ensure fruit set can be well worth the
effort and can be done as follows: Using a small, flexible artist's
brush, transfer a quantity of fresh pollen from the anthers of the
flower of one clone to the ripe stigma of the flower of another clone.
Pollen is ripe when the little ball of anthers is brown in color, loose
and friable; pollen grains appear as yellow dust on the brush hairs.
The stigma is ripe when the tips of the pistils are green and glossy,
and the anther ball is still hard and green. Do not overburden the tree
with fruit, as this will stress the tree, resulting in smaller than
normal fruit, and may cause limbs to break under excessive weight.
Pests
In its native habitat the pawpaw has few pests of any importance. The worst pest is Talponia plummeriana,
the pawpaw peduncle borer, a small moth larva (about 5 mm long) that
burrows into the fleshy tissues of the flower, causing the flower to
wither and drop. In some years this borer is capable of destroying the
majority of blossoms.
Another pest is Eurytides marcellus,
the zebra swallowtail butterfly, whose larvae feed exclusively on young
pawpaw foliage, but never in great numbers. The adult butterfly is of
such great beauty that this should be thought more a blessing than a
curse.
Sometimes the fruit surface may be covered with patches
that are hard and black; this is a fungus infection, but it seldom has
any effect on flavor or edibility. Deer will not eat the leaves or
twigs, but they will eat fruit that has dropped on the ground. Male
deer occasionally damage trees by rubbing their antlers on them in
winter. Outside its native region, the pawpaw is sometimes reported to
be plagued by pests, but this may be because of poor tree health
resulting from the stress of improper soils and an unsuitable climate.
1 Research Assistant, Kentucky State University
2 Founder of the PawPaw Foundation, PO Box 23467, Washington, D.C. 20026
3 Undergraduate student, Kentucky State University
4
Principal Investigator of Horticulture, Kentucky State University, and
Curator, USDA National Clonal Germplasm Repository for Asimina spp.
5
Assistant Professor, Clemson University, Department of Horticulture,
Clemson, SC 29634-0375; President of the PawPaw Foundation
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