HYBRIDS OF THE AMERICAN PAPAW
G. A. ZIMMERMAN
Harrisburg, Pcnusvlvania
THE American papaw continues to
be an horticultural orphan. The
virtues of this interesting plant
were called to the attention of plant lovers in a contest sponsored by this Association in 1916 to find the largest papaw,
tree and fruit, growing in the United
States. In that contest this largest wild
fruit in North America did fairly well,
for the winning papaw weighed twelve
ounces, and was of delicious flavor. Reliable accounts of fruits a third heavier
than this were reported. The plant grows
over the entire eastern United States, except New England and Florida, and its
culture is not difficult. In spite of these
rather challenging possibilities it continues to be ignored.
The experiments in hybridization and
selection of the papaw here reported
appear to be one of the few projects
which trace to the Association's contest of a quarter century ago. The results obtained to date suggest that this
botanical Cinderella is quite capable of
blossoming into a princess of our gardens with very little attention on the \
part of horticulturists.
Botanically. the papaw is somewhat of
an anomaly. First of all, it is no relative
of the tropical papaya (Carica papaya)
which is also sometimes called "papaw".
Among its relatives are to be found some
of the outstanding tropical fruits of the
world, — the Annonas which include the
cherimoya, the custard apple, the sweetsop and the Javanese keppel. The genus
Asimiiui to which the papaw belongs is a
temporate zone representative of a tropical group. The nine or ten other species
of the Asiminas are not promising fruit
bearers of themselves, but as reported below they do offer excellent possibilities
for hybridization with the papaw.
Through the encouragement of Dr.
David Fairchild the writer has continued
his interest in the papaw ever since the
contest in 1916 brought the plant to his
attention. In the intervening years he
83
has gathered at his home at Piketown,
Pa., all of the recognized varieties and
many unnamed selections. Together with
seedlings of the best ones, his collection
comprises probably sixty or seventy varieties in all. Many of these selections
are for breeding purposes only. In fact
the ultimate goal of this venture is the
production of hybrids between the Asiminas and the Annonas. This collection has
been derived from an area extending from
Ontario to Florida and from California
to Massachusetts. (The plants from
California, however, I believe were introduced there from the Central West.)
The best types have come from the river
bottoms of the Susquehanna. the Potomac and the Ohio. One of them, the
Fairchild, a seedling of the papaw that
took first prize is still, I consider, the
best. The Ketter, the first prize winner, is a close second. I might just add
here that for many years the Ketter produced very poor fruit when grafted on
native wild stock but in recent years
since the top has had time to overcome
the root system the fruit is showing up
as the delicious product it really is. This
collection is large enough to furnish ample variations for the breeding work
which has been begun. The pleasing and
valuable qualities of some of these fruits
have far exceeded expectation. The collection includes not only the papaw itself,
A. triloba, but nine other sj^ecies of Asiniiiia or their hybrids, from Florida or
Georgia. These are of value for crossing
because they bring with them the fragrance so lacking in the triloba, a touch of
delicate flavor to the fruit and the beauty
of gorgeous flowers not met with in the
north.
There is a wide variation in the fruit
of the triloba but it will suffice to divide
them into two classes: a large, yellow
fleshed, highly flavored, early ripening
type : and a white fleshed, mild flavored,
late or very late ripening fruit. These are
variable in size : some are very large.
The light fleshed varieties are as a rule a little too mild in flavor, while some of the
yellow fleshed fruits are too highly flavored. Some of them are really delicious
while others are fit only for the hogs
who are not quite so fastidious. The selected varieties have rather large fruit
and mild, delicious flavor. Eaten with
cream and graham wafers they make a
delightful breakfast dish. They are also
excellent in pies. Their food value is
largely in carbohydrates. They are reputed to have medicinal value, but this
is negligible if any at all. They are also
reputed to make some people sick but
this may be due to personal idiosyncraf
cies, just as in the case with strawberries
and tomatoes.
Further interesting discussions on
Asimina triloba will be found in this
JOURNAL for July 1916 and January
1917.
Varieties
The varieties of today will probably be
replaced by other and better ones. But a
few that deserve note at this time are:
(early varieties) Fairchild, Ketter,
Hope's August; (later and very late varieties) Gable. Taylor, Tiedke, Jumbo,
Shannondale, Osborne, Buckman, and
Martin. Martin, although rather small
and not of very good flavor, is interesting
in that it seems able to resist cold better
than any other I have seen. Contrary to
popular belief, papaw fruit will not stand
iow temperatures. A cold wind for a few
days with temperature under fifty will
turn the fruit black and spoil its flavor.
This is not surprising considering the
tropical relationship of the papaw.
Propagation
Contrary also to popular belief, propagation is easy. Papaws graft very readily on any native stock. Any of the methods used in nut culture are satisfactory,
and the scions make excellent growth.
They must, however, be grafted a little
earlier than persimmons or hickories. In
the latitude of Harrisburg, Pa., any time
in April or early May is best. Seeds of
selected Asiminas will give a large percentage of trees bearing excellent fruit.
I have been able to handle seed best by
placing them in moist sand in a can immediately after collecting, closing the can
and placing in a cellar or cave. About
February they begin to sprout. As the
weather gets warm enough the sprouted
seed can be picked out and planted,
either in their permanent places, or in
nursery rows a foot apart to be transplanted later when they are about a foot
high. Larger plants transplant with considerable difficulty. A stake should be
driven and a little shade afforded them
for a year or two. The sprouted seed will
grow about six inches the first year, a
little more the second and after that quite rapidly,
depending of course on the fertility of the soil and the water supply.
Moist but not wet ground is best. Seedlings from good seed are desirable, for
papaws are usually rather short lived.
Thus they are inclined to die back every
now and then and if the trees are seedlings the stumps will soon sprout and
make nice trees again. But with grafted
trees the varieties would be lost and the
trees have to be regrafted. They should
be pruned a little from time to time to
force new bearing wood. I have had
very little experience with fertilizers but
it appears that some lime in acid soil
districts improves the trees.
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Allied Species and Hybrids
According to Small there are ten species of Asimiiia: triloba, parviflora, obovata, tctramerus, incana (speciosa), angustifolia, reticulata, pygma-ea, pulchellus
and rugleii. The last two are said to
have very fragrant flowers. Some plants
of the reticulata also have fragrant flowers. Since the death of Small many
botanists have either questioned or denied the existence of tetramcrus as a
species. However there is a beautiful
plant found in abundance along the east
coast of Florida, north and south of
West Palm Beach that does not quite fit
into any other classification. If I were
to cross the obovata with the reticulata
I would expect just such plants to result.
I am therefore inclined to think they are
hybrids. Be that as it may, nature might
have made a worse combination for
breeding purposes. Photographs of some
of these handsome plants and flowers are
shown herewith.
My first experience with hybrids came
several years ago when I brought with
me from Florida pollen of the large flowered obovata, storing it for six weeks
with calcium chloride. When the papaws
came into bloom I successfully pollinated
the flowers of Hope's August with this
stored obovata pollen. This produced the
trilobovata (Deming). These plants
have been perfectly hardy and have fruited every year when I have taken the
time to hand pollinate them. Unless this
is done they set only an occasional chance
fruit. This hybrid is rather too highly
flavored for my taste, but many people
like it. Other crosses made since then
with the mild flavored, white fleshed
Buckman have tamed down the flavor.
The hybrids have been crossed with several other varieties, and fertilized with
their own pollen, but the resulting plants
are too small for bearing as yet. As a
matter of fact all seeds produced are hybrids because I necessarily make crosses
when I hand pollinate the flowers to get
them to set fruit.
A second cross between the atigustifolia and triloba, producing the trigustifolia is making excellent growth. These
hybrids are interesting because the male
parent, aiigustifolia, has leaves not over
a half inch wide and six or seven inches
long. The hybrid is much like angustijolia in shape of leaf, but it has not yet
flowered. It has been hardy so far.
Crosses were made last spring between
incana and triloba and also between the
reticulata and triloba. The seed of these
will be planted this spring.
Other hybrids, mostly between the better trilobas, are under way and ought to
produce some excellent fruit within a
year or so. It is interesting to note that
by cutting the tops from the hybrid seedlings at the end of the first year and
grafting them into large plants it is possible to get a huge growth which often
sets flowers the next year. These produce
fruit if they are hand pollinated. Cutting
off the tops does not hurt the little plants
in the least. The grafting is a chore but
when a man is getting old it enables him
to see the results of his work, perhaps
before he passes out. It is expected that
these hybrids will form a sort of "half
way station" in crossings with the Annona.
Hand Pollination Necessary
It is becoming quite clear that if we
want fruit in any considerable quantity
from our finer varieties of papaws we
must resort to hand pollination. Dr.
Fairchild informs me that in Egypt Annonas are hand pollinated on a commercial scale. This is a tedious job, but there
seems to be no way to avoid it at press
The Journal of Heredity
ent. I have many trees of fine varieties
of triloba that have never yet set a fruit
unless they were hand pollinated, even
though they were covered with bloom.
This is a poor showing when compared
with the limbs shown in Figure 5 from
these very same trees covered with fruit
so heavily that they are bent to the
ground. Some limbs have broken off
from weight of fruit and yet not a single
fruit set on any other part of the tree. It
seems to make little difference what papaw pollen is used provided it is good
and the flowers are pollinated early
enough. Ordinarily all the early bloom
drops off. The later bloom of some trees
will set an occasional fruit. Many trees
have never set any fruit as long as I have
had them under observation, while a few
others bear fairly and rather regularly.
Once in a while a tree bearing good
fruit has this desirable habit. The Fairchild is notable in this respect although
it never bears the quantity of fruit it
should.
The principal reason for this unfortunate infertility is that the flowers of triloba are nearly always dichogamous,with
the stigma maturing frequently a long
time before the anthers shed their pollen
(protogynous). In view of the fact that
nearly all of the flowers display this cycle
of maturity one can easily see why the
early opening flowers set no fruit. The
late flowers occasionally get pollinated if
an insect or the wind happens to convey
the necessarv pollen from another flower.
This is not at all a frequen'; occurrence
with the triloba for its bloom is inconspicuous and almost foul scented.
I doubt whether the wind is an important factor in the pollination of the papaw, and insects in the north are little
more valuable. I have never yet seen an
insect in the flowers of the triloba that
appeared to have anything to do with pollinating it. In the south, on the large
flowered obovata I have often observed a
beetle of the Trichius group go into the
flower and get caught under the petals,
which evidently scared him. He would
set up quite a whirlwind in the flower
and escape covered with pollen, go to
another flower, crawl in and do the same
thing. In some years hardly a flower of
the obovata escapes such treatment.
Even though the flowers of the papaw
are perfect, they are nearly always dichogamous, so without the aid of the wind
or of insects, it seemj that to plant different varieties together in the hope of better pollination is not likely to be very
helpful.
Our only hope seems to lie in our ability to find special trees which will pollinate their own flowers and bear good fruit.
Such plants can perhaps be produced
through hybridization and selection. The
only other solution seems to be to resort
to hand pollination. It may be possible
to develop hybrids with the excellent
fruit of the triloba, and with the attractive and fragrant flowers of the southern
species so that they will be attractive to
insects or bees. For this purpose I am
using on triloba the pollen of obovata,
whose flowers are sometimes six inches
in diameter, and the reticulata whose
fragrance is equal to that of the petunia.
This combination, could it be combined
with a desirable fruit-type, should be irresistibly interesting to many insects.
The pistils of Asimina flowers are so
easily injured that large insects may
turn out to do more harm than good.
During the operation of hand pollination with a fine camel's hair brush it is
not an uncommon tragedy to find the
stigma on the brush when the job is
done. The stigma will not set any fruit
on a camel's hair brush or on a bumble
bee's legs. However, if carefully done
one can in a very brief time pollinate
enough flowers to give an abundance of
fruit for the family table. A very little
more time would be necessary to put
fruit production on a commercial basis.
After a little experience pollination
does not take so much time as might be
expected. When the fruit ripens the time
seems very well spent. Otherwise, without hand pollination failure is almost certain under any conditions. Fcr the past
two years I had more fruit from my papaw collection than ever before and all
from a couple of hours' work. Two years
ago there was no fruit of any value anywhere in our district other than that
Zimmerman: Papaw Hybrids 91
which developed from that pollinated by
hand. I had made a practice of allowing
native trees that bore well to stand, whatever the quality of their fruits, in the
hope of getting adequate pollination for
the good trees. Xow I am beginning to
doubt whether this if of any real benefit.
Crosses With Annonas
With the Annonas it has been very difficult to get pollen at the right time and
in sufficient quantities since it must come
from Florida or California. However, I
now have in the greenhouse many species
growing and hope shortly to be able to
produce plenty of Aniwiia pollen. Last
spring I pollinated flowers of the Tiedke
and Long John with pollen of the "atcmoya" hybrids and of A. Squamosa and
had a few fruit set from each. Whether
these will prove hybrids must wait the development of the plants from these seed.
It is just possible that we may have to
change the chromosome groups in some
of these species before the ultimate goal
is reached. An interesting discussion on
the Aniwna will be found in the Procccdimjs of the Florida State Horticultural
Society for 1939 by Dr. David Fairchild.
Since 1916 I have succeeded in going
far enough in breeding papaws to realize
what an immense job lies ahead. At the
same time the vision of what can be accomplished has widened in proportion
and I am sure that the end will certainly
justify the work and the waiting. The
possibilities are thrilling and there are
compensations; quite soon the crosses
yield handsome ornamentals, novel and
interesting new creations. If the experimenter takes a little extra time
hand
pollinate until better ways are found, he
will be handsomely repaid with an abundance of luscious fruit.
(CAPTION under Photo)
#1 Photo (see clickable PHOTO link at beginning )
HYBRID TREES PRODUCE ABUNDANT FRUIT
Figur. 1
Three "tnlobovata" hybrids (triloba X obovata) growing at Pikestown, Pa. Like the
native papaw, these produce fruit if hand pollinated but otherwise very few fruits develop.
A—shows a branch which was hand pollinated with its own pollen. B—is a small tree hand
pollinated with Taylor papaw pollen and C shows two limbs of a hybrid tree hand pollinated
with its own pollen.
#2 Photo
PARENT AND HYBRID
Figure 2
A—a small plant of A. angjistifolia. This
bore three flowers, which were pollinated with
tetramcnts pollen. Three fruits set: two fell
off and one matured, having two seeds. These
have germinated and are now growing. B—is
a "Trigustifolia" hybrid (between augiistifolia
and triloba). This hybrid has proven hardy
at Pikestown. Pennsylvania, though this speciman was grown in the greenhouse.
--
PAPAW RELATIVES AND A HYBRID
Figure 3
A—Plant of the "trilobovata" hybrid in flower. The flowers when fully opened are .slightly
over three inches in diameter. The plant is growing at Pikestown, Pennsylvania, and is fully
hardy. B-C—Plants of Asimina rcliculala in flower in eastern Florida. The large fragrant
flowers are more than three inches across. Photograph by Dr. Buswell. C—shows small
plants of rcticulata covered with blooms. This shrub is found growing over most of the state
The flowers of A. oborata (D) are six inches in diameter. This Florida species has the
largest flowers of any member of the genus.
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FLOWERS OF AS1MINA SPECIES
Figure 4
A—The large flower of incana (spccwsa). This species occurs in two varieties, one with
pink petals, and the other with white petals tinged with yellow, of a heavier structure. B—the
flowers of some plants of rcticulata are fragrant as well as showy. C—The largest-flowered
species of the genus, oboz'ata, has flowers as much as six inches across. Nearest relative of the
papaw, parviflora (£>), has a small foul-smelling flower. The papaw itself has an inconspicuous
inflorescence (£) shown here somewhat enlarged. (Photograph courtesy of the U. S. Department of Agriculture.)
--
HAND POLLINATION NECESSARY TO PRODUCE FRUIT
Figure 5
In the large orchard of papaw trees at Pikestown, Pa., very few fruits are produced unless
the flowers are pollinated by hand. This illustration shows details of three trees which have
produced abundant fruit as the result of hand pollination. A—is a Taylor tree hand pollinated
with Taylor pollen. In the ten preceding years not a half dozen fruits have been borne on this
tree unless it is artificially pollinated. The small tree (B) has abundant fruit on three branches
as a result of hand pollination. No fruit appears on the rest of the tree. A limb of the Shannondale papaw ( O heavily covered with fruit as a result of hand pollination. Here again no
fruit was borne on the rest of the tree.
(PHOTO Omitted from Clickable PHOTO LINK ABOVE )
