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Wednesday - February 06, 2013

From: Cleveland, TN
Region: Southeast
Topic: Pollinators, Propagation
Title: Pollinating Pawpaws
Answered by: Anne Van Nest

QUESTION:

We have many good sized pawpaw trees in our area but they never bear any fruit. I've checked them at different times in the fall over the years but no fruit. Someone told me that the flowers were pollinated by flies and it was a good idea to hang a road killed animal in their branches to attract the flies so they would also pollinate the flower to bear fruit. Is there any truth in this or it just an old wives tale? Thanks for your help!

ANSWER:

Asimina triloba (pawpaw) is an interesting small understory tree that forms colonies in the Eastern, Southern and Midwest regions of the United States and the southern tip of Ontario, Canada.  It has long floppy leaves, small drooping purple flowers and large yellowish-green fruit that could weigh up to 18 ounces (500 g). The conspicuous fruit mature in the fall and have a soft, edible pulp that taste like a banana, mango custard. American agronomist E. Lewis Sturtevant described pawpaw fruit as “a natural custard, too luscious for the relish of most people."

It is with no surprise to hear that you are anxious that your pawpaw trees have not had any fruit since this is one of the main features of this native tree. But the pawpaw has a very interesting pollination story and getting that luscious fruit is not without its challenges.

From our website, pawpaws are described as “Not particularly showy, but interesting, the purple, six-petaled flowers are borne singly in leaf axils in early spring before the leaves emerge. Pawpaws seldom set much fruit without cross pollination." In fact the rate of fruit to flowers is very, very low. Mary Willson and Douglas Schemske in their article, Pollinator limitation, fruit production, and floral display in pawpaw (Asimina triloba) write that “Pawpaw reproduces sexually, however, the rate of fruit set is very low (0.45 percent) compared to the number of flowers produced."

More information on pawpaw fruit production can be found on the California Rare Fruit Growers website which confirms that flies do pollinate pawpaw. "Poor pollination has always plagued the pawpaw in nature, and the problem has followed them into domestication. Pawpaw flowers are perfect, in that they have both male and female reproduction parts, but they are not self-pollinating. The flowers are also protogynaus, i.e., the female stigma matures and is no longer receptive when the male pollen is shed. In addition pawpaws are self-incompatible, requiring cross pollination from another unrelated pawpaw tree.

Bees show no interest in pawpaw flowers. The task of pollenization is left to unenthusiastic species of flies and beetles. A better solution for the home gardener is to hand pollinate, using a small, soft artist's brush to transfer pollen to the stigma. Pollen is ripe for gathering when the ball of anthers is brownish in color, loose and friable. Pollen grains should appear as small beige-colored particles on the brush hairs. The stigma is receptive when the tips of the pistils are green, glossy and sticky, and the anther ball is firm and greenish to light yellow in color."

Hand pollinating is possible with a paint brush and a small bowl. Lincoln Smith has posted an article on hand pollinating pawpaws on the Apios Institute website that shows the steps and includes several pictures of the female and male pawpaw reproductive flower parts. Clemson University has also included an article and pictures by Neal Peterson about hand pollinating paw paws on their fruit gardener website.

Regarding your question of hanging road kill in the tree to attract rotting meat-loving flies, The College of Arts and Science at Vanderbilt University confirm that commercial pawpaw growers do use this practice to increase fruit set.  But before you do hang it, please consider your neighbors living downwind!

 

From the Image Gallery


Pawpaw
Asimina triloba

Pawpaw
Asimina triloba

Pawpaw
Asimina triloba

Pawpaw
Asimina triloba

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