Paw paw tree pollination

Paw Paw tree Pollination

Pawpaws seldom set much fruit without cross pollination. In fact the rate of fruit to flowers is very low (0.45 percent) compared to the number of flowers produced.

Poor pollination has always plagued the pawpaw. The 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 variety of pawpaw.

Bees show no interest in pawpaw flowers. The task of pollenization is left to 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.

Papaya

Industry overview

Papaya Australia represents the biosecurity interests of papaya producers and the industry.

Papayas (Carica papaya) are predominately grown in Northern Queensland on the wet tropics of far north Queensland (Innisfail) and the Mareeba district on the Atherton Tablelands west of Cairns. Other growing areas in Queensland include Proserpine and Yarwun in Central Queensland, Gympie and the Sunshine Coast district in SE Queensland. Other commercial production areas include Carnarvon, Kununurra in north Western Australia, the Darwin rural area in the Northern Territory and northern NSW.

Papaya fruit is produced as either red fleshed fruit from hermaphrodite trees, which the industry label as papaya or larger yellow fleshed fruit from dioecious trees which the industry label as pawpaw. Papaya trees have multiple sources of pollination (eg bees, hawkmoths etc) and some cultivars are self-pollinating. Pawpaws make up approximately 60 per cent of the total production with the remainder of production based on red fleshed varieties. The crop is harvested and available all year round and can be purchased nationally from all major supermarkets and smaller independent fruit markets.

Papaya in flower. John M Randall, The Nature Conservancy

Pollination information

The papaya, originating from the tropics of the Americas, is now widely cultivated around the world, particularly in the tropical climates of Asia, Africa and Polynesia. The size and shape of the papaya depends on the variety but most are round, pear-shaped or oval. There are two distinct papaya plant types. Most cultivars are dioecious having both male and female flowers on separate plants and require both plants to produce fruit. However, some are gynodioecious, meaning they have flowers that are either female or bisexual with both male and female parts on the same flower, allowing self-pollination.

For papaya fruit to develop, pollen must be transferred from the staminate (male) flowers to the pistillate (female) flowers. The fruit may produce 1,000 or more seeds and so well over 1,000 pollen grains must be deposited on the stigma while it is receptive. Fruit with less than 300 seeds is usually not marketable, and the more seeds the larger the fruit.

Hawk moths (Lepidoptera: Sphingidae: Macroglossinae) have been reported as one of the main pollinators of papaya in Queensland where around 94 per cent of Australia’s papaya production occurs.

Several other pollination agents have been suggested in the literature for the more common dioecious varieties including wind, gravity and insects; however, research into honey bees and the importance of their role as pollinators has been somewhat inconclusive. Collectively the research suggests that insects do play a significant role, and that honey bees are capable of improving the pollination and fruit set of most papaya cultivars given their history as an easily managed pollination agent. There is a significant body of earlier research which describes the pollination of papaya by insects, however, results vary as to which insects (if any) are the most important. Some have considered wind to be the primary agent for pollination while others argue a combination of wind and insect pollination is needed for optimal pollination.

Conflicting evidence persists with reference to the pollinating capabilities of honey bees in papaya orchards with more recent research by finding that attractive nectar produced by male flowers around the rudimentary pistal is out of reach of the bees because of the long tube. However, more recent research highlights the importance of insects in general in the pollination of papaya.

Papaya tree bearing fruit. Bigstock

Value of production and distribution

The value of Australian papaya production was estimated at $20 million in 2005–06 season, with $18.4M from Nth Queensland, $1.2M from Western Australia and the Northern Territory and $0.4M from central Queensland and SE Queensland. Industry R,D & E issues are dealt with by the national association Papaya Australia in association with HAL. Papaya growers contribute a levy of 2c/kg for fresh fruit (24c per 12kg carton) for R&D and marketing.

Industry production plummeted (1,036,109 cartons in 2005–06 to 561,377 cartons in 2006–07) following Cyclone Larry in March 2006 which decimated the industry which was then predominately located on the Wet Tropical coast around Innisfail. Atherton Tableland producers quickly responded with increased production and now produce in excess of 50% of the production in far north Queensland. Current production is estimated at around 1.3 million cartons worth approximately $25M per annum. Whilst there are around 80-100 papaya growers, the bulk of production is dominated by a few larger growers.

From a biosecurity perspective, only 17 businesses are registered to move papayas interstate. There are also a number of growers that sell produce in roadside stalls, and many papaya trees are located in backyards across northern Australia.

The above text about pollination is an excerpt from Papaya – Pollination aware fact sheet (2010). Agrifutures Australia Publication No. 10/081

The industry overview and information on the value of production and distribution is from the Industry Biosecurity Plan for the Papaya Industry, Plant Health Australia, Canberra

What Sex Are Pawpaw Flowers: How To Tell Sex In Pawpaw Trees

The pawpaw tree (Asimina triloba) is native from the Gulf Coast up to the Great Lakes region. Not commercially grown, or rarely, the pawpaw fruit has yellow/green skin and soft, creamy, almost custard-like orange flesh with a delicious sweet flavor. One reason this delicacy is not commercially grown has to do with pawpaw flower sex. It is difficult to know what sex pawpaw flowers are. Are pawpaws monoecious or dioecious? Is there a way to tell the sex in pawpaw trees?

How to Tell Sex in Pawpaw Trees

Tasting like a cross between a banana and a mango, pawpaw trees can be fickle with regards to what sex the pawpaw flowers are. Are pawpaws monoecious or dioecious?

Well, they are definitely not entirely dioecious or monoecious for that matter. Pawpaw flower sex is something rarer. They are termed trioecious (subdioecious), which means they have separate male, female as well as hermaphroditic plants. Although they have both male and female reproduction parts, they are not self-pollinating.

The pawpaw’s blossoms are protogynaus, which means that the female stigma matures but is not receptive at time that the pollen is ready for fertilization.

Pawpaws are most often propagated via seed, and their sex cannot be determined until they flower. This can be a problem when raising the fruit for commercial sale. It means that few trees will actually produce and yet the grower is cultivating and investing time and money to wait and see which trees will fruit.

Furthermore, under stressful conditions, the dioecious plants may convert to hermaphrodites or the opposite sex, and monoecious plants may change the ratio of their male to female flowers. All this makes determining the sex of pawpaws anyone’s guess.

Of course, there are other reasons the pawpaw isn’t cultivated commercially despite its rich nutritional value – high in protein, antioxidants, vitamins A and C, and several minerals. The fruit has an odd bean-like shape that doesn’t fare well with the sweet custard inside and it also doesn’t handle well.

This means the delicious fruit will probably remain the province of eastern U.S. dwellers and those determined to grow pawpaw. And for those intrepid growers, pawpaws are also self-incompatible. This means that they require pollination from yet another unrelated pawpaw tree.

Pawpaw

Additional information:
There has been great demand for pawpaw recently, not only because of its appealing ornamental characteristics and tastyfruit, but also because of its potential as a source of organic insecticide and for use in cancer therapy. An insecticide can be made from the tree’s ground-up bark and twigs. Extract from pawpaw can overcome the ability of some cancer cells to reject chemotherapy.

On dormant stems, pointed buds are vegetative while flower buds are round and fuzzy. Cross-pollination is needed to get good fruit set in pawpaw. Pawpaw fruit develops in clusters. Fruit with orange flesh is considered the tastiest. The fruit attracts wildlife (especially raccoons, opossums, gray squirrels and birds). It also attracts the beautiful zebra swallowtail butterfly, giving added incentive for wildlife lovers to obtain the tree. It is an excellent source of vitamins A and C, and is high in unsaturated fats, proteins and carbohydrates. Pawpaws contain more potassium, phosphorus, magnesium and sulfur than apples, grapes or peaches.

The first reference to pawpaw came in writings of Hernando DeSoto’s expedition to the Mississippi Valley in 1541. The fruit, a favorite food of American Indians, was used to feed DeSoto’s conquistadors. Early North American settlers used the fruit to make jelly, and the tree’s inner bark to string fish. Indians in Louisiana used the inner bark to weave a fiber cloth.

Although pawpaw is in great demand, it does have a disadvantage. Its fruit and foliage produce a great deal of litter. Planting only one tree can help alleviate the problem, as lack of cross-pollination leads to production of less fruit. Because they are broader at the tip than at the base the leaves droop, giving pawpaw a “sleepy” or tropical appearance.

The genus Asimina includes the only temperate members of this family. The tree, which is hard to transplant because of its wide root system, should be balled-and-burlapped and moved when it is less than 6 feet tall.


Here is a most unusual flower that hangs like a bell, sometimes in rows.

It has three deep maroon petals, three green sepals, and if you look inside three curled maroon petals surround the pistil. The flowers are small when they first bloom, but grow to two inches long.

This is the flower of the Common Pawpaw tree (Asimina triloba), blooming right now in western Pennsylvania.

Though named “common,” pawpaw is an uncommon understory tree that grows in hardwood forests and bottomlands. I found pawpaw trees blooming last weekend in Schenley Park and at Enlow Fork.

Some people prize pawpaw for its 4-inch long lumpy fruit that has the consistency of mangoes and the taste of bananas. I ought to like it, but I’m not fond those two tropical fruits. If you try it, don’t eat the seeds (see the link above).

I was able to identify the flower because Marcy Cunkelman sent me this photo a year ago. When I saw the pawpaw blooming I remembered her picture but not the name of the tree, so I looked it up when I got home. Thanks, Marcy!

(photo by Marcy Cunkelman)

  • FAMILY: ANNONACEAE
  • ALTERNATE COMMON NAME: Kentucky banana, custard-apple
  • LEAVES: deciduous, alternate, simple, short-petiolate, upper surface of petiole grooved, blades thin and papery, obovate or oblanceolate, 6-12” long; upper surfaces of young blades sparsely reddish appressed-pubescent, lower surfaces densely so; leaves with a green (bell) pepper odor when crushed
  • FLOWER: perfect (bisexual), maroon, ca. 1.5” in diameter, born in axils of leaf scars, nodding; sepals in a single whorl of 3(-4) distinct members, deciduous; petals in 2 unequal whorls each of 3(-4) members, petals of the inner whorl smaller than those of the outer; flowering in early spring just as new leaves are emerging
  • FRUIT: greenish-yellow berry 3-6” long, oblong-cylindric (somewhat sweet potato-shaped); seeds shiny, bean-shaped; berry edible, matures in August and September in our area
  • TWIGS: new shoots generously dark brown-hairy, aging smooth and gray-brown; terminal buds dark brown, hairy, naked, to ca. 0.5” long
  • BARK: smooth with small warty processes, brown with gray splotches
  • FORM: shrub or small tree, to ca. 40’ tall, colonial growth from root suckers
  • HABITAT: shady mesic forests with fertile soil, on old and young landscapes
  • WETLAND DESIGNATION: In the “Western Gulf Coast Subregion” of the Atlantic and Gulf Coast Plain Region, this species is Facultative Upland (FACU): Usually occurs in non-wetlands, but may occur in wetlands. In the remainder of the Atlantic and Gulf Coast Plain Region, this species is Facultative (FAC): Occurs in wetlands or non-wetlands
  • RANGE: eastern US
  • USES: edible fruit; ornamental (edible landscaping)
  • WILDLIFE: fruit eaten by opposum, raccoon, fox, and squirrel; Asimina species are the exclusive hosts for zebra swallowtail butterfly (Eurytides marcellus) larvae
  • Best Recognition Features:

    1. colonial shrub-small tree of rich-soil forest understories
    2. large brown hairy naked terminal buds
    3. dorsal surface of petiole grooved
    4. large papery obovate leaves with bell pepper smell when crushed
    5. maroon flowers appearing in early spring
    6. fruit a sweet potato-shaped yellow-green berry

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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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