Considering Pawpaw Biology to Inform How to Grow Them
Considering Pawpaw Biology to Inform How to Grow Them
The pawpaw has an interesting biology. Establishing pawpaw seedlings need shade because their young shoots and leaves can scald or burn in full sun. Mature, fully grown plants like the sun. Given enough heat and the right conditions for idiosyncratic pollination, the pawpaw can grow the largest edible fruit in North America. Yet, most Americans are not aware of, let alone have ever eaten, a pawpaw fruit.
In a blog on grafting pawpaw for work at Raintree Nursery, I promised to revisit the fascinating pawpaw, devoting more writing to its biological quirks. In that previous blog post, I covered pawpaw diversity and relatedness and their cultural significance. Well, here we are again talking about one of North America’s most unusual fruits.
The pawpaw is worth growing because it has a big, delicious fruit unlike anything else that can be grown in the same temperate climate. The oblong fruit can be five inches long and has soft, custardy pale yellow flesh with flavors of banana, vanilla, melon, and mango.
Fig 1. Pawpaw in it's natural habitat.
Image credit: National Parks Service
Fig 2. A pawpaw fruit.
Fig 3. A ripe pawpaw fruit bisected.
As with understanding and cultivating any plant, the pawpaw grower should start by learning about the species’ native habitat and habits. Pawpaws are native to eastern North America, occurring from the Mississippi River Valley east and from southern Ontario south into the Gulf Coastal Plain. Their typical habitats are fertile, well-drained bottom-lands and mixed forest uplands, including forest edges and along streams. Either way, a fairly dense forest canopy is involved, which keeps strong sunlight from penetrating to the forest floor. Young pawpaws do best under a canopy as they are sensitive to ultraviolet light and don’t thrive in full sun for their first couple years. Reminding me somewhat of the slow replacement of sun-loving Douglas-fir with shade-tolerant western hemlock in the Pacific Northwest’s old growth forests, shrubby pawpaws can only become canopy members through forest succession . They might rarely succeed into the canopy or an opening within it (maybe helped by a small disturbance like wind throw of overhead trees or a single dominant tree falling). There, out of the shade, they have become old enough to stand the sun. More commonly, without these conditions, pawpaws stay in the understory. Pawpaws can grow to 25 feet tall. They can spread clonally (instead of by seed), forming bushy patches. The home orchardist can achieve analogous results to natural pawpaw succession by putting temporary shade structures (for example using greenhouse shade cloth) up over young pawpaws for their first two years. One could also plant pawpaws in the shade of trees the pawpaws will either eventually overtop or that will be removed later, such as for firewood. Because a pawpaw can grow as a shrub or multi-stemmed, short tree (they’re no redwood!), pruning could be used to determine a desired height within that range.
The native habitats of pawpaws imply that these plants like moist, but well-drained, loamy soil. They tolerate USDA Zones 5-8. This says something about their cold-tolerance (down to -15 degrees Fahrenheit), but not much about heat requirements. At Raintree (USDA Zone 8), there is often not quite enough hot days into the early fall to ripen the fruit.
Another factor pawpaws fruit production is pollination. They have unusual flowers.
Many flowers show off their sexiness with bright colors and sweet scents. Everyone knows flowers like that. Many in contrast hardly look to people like flowers at all because of their dullness in color and scent (like the flowers of many hardwood trees, like maples or oaks, or the complex, but usually muted, compound flowers of grasses). The pawpaw flowers does something rather different. Some plants have evolved flowers that may big and flashy, but the colors seem a little less than cheerful and the odor, if detectable, smells not sweet, but instead stinky. Mimicking a carcass, stinky, sultry flowers lure carrion or dung feeding insects to them. Perhaps the most famous example of this phenomenon is exhibited in the world’s largest flower, the Rafflesia of southeast Asia. Many flowers have converged upon attracting dung or carrion feeding insects for pollination by producing distasteful smells and rotten colors. The Rafflesia takes this strategy to an extreme degree. The separate male and female flowers, some measuring three feet wide, smell to high heaven, have an appearance like rotten flesh, and attract hordes of flesh-eating flies.
Pawpaw flowers fortunately barely smell at all, but they do strive to attract flies and beetles with their fleshy browns and reds. Maybe it would help if they stank more! Pollination (and thus fruit set) can be uncertain in pawpaws. Our pawpaws at Raintree seem to share their pollen just fine, but some growers can have trouble getting good pollination. I’ve even read suggestions to place animal carcasses among the pawpaw during flowering to attract flies and beetles. I guess some chopping and active composting with a good mulch later could cycle some of that nutrition into the orchard soil. If that seems crazy, then the grower should consider planting at least a few pawpaws close together so that the few local flies might get the job done! Whatever the case may be with pollinating insects, pawpaw flowers are protogynous, which means the stigma (female part) becomes receptive before the anther (male part), disallowing self-pollination or “selfing”. An individual pawpaw plant is self-incompatible to some extent as well. This means that different flowers on the same plant do not easily pollinate each other to make fruit. Many flowering plants have self-incompatibility as a defense against inbreeding. More than one individual pawpaw tree is needed for successful pollination.
Given the right conditions. growing a pawpaw patch could be an unusual and rewarding addition to the home orchard.
Happy growing! Because, after all, change is the only constant. -Xander Rose
Ames, Guy. 2018. Cornell Small Farms Program. https://smallfarms.cornell.edu/2018/01/pawpaw-a-tropical-fruit/> Accessed Nov. 12 2020.
Bordelon, Bruce. 2001. Purdue University Cooperative Extension Service. “Growing Pawpaws.”
Matthews, Elizabeth. 2017. National Park Service. “Pawpaw, Small Tree, Big Impact.”
Morton Arboretum. 2020. “Pawpaw.”