By: Anya Osatuke
Pawpaw (Asimina triloba L.) is an understory tree naturally occurring in woodlands in eastern North America. Pawpaws are the northernmost member of the plant family Annonaceae, a mostly-tropical family of roughly 2,400 species whose other constituents include the delicious tropical fruits cherimoya (Annona cherimola) and guyabana (Annona muricata), as well as the aromatic ylang-ylang tree (Cananga odorata).
The native range of pawpaw extends from Florida to Ontario, and from Missouri to Pennsylvania. Pawpaws are rare in their ability to tolerate cold winters, although at the northernmost edges of their range, most pawpaws primarily spread clonally through suckers off their root system, as the season is too short for their fruit to mature. The suckering habit of pawpaws contributes to their longevity in the wild. The wood of pawpaws becomes brittle quickly, and each individual trunk will last for 20 to 30-odd years before declining; the roots, however, can live on at least a hundred years.
The Brooklyn Pawpaw
Picture 1: A 99-year-old pawpaw tree at Brooklyn Botanic Garden. This tree was planted in 1923, and the photo taken in 2022. None of the trunks have lived out the full hundred years, but the roots are actively pushing out new growth.
In Ohio, where I am from, I visited large stands of pawpaw growing underneath tall maples in old growth forests. Amongst the dense clusters of pawpaws, young beeches and spicebush would grow. At the base of the trees, dry leaves would blanket the ground. In younger woods, where the ground is thickly settled with low-growing, leafy vegetation and grasses, pawpaws grow singly or in small groups, interspersed amongst numerous other tree species, including black walnuts and hickories that secrete phytotoxic substances, juglones. Pawpaws are resistant to these substances. Naturally occurring pawpaws tend to be reedy and thin, accustomed to low light conditions throughout their life. In the woods, I’ve never met a pawpaw trunk that I couldn’t grip all the way around with my two hands. Woodland pawpaws bloom profusely in the spring, with beautiful maroon flowers that droop downwards on bare branches. The flowers are pollinated by insects that enjoy carrion: beetles, gnats, and flies. The fruit tend to be small, seedy, and richer in scent than in sweetness.
Picture 2: A grove of pawpaws in bloom on May 5, 2023 in Hueston Woods State Park, Ohio.
In the many creeks that feed in and out of woodlands, pawpaws can be found near the creek-bed in raised embankments, and their seeds underwater amongst the river stones. The fruit is buoyant, though the seeds are not. Pawpaw seeds are heavy and smooth, about the size of a large lima bean. The seeds are oily when crushed, and it’s tempting to grind them up or extract the oil from them. This shouldn’t be done, though, because the seeds are rich in toxins called annonacins. Interestingly, the skins of fruit, leaves, and unripe fruit are also rich in annonacins and can cause nausea or vomiting if ingested. Some varieties of pawpaw have higher annonacins than others even when ripe, and sensitive individuals may find that they cannot tolerate most pawpaw fruit. Cooking the fruit in a way that dehydrates it—for instance, baking a custard or making fruit leather—concentrates the annonacins unequivocally and is a recipe for sick feelings. But back to the seeds. The oils in pawpaw seeds can go rancid quickly if the seeds dry out, killing them. Kept somewhere cool and wet, pawpaw seeds can stay dormant for years. Even in a great growing environment, pawpaw seeds often take longer than we’d expect to sprout: it’s normal to see pawpaw seedlings pushing the seed capsule off of their leaves in August or September.
Picture 3: Illustration of pawpaw seedling sprouting in its typical, root-first fashion. By Anya Osatuke.
Sprouting pawpaws from seed is a labor of love. Due to their long dormant period and their intolerance of drying out, pawpaw seeds are very easy to forget in the garden. After pawpaw seedlings emerge aboveground, they’ve already formed a taproot nearly a foot long. This makes transplanting pawpaws a difficult task: if the taproot breaks, they die. When replanting a young tree from a garden center, the taproot should be handled gently as the tree is placed in its permanent spot.
I occasionally see pawpaws for sale in garden centers here in upstate New York. Usually, only one tree is left in stock. Sometimes it is waist-high, sometimes it is a little whip that doesn’t reach from my wrist to my elbow. If the label simply says “Pawpaw,” I’ll assume that it’s a sprouted seed, whereas if a variety is indicated (‘Potomac’, ‘NC-1’, ‘Mango’, etc.) then I’ll know that it is a grafted tree. Grafted pawpaws are typically more expensive than ungrafted ones. We pay for the predictable fruit characteristics, the size, sweetness, and flavor. In my opinion, a grafted tree is a worthwhile investment for those who intend to eat the fruit. A larger tree may pay off as well, as it is more likely to survive the first winter.
Grafted pawpaws are more delicate during establishment than ungrafted trees. Something about the graft union makes the trees especially vulnerable to sunburn, and trees planted into direct sun often start losing leaves. This stress can be enough to kill or severely stunt the sapling, so we recommend to shade the trees for several years. One option is to place a tomato cage with a bedsheet around each side, and another is to plant the seedling underneath an older tree that we plan to remove in 5–10 years. Ungrafted pawpaws generally do fine without this support. Once the trees begin to produce fruit, being in full sun helps them produce bigger, sweeter fruit. I also would recommend a tomato cage setup if the trees are planted in an area frequented by grazing animals, such as deer or goats. While pawpaws have a reputation for having toxic, unpalatable, leaves, each critter must form this opinion for themselves. When the pawpaw is small, a single taste can knock each sapling back a season or more.
When establishing, pawpaw trees are slow to exit dormancy after winter. They may look dead even in June, and in many trees, the leaf buds are black in color. Living trees will have supple, pliable tissues and velvet-soft leaf buds. Ones that are truly dead—killed by flooding, drought, or a late frost, perhaps—will have brittle tissues that snap easily when handled. Even so, the roots may still be alive. Cutting away dead tissue and waiting a season or two may yield a tree after all.
Pawpaws do not need much fertilizer to survive and establish, but commercial growers find that fertilizing them similarly to apples can help speed growth and maturation. In his book, Kentucky-based pawpaw grower and nurseryman Blake Colthron recommends a high-input fertility program from March through June in zone 6. In Upstate New York, where I currently live, we recommend a similar timeline of fertilizer inputs, especially nitrogen, for growers of perennial woody crops. Stopping nitrogen applications after the Fourth of July will encourage any new tissues that grew over the season to harden off.
Getting a pawpaw to fruit is not very easy. In a study of woodland pawpaws in the Midwest, Lagrange and Tramer (1985) found that very few flowers ever become fruit: from 0% to 3.5%. A combination of low insect activity and light and nutrient limitation were suspected to contribute to these low yields. In the landscapes we manage, introducing light and nutrients can help support fruit set, but it’s hard to imagine beating a forest in terms of insect activity. After concluding a pawpaw pollination study at Cornell University, Dr. Ian Merwin concluded that a planting needed to have at least 50 trees to attract sufficient pollinators with the scent of its flowers. Some growers I know confess to hanging roadkill in their small plantings to encourage detritivorous pollinators—and it works—but in South America, the relatives of pawpaw are hand-pollinated with soft paint brushes.
With young trees, even getting to flowers is a challenge. I’ve found it takes roughly eight to ten years. Likely, there is an interaction between the rootstock and the grated wood in grafted varieties, and the growing environment surely will influence how quickly the tree will grow and whether the fruit buds will survive the winter. Even once flowers emerge, it’s normal for the blossoms and immature fruit to drop off before ripening. I don’t know any way to avoid it. At least, it isn’t terribly hard to tell apart leaf buds from flower buds. Leaf buds are long and thin, and flower buds are fuzzy and plump, shaped like an onion bulb. Flower buds form on one-year wood, which makes the fruit hang close to the ends of each actively-growing branch. The transformation from flower to fruit in pawpaw fascinates me. Each flower contains multiple carpels, which separate out into a cluster of fruit akin to a bunch of bananas.
Picture 4. Illustration of pawpaw fruit development after pollination of the flower.
Pawpaws are not self-fertile. Although each flower produces pollen and has a stigma to receive it, the stigmas of pawpaws mature and die off before the pollen is released. The solution to this problem is to plant multiple varieties of pawpaw, as slight genetic differences typically result in differences in the timing of flower maturation. Theoretically, encouraging the growth of suckers from the rootstock of a grafted variety is one way to introduce genetic diversity into a planting. However, while the trees are young, this may divert resources from the single “desirable” trunk, and result in more fruit overall, but a preponderance of less delicious fruit. Another option is to invest in several grafted trees, and surround them with ungrafted seedlings. Or to graft your own trees: the whip and tongue method has been found to work the best. Dr. Charles West partnered with Northeast Organic Farming Association of New Jersey to make a YouTube tutorial on grafting.
As you may recall, most pawpaw trunks live for 20–30 years before declining in their old age. In an orchard setting, this is true as well. To avoid peaks and valleys in production, I’d suggest choosing a root sucker around year 10 or 15 to replace the trunk. Sometimes this is not practical due to the corresponding accommodations in mowing, pruning, and motorized transport throughout the planting. In that case, if continuous production is the goal, I’d suggest starting a new planting in another fitting location, also around year 10–15.
Pawpaws have a reputation for getting few diseases, but they do get a few diseases. The first ones to be aware of are fungal leaf spots and blights, which present as brown or black discoloration on the leaves, and then travel to the fruits, where they make the fruits crack open before they are ripe. To reduce the spread of this disease, it is helpful to promote good airflow in the planting, and to keep moisture off of the leaves when watering. If a fungus manages to get into a planting, physically removing leaves and rotting fruit in the autumn can remove the disease before the following year.
Our understanding of pawpaw diseases recently grew with the identification of 2 viruses that attack these trees. Tomato ringspot virus and tobacco ringspot virus are both spread through soil, where a species of root-feeding nematode dwells. These nematodes also eat the roots of many weeds and commercial crops, such as apples. This virus is widespread and has been detected in plantings in New York as well as Maryland. Diseased trees cannot be cured, but the virus does not pass along into the seeds. The virus causes temporary lapses in tree productivity, as their leaves grow malformed and discolored for part of a season. As trees age, the virus appears more and more frequently, and in old plantings, it can speed the decline of the trees.
To tie all of this together: pawpaws are a versatile tree that can contribute to a woodland in a landscape, minimally managed, attracting native pollinators and feeding wildlife such as possums, raccoons, and wild turkeys. Or, pawpaws can be grown in an orchard setting, protected from frost, from the sun when young, and fertilized often to encourage thick trunk growth. In both of these scenarios, keeping young trees watered for the first few years is critical for establishment. Protecting from grazing animals is important as well, until the trees can spare a few branches without dying. If fruit production is a goal, plant as many trees as possible and space them widely to discourage rots. Plan on a trunk to live for roughly 25 years.
Anya Osatuke, with a profound background in berry and small fruit production, collaborates closely with the New York State Berry Growers Association and various fruit specialists to craft and execute programs while adeptly addressing challenges encountered on commercial farms. Her academic journey saw her acquire an M.S. in Horticulture from Cornell University under the guidance of Dr. Marvin Pritts. Her research primarily delved into field management and site characteristics, exploring their impact on strawberry quality and fruit yield in New York State berry farming. Complementing her horticultural expertise, Anya holds a B.A. in Botany and Russian Studies from Miami University.
Resources:
Pawpaw Workshop with Dr. Charles West. NOFA-NJ. May 2020. Link: https://www.youtube.com/watch?v=UEHIG12EmA0&ab_channel=NOFA-NJ
Pawpaws: The Complete Growing and Marketing Guide. Blake Colthron, 2021. New Society Publishers.
Lagrange, R. L., and Tramer, E. J. 1985. Geographic variation in size and reproductive success in the paw paw (Asimina triloba). The Ohio Journal of Science 85(1): 40-45. http://hdl.handle.net/1811/23052