It is true that some cultivars perform better than others when multiple trees are present. This is an important fact for orchards. It is also true that every tree tested in isolation has produced at least a partial crop. In addition it has been observed that in maturity two adjacent backyard trees will produce too much fruit.
And as an epilog, there are sellers that like to point at the “partial production” as a reason to buy at least two plants from them.
There is an interesting discussion of Pawpaw flower fertility beginning at the bottom of page 1897 in this article: https://doi.org/10.3732/ajb.1700319
Robert Brannan is known for his studies of crop utility - including Pawpaws. Here a recent paper of his discussing Pawpaw research efforts and established orchards outside the U.S.
Here is a figure illustrating the shortest (2-6) and farthest (20-22) genetic distances between cultivars examined by H. Huang in 2003. Distances are number of marker mismatches, so the larger the number the less related.
Most of what you have read about needed chill hours in pawpaws is simply wrong. There are a lot of myths that have grown up about pawpaws that are simply not fact. There has long been a myth that you can’t grow pawpaws in zone 9 at all no matter how many chill hours you get. I grew pawpaws for almost 30 years in zone 9 in the California Central Valley. In some years we were effectively zone 10. All of my pawpaws leafed out and fruited every single year. During that same time others grew multiple cultivars in SoCal in zone 10 and no one ever had any problems with chilling issues.
Just adding to the conversation….
There may be other variables at play.
The legendary Bill Whitman was unsuccessful growing them in south Florida, zone 10.
I’m not sure about the age of the oldest trees that were received or where any of the plants,especially the young ones,were placed,in relation to the Sun.Strong,direct sunlight has been known to damage new seedlings.I can imagine southern Florida has some of that.
I’m down the road a bit east in San Marcos. How many years have you had your pawpaws? Mine are at least 7 years old and I would hesitate to call them saplings. I have yet to have any fruit. If you need genetic material I have a KSU Atwood, KSU Chappell, a Mango, a Peterson Shenandoah and Peterson Tallahatchie along with the root stock from an earlier Shenandoah that died above the graft. I like those deep pots, where did you purchase them?
These sticks are at least 6 years old and none of them is over 3 feet high, probably not even over 2 feet! The only one that has ever flowered is the Allegheny cultivar. It flowered the spring that I planted it, with no results, and this year finally again. They seem to do better in containers than in the ground here.
With sparse amendments, weekly irrigation and no modifications for percolation the San Marcos soils are good for natives like mallow, yarrow, chaparral, evergreen oak - and not much else in my experience.
So far in the leaf-out “race”, the Overleese, Sunflower, Susquehanna, Taylor, Wabash, and NC-1 (not shown) are ahead and the remainder are pushing out 1/4" to 1/2" dark red leaf points. Overleese is the only cultivar with flowers at this time.