Pawpaw is a rare Carolinian species of tree native to the United States and eastern Canada. In Ontario, it is mainly found in the south near Lake Erie and the Niagara region. It is a small deciduous understory tree, rarely growing to 15 metres in height. It produces large yellow-green edible fruits that taste similar to bananas and mangoes, though the bark, leaves, and seeds contain the neurotoxin annonacin. It has large leaves with drip tips, an adaptation more characteristic of tropical rainforest plants. Asimina triloba has the most northern range of all species in the Asimina genera, with close tropical relatives being the Custard-apple, Ylang-ylang, and Soursop.
Pawpaw flowers are reddish-purple or maroon and appear in the spring. They are sometimes described as having a yeasty or rotten smell. They are unable to self-pollinate to fertilize flowers on the same tree which has led to reduced fertilization success. Pawpaw also forms clonal patches, meaning multiple trees growing from the same plant that are all genetically identical. For this reason, it is possible to have fruitless patches of Pawpaw.
Pawpaw produces large fruit and seeds and is recognized to have evolved alongside large mammals, relying on megafauna for long-distance seed dispersal. Mammoths and Giant Ground Sloths would have eaten the fruit and transported the seeds in their dung until the end of the glacial episodes when most megafauna faced extinction. Near the end of the Pleistocene era, humans moved into the role of seed dispersers alongside bears and small mammals like foxes, squirrels and raccoons.
Historical evidence suggests that the Haudenosaunee first introduced Pawpaw trees to Southern Ontario by transporting seeds from fruit harvested farther south. The Pawpaw was a trade item among multiple Indigenous groups and the trees are found along major Indigenous trade routes which bolsters this theory. Humans continue to play an important role in Pawpaw seed dispersal today with intentional wild and horticultural seed plantings.
As climates continue to warm we can expect Carolinian species to extend their range north, becoming more common in our watershed. With several Pawpaw trees already planted in the Arboretum at Glen Elbe, we are doing our small part to help them along their way!