Native to North America from northern Florida to southern Ontario in Canada and westward to Nebraska. Although it's the largest tree fruit native to continental USA, it has mainly been grown for private or local consumption rather than larger scale commercial marketing.
Asimina is the only genus in the pan-tropical Family which favours a humid temperate climate. Each year it requires about 800mm of precipitation with most in spring and summer, and 400 chill hours to break bud dormancy going into spring. There are some named low-chill varieties. It is very cold-hardy when dormant, withstanding temperatures down to -25°C, and does not do well in coastal environments.
Shrub or small deciduous understory tree (5-10m high) with a lush drooping pyramidal canopy. The alternate dark green glabrous leaves are obovate-oblong, 15-25 X 8-12cm. Before abscising each autumn they present a spectacular bright yellow display. The species is notorious for producing many suckers, hence the old American folk song about picking pawpaws 'way down yonder in the pawpaw patch'.
A member of the large Annonaceae Family which is distributed worldwide and has 108 genera and 2400 species; Asimina contains 8 species. Tropical/sub-tropical relatives valued for their delicious fruit are the hybrid custard apple atemoya, cherimoya, sugar apple, soursop, sweetsop and biriba. Common names are quite often botanically misleading; eg other names for A. triloba in the US are custard apple and tree banana, and Australians call Carica papaya pawpaw.
These can vary from sandy to clayish, with a preference for well-drained, moist and slightly acidic fertile loams. Soils rich in humus are needed for good and regular fruit production. Asimina can tolerate some waterlogging.
Seeds are partly recalcitrant and do not tolerate desiccation. Stratification is necessary to break dormancy, either in-ground over winter or in the fridge at 1-4°C for 3 months and kept in a zip-lock bag with moist sterile peat moss. Provided no microbial infection or desiccation occurs, they can be kept in this manner for some years without loss of viability. Great care has to be taken with delicate roots when potting on or planting in-ground. Growth in the first 1-2 years is slow and plants should be shaded. Vegetative propagation is of course the ideal, and most grafting techniques are successful while root suckers and cuttings have proven difficult. Currently there are no named rootstocks, so seedlings are used.
There are dozens of named cultivars in the US and some breeding work has been done in New Zealand. Superior fruiting cvs should be obtained if possible, but you'll probably have to make do with seedlings.
Vegetative and flower buds occur on the previous year's wood at different branch nodes. The vegetative ones are narrower/more pointed than the flower buds that are fatter and covered with a brown pubescence. In spring, pendulous flowers appear before new leaves and are carried on 4cm-long peduncles. Flowers are perfect, 4cm in diameter with 2 whorls of 3 X 3-lobed petals, initially green then turning dark maroon and curving slightly backwards, and with a nectary at the base of the inner whorl. They are protogynous, ie stigmas are receptive before pollen is released. Poor fruit set is often due to poor pollination (<1%) but in this case can't be resolved by hand pollinating from the same tree as they're self-incompatible; pollen has to come from a genetically different tree. Almost all trees behave in this way, but a few instances of self-compatibility have been suggested. Flies and beetles are attracted to the unusual, yeasty, fermentation-like smell of the flowers and act as the main pollinators; bees are not interested. Hand pollination is usually required to achieve good fruit set, which can then be quite heavy. Pollination is enhanced by misting. The gynoecium has 3-7 carpels and can form clusters of up to 9 fruit, then appearing similar to a banana hand (hence one of the common names).
After being shaded for the first few years, trees should be planted in full sun and ensuring that their watering needs are met. Rigorous fertilisation practices have not been developed, but monthly application of NPK with trace elements during the spring growth period has achieved good results. Weed control is important in the establishment years; mulching can minimise this and also conserve soil moisture levels.
Good but they should be protected from strong winds.
Plants should not be pruned in the first year. As they grow, they commonly exhibit strong apical dominance with narrow crotch angle branches, and these should be trained to more horizontal positions for strength and increased fruiting. Mature trees should be kept skirted to a height of 70-80cm. Late winter and early spring is the best time for pruning, and periodic selective thinning will help maintain yield and vigour.
A large cylindrical-oblong berry, 3-12cm long X 3-8cm wide, weighing 100-1000g with thin green skin that turns yellow or brown as it matures, and creamy-white to yellowish-orange flesh. Ripe fruit from superior named cultivars have a smooth custard-like texture and an excellent aroma and taste described as a blend of banana, mango and pineapple. Seedling fruit can be poor in quality and size, often with a bitter-sweet aftertaste. There are 2 rows of dark brown seeds, 12-20, bean-shaped and 3cm long. Fruit pulp is low-acid with high carbohydrate content (up to 20%) making it very sweet, and there are also good levels of antioxidants, Fe and Mg. Like apples, cut pawpaw flesh can darken (oxidise) over time. There have been reports of allergies in some people.
Seedlings may take 6-8 years to begin cropping; grafted plants may start in 3 years with regular cropping by 5-6. Fruit is picked mainly in autumn and spread out over 2-3 weeks, good for home growers but more costly for commercial orchardists. Seven-year-old named cultivars with good hand pollination can produce 10-20kg/tree, seedlings much less. Asimina is a climacteric fruit and will not ripen if picked before maturity. At ambient temperature, soft ripe have a shelf life of only 2-3 days, which is a significant handicap for commercial development. However, if mature and only just starting to soften they can be stored in a fridge (4°C) for about a month, and afterwards they will ripen normally at room temperature.
It is eaten in many ways: fresh out-of-hand or using the pulp in desserts such as ice cream, custard, sorbet, mousse, mustard and pawpaw sweet potato patties. Pawpaw can be substituted for banana in most recipes. The skin and seeds are not eaten, the latter being emetic and containing toxins that impair digestion.
Twigs, bark and leaves of asimina contain natural insecticides and toxic alkaloids, so the plants are reasonably well protected from most pests and diseases. However fallen fruit may be eaten by animals.
If you can source these here they will likely be seedlings, but named cvs would obviously be best and you'll probably need to have multiple plants for cross-pollination. Either way you should be prepared to hand-pollinate for good yields. Keep in mind the chill hour requirement; in the Perth coastal areas this will be a stretch but will be better inland or further south. Sucker removal will be a constantly recurring issue, and if not managed you could finish up with a clonal patch taking over the whole garden. High perishability can limit storage times.
More info: Asimina reproductive processes