Asimina triloba
Asimina, asimoya, pawpaw
Origin
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.
Climate
A Triloba 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.
Plant Description
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 pawpaw ‘way down yonder in the pawpaw patch’.
Relatives
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.
Soils
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.
Propagation
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.
Cultivars
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.
Flowering and Pollination
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. 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 really 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 several fruit, then appearing similar to a banana hand (hence one of the common names – poorman’s banana).
Cultivation
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.
Wind Tolerance
Good but they should be protected from strong winds.
Pruning
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.
The Fruit
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.
Fruit Production and Harvesting
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 but will not ripen if picked before maturity. At ambient temperature, soft ripe fruit 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.
Fruit Uses
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 papaya 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.
Pests and Diseases
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.
Comments
If you can source these here they will likely be seedlings, but named cvs would obviously be best. 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 Information
Asimina reproductive processes
In earlier times in the US there was a kids rhyme that went ‘… way down yonder in the pawpaw patch’ and maybe it’s still sung in some rural areas where there are native stands of Asimina triloba that haven’t succumbed to urban sprawl or agricultural clearing. It’s called pawpaw there and not to be confused with what Australians usually call pawpaw, namely Carica papaya. The species is the largest edible tree fruit native to North America and although it has commercial potential it still has not really taken off there or elsewhere, even though the fruit is pleasant tasting; Brix is 15-20% when ripe. The USDA maintains a germplasm collection at Kentucky State University for breeding work and other studies with the goal that it might be developed into a more important crop in the future. The ‘pawpaw patch’ ditty stems from the fact that plants sucker freely in suitable climates and conditions so that whole clumps of trees that were commonly seen in the past were in fact clones of a single or few trees. Compared to more commercially-successful fruit tree species there has been far less research on its properties and behaviour, and as a result there are various confusing statements regarding reproduction processes – flowering properties, self-compatibility, the need for cross-pollination, whether fruit yield is pollinator and/or resource limited, seed viability, possible yields etc.
If we consider first the question of genetic self- and cross-compatibility. Flowers in Asimina are hermaphrodite and exhibit protogynous dichogamy, a common feature of pollination biology in the Annonaceae family. Inflorescences occur as single axillary flowers with each gynoecium having 3-9 ovaries and each of these containing 5-20 ovules; fertilisation of these ovaries can lead to multi-seeded fruit being formed in clusters. Most dichogamous fruiting species have a two day cycle, with in the case of protogynous behaviour the female organs being effective on one day and male the following with an interim period where neither sexual phase is active. However for this species the process is greatly slowed down, with stigmas being receptive for 3-15 days and anther release of viable pollen occurring for 2-3 days afterwards. Such slower dichogamous cycling is more common in temperate species like Asimina than in tropical and sub-tropical species. Temporal separation favours cross-pollination and can minimise inbreeding depression and the possible build-up of deleterious mutations and alleles in progeny. However depending on environment, there is sometimes a brief overlap period of 1-2 days where both organs are effective, and self- pollination then becomes possible.
Flower production in the species is prolific, but natural pollination (usually by beetles with smaller contributions from flies and thrips) in single trees results in low fruit set and yield (2-4%) and there is little improvement when done by hand. Fruit abortion following fruit set is not usually a problem, but seed counts in these fruit are low (3-5), with many aborting or being sterile. This low productivity is not the result of inadequate pollen supply as hand pollination can deliver 30-70 pollen grains per stigma. Pollen is released from anthers as tetrads and germination requires a supportive stigma environment and secretions, with normally only one of these four grains forming a pollen tube that grows towards an ovary. Many of these abort along the way and do not result in fertilisation. Together these findings indicate there is a degree of genetic self-incompatibility in the species, probably polygenic, and unlike most species in the Annonaceae family. There are a few named cultivars where self-fertilisation appears to be less of a problem, but yield, fruit quality and seed count is still less than what can be achieved with cross pollination. Naturally-mediated cross-pollination can produce 5-10% fruit yield, and this can then be increased to 10-40% with hand pollination, indicating yield in this situation is normally pollination limited. A separate factor limiting yield is maternal resource availability through tree health and vigour, as larger stems support more fruit through to maturity.
Currently there are 11 recognised species in the genus Asimina, with several natural hybrids between them. Molecular genetic work has established the genus can be divided into three taxonomic clades, with A parvifolia (the dwarf pawpaw) and A triloba both in the same clade. This close relationship is supported by many common morphological features between the two, and a natural hybrid in North America has been characterised and officially recognised (A triloba X A piedmontana). Parallel research on parvifolia reproduction (out-crossing, pollination limitation, selfincompatibility etc) has shown similar behaviour to triloba.
Asimina is therefore a facultative xenogamous species strongly favouring cross-pollination, with self-pollination essentially being a back stop that can produce a small number of fruit but with only a few seeds that are mainly infertile. If you are intending to grow the species in the south west of WA you’ll have more success if you plant (or graft) more than one plant from different genetic stock, and you should also be prepared to hand-pollinate to optimise quality and quantity of fruit. Named varieties may give better results but will still do best with crosspollination, and they’re difficult to source in WA.