Pawpaw is native to Central American lowlands. It is nowadays grown in most countries within 25 degrees of the equator, with annual world production exceeding 7MT.
It is a tropical plant that nevertheless can do very well in sub-tropical climates. Ideally, temperatures should be 21-33°C and humidity above 65%. If temperature is below 12-14°C for several hours overnight, growth and productivity is compromised. Dioecious cvs are more reliable and better performers in sub-tropical climates than the hermaphrodite types. Yield is best with a minimum of 100mm per month rainfall or irrigation.
It is a large, rapidly-growing, tropical, herbaceous, hollow-stemmed tree, usually with a single trunk unless damaged. The large leaves are palmately lobed on long petioles. Older leaves senesce continually with most of the canopy at the apex.
Pawpaw is the most important species in the Caricaceae Family. There are a number of other species of Carica. Babaco is a small fruit-producing tree that is a hybrid of two other species in the Family.
A variety of soil types can be used provided they are well-drained. Substantial amounts of organic matter should be added to sandy soils. Preferred soil pH is 5.5-6.5.
Almost universally by seed, as they germinate readily, and if properly dried, can be stored for lengthy periods. Seedlings may be ready for in-ground planting within several weeks of germination. Cuttings and grafting can be used but are often not worth the extra effort. Rigorous sanitation and bottom heat are necessary with cuttings. Grafting can be used to transform surplus male plants into known females.
The best strategy for home growers is to use seed from favourable fruits produced locally, as it is then known that they perform under prevailing conditions. If the tree form can be seen in fruiting, select those where the fruit has a longish peduncle allowing it to form properly, instead of those where numerous fruit are all tightly bunched together, resulting in deformations and difficulty in picking. Retail-sourced seed could have been from fruit trees that favoured other climates.
Tropical countries generally prefer to grow hermaphrodites, and Solo is one of the better-known ones. Hawaii has authorised sale of transgenic plants to combat papaya ring spot virus which threatened the viability of the whole industry there. The genome has virtually been established.
Sexual behaviour in pawpaw is complex. In essence, there are three main types of flowers: staminate, pistillate and hermaphrodite. But in reality, there are many more variants than this, particularly amongst the hermaphrodites. Plants can be polygamo-monoecious or polygamo-dioecious. Flowers are borne on cymose inflorescences that arise from leaf axils. Pistillate flowers have an ovoid ovary with short peduncles while staminate are tubular with 10 stamens and clustered on long racemes; the hermaphroditic form is between the two.
Environmental factors (eg low temperatures, excessive N or moisture, plant damage) can cause undesirable changes in behaviour of staminate and hermaphrodite plants, whereas pistillate trees are much more stable.
Pollination is thought to be by moths; bees may be seen working staminate flowers but pistillate flowers are not attractive to them. Hermaphrodite flowers are self-pollinating (cleistogamous).
The sex of seedlings is not known till flowering, so usually 2-3 are planted in a single hole, and once identified, the surplus can be removed by cutting them off at ground level. In reasonable proximity, one male is sufficient to pollinate several females. Space them at least 2m apart. Endeavour to keep them actively growing through all the warmer months of the year. They need a constant supply of nutrients to maintain growth and production, particularly N and K, but also micronutrients. When fruiting, they prefer high K and low P fertilization; yield starts to decrease after 3-4 years and commercial plants are usually replaced with new stock.
With strong winds the main damage is extensive shredding of the large leaves. However more violent storms or cyclones can break branches or even uproot the whole plant.
They are self-pruning, with older leaves continually abscising. After a few years the plant may become too tall to access fruit easily; at this stage it can be headed back, wherafter it will regrow with several lateral branches. Although the number of fruit may then be increased, fruit size is generally smaller.
Pawpaw is a fleshy berry from 200g-5kg. Shape is sex-linked, with female flowers producing spherical-ovoid fruit and hermaphrodite more cylindrical or pear-shaped fruit. There is a central ovoid cavity normally with hundreds of black seeds. Some fruits set parthenocarpically. Male plants occasionally bear small fruits of moderate quality.
The thin fruit skin changes from green to yellow-orange when mature. Flesh colour can be yellow-orange to reddish, and unless true-breeding lines are used, the red flesh types are not stable. Cut or damaged immature fruit exude a milky latex which decreases as they ripen. Fruit flavour can vary between different plants. The flesh contains 8-16% sugars with many aromatics contributing to flavour, and it also has good levels of vitamin A and C.
Fruit can be available for much of the year and should be harvested when about a third has changed colour. They will then ripen at room temperature over several days. As a tropical fruit they will suffer chilling injury if stored below 10°C.
Normally eaten fresh, but is also eaten green as a vegetable.
Fruit flies. Papaya ring spot virus can destroy whole plantations, as can a phytoplasma-induced dieback when trees are under stress. Post-harvest, anthracnose can rapidly destroy whole fruit.
Along with bananas, pawpaw is one of the most productive fruit trees in terms of required footprint. Sourcing plants is simple and cheap via self-germinated seedlings, and with our dry WA climate, fungal infections are much reduced compared to those in the more humid tropics. Take care that soil is well-drained and maintain growth through watering and fertilizing.