Pawpaw (Asimina triloba) is native to North America, where it grows naturally as part of mixed forest stands. As the only genus in the Annonaceae family, it grows in temperate climates, while other species, such as papaya, require a tropical climate. In Central Europe, it can be cultivated, particularly in wine-growing regions. With continued global warming, other fruit-growing locations are also possible. As an interesting, exotic-looking plant, Asimina triloba, due to its many positive characteristics, could and should increasingly find its way into special nursery assortments such as fruit gardens, exotic gardens, or novelty plants.
In the search for new fruit tree varieties, increasing demand for Asimina triloba can be observed, initially in the hobby sector. Reasons for this include its weak and slow growth, which allows this frost-hardy and disease-resistant deciduous tree to remain small or even be kept small. Its ornamental value is expressed in bell-shaped, purple-violet flowers and its autumn color with yellow foliage. And last but not least, the edible fruits, which ripen from late August to mid-October, are reason enough to take a closer look at this innovative fruit.
Years of Experience in Veitshöchheim
In the spring of 1998, five trees of five varieties were planted as biennial grafts (rootstock: seedling) at a distance of 4 m x 3 m at the Bavarian State Institute for Viticulture and Horticulture in Veitshöchheim. The single-stemmed tree initially grows very slowly, branches well (predominantly at a 45° angle), and produces short new shoots 15 to 30 cm long each year. During tree training, only the competing shoot was removed in the first few years. However, it takes three years for the first flowers to appear. These appear individually on the one-year-old wood before the leaves emerge as purple bells that can be up to 5 cm wide and are intended to attract insects for pollination. The flowering period is around mid- to late May, i.e., after the Ice Saints and after the apple blossom.
Asimina triloba is dependent on cross-pollination. Only a few varieties are classified as partially (e.g., 'Sunflower') or fully self-fertile (e.g., 'Prima'). In commercial cultivation, two varieties should always be planted. The flower contains several ovaries, so that up to five individual fruits can develop in a cluster from pollinated organs. Nevertheless, fruit set and thus yield are rather low. Certain insects and pollen-carrying beetles, which ensure better pollination in North America, are absent in Europe. Bumblebees and honey bees are also found here, but the "stinky" smell of the flowers makes them unattractive to our main pollinators. Therefore, hand pollination is recommended, although this is unlikely to be practiced in commercial cultivation. In any case, only fully or partially self-fertile varieties such as 'Sunflower' or 'Prima' should be planted, where the yield potential is generally higher.
The ripening period is late August to mid-October. Two to three, or even four, pickings are required during the approximately three-week ripening window. The exact time of harvest is somewhat difficult to determine. It has been proven that the skin color of the elongated-oval to club-shaped fruits changes from green to a light yellow-green. Harvesting should be carried out carefully, as the skins are very susceptible to bruising, which can also occur from friction between fruit hanging in bunches. The fruits must be stored in a cool place (2 to 3°C), otherwise they will ripen quickly and should be consumed as soon as possible. This is due to the similarity to the "real" banana – as is the fact that the skin quickly turns yellow to brown, and the flesh becomes soft, very sweet and even slightly fermented due to the rapid breakdown of the fruit acids.
The taste is sweet, the aroma a mixture of banana/mango/melon with a slight hint of vanilla – thus fruitier than banana. The consistency of the creamy-white to yellowish flesh is similar to that of a banana or even an avocado. The fruits are eaten without the skin: either peeled or – similar to a kiwi – sliced and then scooped out. However, the bean-sized seeds, which are distributed throughout the fruit, are a nuisance. In addition to being eaten fresh, they can be made into puree for use in ice cream, milkshakes, and mixed juices – essentially like a banana. A sponge cake with pawpaw puree has a fruity flavor. The use of the seeds or leaves, for example, for tea, creams, or pharmaceutical purposes, should be explored.
Yields
Yields were assessed for 'Sunflower' and 'Overleese' in the first few years and extrapolated based on fruit count and average fruit weight, and recorded precisely from the 9th year onwards. The other varieties declined significantly.
Average yield of the "Sunflower" and "Overleese" varieties in kg per tree from the 3rd to the 8th year after planting:
3rd year: 1 kg, 4th year: 2 to 3 kg, 5th year: 4 to 5 kg, 6th year: 5 to 6 kg, 7th year: 6 to 8 kg, 8th year: 7 to 9 kg
Yields vary depending on the variety, training (crown volume), and pruning. To maintain tree height, more intensive pruning is required. Consequently, the yield potential remained at 7 to 10 kg in years 11 to 13 after planting. The late frost year of 2011, with only 3 kg of fruit per tree, was followed in 2012 (year 15) by "record tree yields" of up to 34 kg ('Sunflower'), up to 25 kg ('Overleese'), and 12 kg for 'Prima', which was only in its fifth year. In 2012, 'Davis' produced fruit for the first time with very convincing fruit quality. The somewhat lower-seed varieties from Neal Peterson/USA were not yet in production. From a large number of seedlings, six were shortlisted.
Growth and Tree Characteristics
The varieties grafted onto Asimina triloba seedlings grow (very) slowly in the first five years under the Veitshöchheim conditions (sandy loam, soil value 28, humus content around 1%, pH 7.2–7.3, dry summer location, 8th year with drip irrigation). Supplementary irrigation from the beginning would certainly have increased growth and possibly also initial yields. The well-branching trees form pyramidal, dense crowns when trained as spindles. The side shoots, which usually branch out at 30–45° angles from the central axis, grow approximately 15–30 cm annually.
Apart from removing competing shoots and diverting steeper side branches, no further pruning or shaping work has been required so far. The large, approximately 20 to 25 cm long leaves densify the crown, which was thinned out after the 10th year, thus reducing the tree's height. Another more severe pruning took place after the 15th year (2012/13).
No pests or winter frost damage to wood or flower buds occurred (lowest temperatures: down to -26 °C; repeated years of -18 °C were easily survived). Only the extreme late frost on May 3/4, 2011, led to flower damage and reduced yield. Since no fungicide or insecticide treatments were required, Pawpaw appears predestined for organic cultivation. Specific pests occur in the USA, which reportedly do not cause serious problems.
Fertilization can be based on that of pome fruit. Depending on the vigor of the location and the intended pruning intensity, planting distances can be 4.0–4.5 m x 2.0–2.5 m; even less for slender spindle trees. To counteract slow growth in the first few years, supplemental irrigation is recommended on light soils or in dry areas, as well as a good nutrient supply.
Besides cultivation in wine-growing regions, other fruit-growing areas are also being considered due to global warming. Serious disadvantages include the short shelf life, browning of the fruit skin, significantly lower, hesitant and delayed yields compared to apples, problems with pollination, procurement (especially large quantities), and the price of young trees – at least in Europe (low supply, slow growth of seedling rootstocks, poor grafting rates).
Evaluation
As an interesting, exotic-looking niche product, Asimina triloba could and should be cultivated in Germany due to its many positive characteristics – at least for the local market/direct sales. This is supported by its novelty value, exoticism, robustness, easy cultivation, potential for organic cultivation, low pruning effort, favorable ingredients (positive levels of amino acids, higher levels of vitamins A and C than, for example, apples, but also high in calories!), and the positive, promotionally effective name "Indian banana."
