Asimina triloba – American Pawpaw

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Sheryl  I bought back some seed from California given to me by Roger Meyer  – really enjoyed the taste of the fruit. Kentucky State University area conducts cultivar testing and growing in an experimental orchard and they have an excellent website:  John Prince, one of our members in New Zealand says he made one of his best ever ice-creams from this fruit.

Kentucky State University has the only full-time Asimina triloba (L.) Dunal research program in the world as part of the KSU Land Grant Program. Research efforts are directed at improving seed and clonal propagation methods, developing orchard management recommendations, conducting regional variety trials, understanding fruit ripening processes, developing fruit storage techniques, and germplasm collection and characterization of genetic diversity. KSU is also the home of the PawPaw Foundation, a nonprofit organization dedicated to the research and development of Asimina as a new fruit crop. (Asimina is known as PawPaw in the USA).Contact Dr. Kirk Pomper:  

The USDA National Clonal Repository for Asimina spp. Since 1994 Kentucky State University has served as the USDA National Clonal Germplasm Repository (NCGR), or gene bank, for Asimina spp. as a satellite site of the NCGR repository at Corvallis, Oregon. There are over 1,700 accessions (trees) from 17 states that are planted on 8 acres at the KSU farm and they are attempting to evaluate the genetic diversity contained in wild Asimina populations across its native range so that unique material can be added to the KYSU repository collection; this potentially rich source of useful genetic traits will be used in breeding efforts.

Climate  Asimina is a tree of temperate humid growing zones, requiring warm to hot summers, mild to cold winters, and a minimum of 32 inches (81 cm) of rainfall spread rather evenly throughout the year, with the majority falling in spring and summer. It can be grown successfully in USDA plant hardiness zones 5 (-15o F/-26o C) through 8 (15o F/-9o C). They grow wild over a wide range of latitude, from the Gulf Coastal plain to southern Michigan. However, the trees may not receive adequate chilling hours if planted too close to the Gulf Coast. Most named cultivars originated in the Midwest, which is the northern portion of the Asimina’s range. A national regional variety trial is underway to determine which varieties perform best in different parts of the country, and results should become available in the next several years. In the meantime, for best results, choose cultivars that were selected in a climatic zone and latitude similar to the area where they will be planted.

Site, soils, and habitat  Although the Asimina is capable of fruiting in the shade, optimum yields are obtained in open exposure with some protection from wind (on account of the large leaves). Germinating seedlings, however, will not survive under those conditions because they are extremely sensitive to full sunlight which can kill them. (Containerized seedlings may be grown without shade in a greenhouse.) Shading for the first year and sometimes the second is normally required outside and it is for this reason that Asimina are almost always found in nature as an understorey tree. The soil should be slightly acid (pH 5.5-7), deep, fertile, and well-drained. Good drainage is essential to success. Asimina will not thrive in heavy soil or waterlogged soil. In habit it is a small tree, seldom taller than 8 mtrs. – 25 feet. Grown in full sun, the tree develops a narrowly pyramidal shape with dense, drooping foliage down to the ground level. In the shade it has a more open branching habit with few lower limbs and horizontally held leaves.

Seed Propagation   The seed is slow to germinate but it is not difficult to grow seedlings if certain procedures are followed. Do not allow the seed to freeze or dry out, because this can destroy the immature, dormant embryo. If seeds are dried for 3 days at room temperature, the germination percentage can drop to less than 20%. To break dormancy, the seed must receive a period of cold, moist stratification for 70-100 days. This may be accomplished by sowing the seed late in the fall and letting it overwinter; the seed will germinate the following year in late July to late August.

Another way is to stratify the seed in the refrigerator (32o– 40o F/0o– 4o C). In this case the cleaned seed should be stored in a plastic ziplock bag with a little moist sphagnum moss to keep the seed moist and suppress fungal and bacterial growth. After stratification the seed should be sown 1 inch (2.5 cm) deep in a well-aerated soil mix, pH 5.5-7, with an optimum temperature of 75o– 85oF (24o– 29oC). Use tall containers, such as tree pots (ht. 14″-18″/35-45 cm) or root trainers (ht. 10″/25 cm), to accommodate the long taproot. The seed will normally germinate in 2-3 weeks, and the shoot will emerge in about 2 months. Germination is hypogeal: the shoot emerges without any cotyledons. For the first two years, growth is slow as the root system establishes itself, but thereafter it accelerates. Trees normally begin to bear fruit when the saplings reach 6 feet, which usually requires five to eight years.

Vegetative or clonal propagation   Clones are easily propagated by a number of grafting and budding techniques, such as whip-and-tongue, cleft, bark inlay, and chip budding. The only method that does not produce good results is T-budding. Softwood and hardwood cuttings have proved virtually impossible to root. Although it is common for the tree to sucker from the roots, propagating a clone by transplanting root suckers is often not successful. They are ordinarily quite difficult to transplant. They have fleshy, brittle roots with very few fine hairs. Experimentation has shown that, to be successful, transplantation should be done in the spring at the time that new growth commences, or soon after. (This is basically the same as for magnolia.) If many roots are lost, it may be desirable to prune the top to bring it into balance with the remaining roots. Grafted trees may bear fruit in as few as 3 years.

Purchasing trees   Since seedlings are not identical to their parents, fruit quality cannot be guaranteed.  Since they retain the clonal identity of the parent, fruit quality is assured, given adequate cultural conditions. If fruit production is desired, purchase at least two genetically different trees (i.e. two different cultivars, or at least two seedlings), to ensure that cross pollination can occur. Container-grown trees may have a higher establishment (survival) rate than field-dug trees; Asiminas have a long tap root and delicate root system that can be damaged by digging.

Caring for young trees   When planting trees, allow 8 feet (2.5m) between them. Water newly planted trees immediately after planting, and as needed throughout the growing season. Trees require adequate soil moisture, especially in the first two years after planting. Transplant shock may be reduced by providing temporary partial shade to newly planted trees. Application of a balanced fertilizer (20N-20P-20K) every few weeks during the first half of the growing season may also improve growth and establishment of young trees.

Pollination: natural and artificial   Pollination can be a limiting factor in Asimina fruit set. The flowers are protogynous, meaning that the stigma (the female receptive organ) ripens before the pollen, and is no longer receptive when the pollen is shed. Thus the flower is designed not to be self-pollinated. In addition, the trees are self-incompatible, usually requiring pollen from a genetically different tree in order to be fertilized. Finally, the natural pollinators of the Asimina –various species of flies and beetles–are not efficient or dependable. Although it requires a little extra labour, hand pollination to ensure fruit set can be well worth the effort and can be done as follows: Using a small, flexible artist’s brush, transfer a quantity of fresh pollen from the anthers of the flower of one clone to the ripe stigma of the flower of another clone. Pollen is ripe when the little ball of anthers is brown in colour, loose and friable; pollen grains appear as yellow dust on the brush hairs. The stigma is ripe when the tips of the pistils are green and glossy, and the anther ball is still hard and green. Do not overburden the tree with fruit, as this will stress the tree, resulting in smaller than normal fruit, and may cause limbs to break under excessive weight.

Pests    In its native habitat the tree has few pests of any importance. The worst pest is Talponia plummeriana, the Asimina peduncle borer, a small moth larva (about 5 mm long) that burrows into the fleshy tissues of the flower, causing the flower to wither and drop. In some years this borer is capable of destroying the majority of blossoms. Another pest is Eurytides marcellus, the zebra swallowtail butterfly, whose larvae feed exclusively on young Asimina foliage, but never in great numbers. The adult butterfly is of such great beauty that this should be thought more a blessing than a curse. Sometimes the fruit surface may be covered with patches that are hard and black; this is a fungus infection, but it seldom has any effect on flavour or edibility. Deer will not eat the leaves or twigs, but they will eat fruit that has dropped on the ground. Male deer occasionally damage trees by rubbing their antlers on them in winter. Outside its native region, the tree is sometimes reported to be plagued by pests, but this may be because of poor tree health resulting from the stress of improper soils and an unsuitable climate.

Chester Dott – Forbidden Fruit Nursery – Mullumbimby:      Plant it the same as papaya, good drainage; do not disturb the taproot.

Tom Ling:  I have 4 seedlings growing in full sun through late Summer and Winter without any problem. 2 of them still have their first pair of leaves. The other 2 have lost the leaves in the Winter. I have others in full shade. The average temp in Winter here is 22C. There is a web page I read long time ago describing experiments on the effect of pot depth to the vigour in growth.  It has a few photos showing seedlings with roots.  All my 10 or so seedlings have varying length of tap roots.  The longest one is about 15cm 4 weeks after casting off the seed pod. I also noticed that if a seedling has difficulty splitting and casting off its seed pod then it is most likely to have stunted malformed roots.  You can try manually to split open the seed pod but you may end up breaking the tip.  Some may try to grow again without the broken tip.

One technique to try to induce the splitting is to use a eye dropper to wet the seed pod constantly. I think it is best to sow directly in the ground as it doesn’t like transplanting from some other pages I read.  But my experience is they don’t mind that at all. The seedling will lose all leaves in Winter and may appear dead with shriveled look on the stem but come Spring, the leaves will grow back.  I use half plastic bottles to protect them from cold.  Ignore what web pages say about sensitive to transplant and sunlight.  Some seedlings do like sun but play safe in the first 2 years.  Quite easy to transplant.  Emerging seedlings have tap roots from 10 inches to a few inches depending on the robustness of the shoots.  Seed pods will stick to shoot tips for 2 to 6 weeks or forever depending on the weather and health of the shoots. After 4 weeks if pods are still unopened, prise open with finger nails. If tips break as a result, keep the shoots over Winter in a neutral location outside.  The broken tip shoot of 1 inch tall is capable of producing new branches in the coming Spring.  This is one of the few species that can do this. Some seeds will lie dormant for 1 year before germinating.   

Marcello:  One reason I read for why this tree is hard to grow on a commercial scale is that when the tree’s female stigmas are ripe for pollination, male blooms are often just getting ready to manufacture pollen so it appears to be a question of ‘missed’ timing for extemporaneous pollination if left to nature’s devices to effect fertilization. In fact, crops are often hand pollinated to circumvent precisely this problem.  

Sergio Duarte – Portugal:  I am using custom made bags that are 60cm tall and I can tell you that the seedlings produce a tap root of at least 25cm in length, even before they produce the first pair of leaves. Some people state that with care it is possible to get Asiminas to live after the main tap root has been pruned so the tap root seems to be important but is not completely vital. I think the main problem is that Asimina triloba is said to require some winter chill to break dormancy. 100 hours was the number that came up on another forum when I asked about it but I don’t think anyone knows exactly how much for sure. You will have a better chance if you plant them at your highest elevation.

Mike Nave –  Sacramento, California     Despite their well known preference for water and preference for cool shaded and even wet areas, I have seedlings that do well with little watering (probably because of that taproot) and do not seem particularly affected by our summer heat and wind. My grafted ones do have problems with the heat here and are not exceptionally vigorous but the seedlings have virtually no problems at all. They grow well in heavy soil, light soil, pots etc. They will push the taproot through almost any pot in one season but I have experienced no problems with cutting the taproot to repot or to plant out.The only pests I have ever seen on my Asiminas are some small insect that snacks on the leaves when they are first emerging. Nothing seems to bother the leaves once they are mature. Snails will attack the fruit only if they suffer some kind of damage. Birds do not seem to understand that the fruits are food although they attack all other fruits on my property. Ants and aphids ignore the trees, although ants will infest the fruits if they fall to the ground and break open. I do occasionally get some kind of very small scale insect but it does no damage. Despite their well known preference for water and preference for cool shaded and even wet areas, I have seedlings that do well with little watering (probably because of that taproot) and do not seem particularly affected by our summer heat and wind. My grafted trees do have problems with the heat here and are not exceptionally vigorous but the seedlings have virtually no problems at all. They grow well in heavy soil, light soil, pots etc. They will push the taproot through almost any pot in one season but I have experienced no problems with cutting the taproot to repot or to plant out.

Elwyn Hegarty:  This is the plant known as Pawpaw in the States that has been credited with some anti-cancer qualities. There has been considerable confusion because of the common name and the fact that Asimina is in Annonaceae and no relation to what we would call Pawpaw (Carica papaya, Caricaceae) here in Australia. Both come from parts of tropical America.  This confusion was compounded a few years ago when New Scientist published a letter about Asimina illustrated by a photo of Carica papaya – they did correct this in the next edition, though.  We had occasion to look into this scene about the same time for a client who was using papaw leaf and tried to find out whether any of the health claims then being made for our papaws had been confused with those then being validated for Asimina, as seemed possible.  Both contain bioactives in various parts of the plant.  If the medicinal use of Asimina comes up, as it probably will if you are distributing seed, perhaps we’d better take another quick look at the information we had found at the time, and then see what’s been discovered about medicinal qualities of both plants in the interim – you may have updates following your trip, too. (There’s the same problem with cat’s claw herbals, at least two different, unrelated, plants are known as cat’s claw).

Notes from Donald Cobb in The Nutshell (Northern Nut Growers Association Sept.1998)  I attended my first meeting of the above association and learnt about meristematic dieback of greenhouse grown Asimina. If after planting your seed and the seedlings have that wilted look in our hot weather, it could be suffering from too high a soil pH. If it’s around 6.8, it’s too high. Not having sulphuric acid on hand or the mathematical background to do calculations for its use, I decided to use what I had at hand to acidify the soil; white distilled vinegar and a soluble complete fertilizer formulated for acid loving plants. I added 1/3 cup vinegar and 5 tablespoons of fertilizer to 20 litres of water. I saturate-watered the Asimina seedlings with this solution and again a week later. Four days after the first watering, the seedlings new leaves were erect and bright green. Two weeks after the second soaking, the seedlings health and vigour were much improved and the soil pH tested 5.8. Some of the seedlings had doubled in size in just three weeks. I am sure I would have lost many of these seedlings if I had not acidified the soil.

Just remember that when planting seed, they require a very very deep pot. 

Article collated by Sheryl Backhouse