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each year. Excellent growth has been achieved with granular ammonium nitrate fertilizer (34-0-0) broadcast under pawpaw trees in early spring at 30–60 g N/tree applied before budbreak.
High-sodium conditions are potentially damaging to salt-sensitive fruit crops (Picchioni et al. 1990, 2000; Picchioni and Graham 2001). Thus, the intrinsic difficulties in field transplanting A. triloba (Callaway 1990) could be increased with sodium stress. For a southern A. triloba ecotype exposed to high-sodium conditions, the addition of gypsum improves field growth and survival rates, and increases fertilizer-N recovery during early orchard establishment (Picchioni et al. 2004).
Most pawpaw genotypes naturally develop a strong central leader. Branches can often develop sharp crotch angles in relation to the trunk. Trees should not be headed at planting and no pruning is required the first year. Pruning is conducted in late winter-early spring and consists of removing low branches to a height 60 to 90 cm on the trunk. Training to more horizontal scaffold limbs increases scaffold strength and reduces limb breakage which may occur under heavy crops.
Flowers are strongly protogynous and are likely self-incompatible (Willson and Schemske 1980), although ‘Sunflower’ may be self-fruitful. Pollination is by flies (Diptera) and beetles (Nitidulidae), and possibly other nocturnal insects (Kral 1960; Faegri and van der Pijl 1971; Willson and Schemske 1980). Seedlings normally begin to flower when they reach a height of about 1.8 m, but may not set fruit; cropping is achieved at five to eight years. Grafted pawpaw trees often flower within 3 years after planting, but often fail to set fruit (Bratsch et al. 2003; Merwin et al. 2003; Pomper et al. 2003d). Failure of trees to set fruit could be due to inadequate pollination or inadequate canopy to support fruit development. Usually 5 to 6 years are required for grafted trees to begin production, although some cultivars such as ‘PA-Golden (#1)’ may produce crops 4 to 5 years after planting (Pomper et al. 2003d).
The normal bloom period for pawpaw may last from 3 to 4 weeks. Thus, harvest is often extended over a similar time frame. Each fruit cluster develops from an individual flower, and fruit within a cluster often ripen at different times (Fig. 7.1). In the spring of 2002, flowering, fruit set, fruit drop, and ripening characteristics were examined in six-year-old trees for 'PA-Golden (#1)', 'Wilson', and 'Sunflower' in Kentucky. The flowering period of individual trees for each cultivar extended from April 15 until May 1 but 75% of all flowers bloomed from April 15 until May 1. Half of th trees of each cultivar examined in 2002 dropped about 50% of their cluters between May 8 and July 11. The average fruit set was 25% and this responded to 21, 9, and 7 fruit clusters per tree for 'PA-Golden', 'Wilson', and 'Sunflower', respectively. Fruit ripening periods for all cultivas reflected the long flowering periods of the cultivars. Harvest periods extended from August 20 to September 6 for 'PA-Golden', from August 22 to September 9 for 'Wilson', and from August 23 to September 21 for 'Sunflower'.
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April 15 until May 1! but 75% of all flowers bloomed from April 15 until May 1. Half of th trees of each cultivar examined in 2002 dropped about 50% of their clutters between May 8 and July 11. The average fruit set was 25% and this corresponded to 21, 9, and 7 fruit clusters per tree for 'PA-Golden', 'Wilson', and 'Sunflower', respectively. Fruit ripening periods for all cultivas reflected the long flowering periods of the cul-tivars. Harvest periods extended from August 20 to September 6 for 'PA-Golden', from August 22 to September 9 for 'Wilson', and from August 23 to September 21 for 'Sunflower'. Hughes to the lui�tian noway wielda non tree are often low (Peterson 1991)
In cultivation, pawaw yields per tree are often low (Peterson 1991). For the cultivars in the above 2002 study, yield averaged 6.4, 2.0, and 3.7 kg per tree for 'PAGolden', 'Wilson', and 'Sunflower', respectively. In another study, yields per tree in the 7th year were: 4.4 kg for 'Sunflower', 2.3 kg for 8-2 (and 2.2 kg for 'PA-Golden' (Pomper et al. 2003d)). The tropical Annonaeae relatives of the pawpaw, such as cherimoya, sweetsop (sugar apple, soursop, and atemoya also have low yields, due to low rates of natural pollination (Peterson 1991; George et al. 1992; Pena et al. 1999). In commercial plantings, these tropical pawpaw relatives are hand pollinated to increase yields (Peterson 1991; Pena et al. 1999). Low rates (<5°) of fruit set have also been noted in wild paw-paw patches (Willsomnd Schemske 1980; Lagrange and Tramer 1985).
In the wild, pawpw trees are usually found in the understory of hardwood forests. Lcv light levels in the understory likely result in reduced photosyntha partitioning to fruit that may cause low fruit set. Pawpaws in the wild ften produce many root suckers that could potentially result in large clnal pawpaw patches contributing to poor fruit set because of self-incomatibility. Pollinator limitation can also cause low fruit set in wild pawaw patches (Willson and Schemske 1980). Low pollinator activity is uually observed on cool, cloudy spring days. Since the pawpaw flowers re strongly protogynous (Willson and Schemske 1980), lack of pollenvailability from other pawpaw genotypes could also limit pollinatior Pawpaw growers report that placing carrion in buckets among pawpw trees has resulted in improvements in fruit set (L. Sibley, pers. com.) supporting the theory that pawpaw flowers may be pollinated byarrion flies. However, fruit set was 15 to 35% in KSU orchards in 199 in nine-year-old seedlings where many pawpaw genotypes are in clos proximity and flies are abundant due to nearby cattle. Pollinizer relationships between pawpaw cultivars need to be determined. Fruit setan be achieved by hand cross-pollination (Peterson 1997), and needso be evaluated as a method to increase fruit set.