Pawpaw seed used in container production must be treated with care to optimize germination. Thompson (1974) suggested that pawpaw seed be stratified at 5 circ C (41.0 °F) for eight weeks to increase germination. Finneseth et al. (1998a) found that stratification of pawpaw seed for at least 7 weeks at 5 circ C increased germination from about 1% to 70%.
Pawpaw has a recalcitrant seed that does not tolerate desiccation, and it only has a relatively short period of viability. It also does not tolerate subfreezing temperatures. Finneseth et al. (1998a) found that desiccation of pawpaw seed reduces viability by 50% when seed moisture content declines from 37 to 25%. Storing pawpaw seed below freezing (-15 °C) will kill the seed (Pomper et al., 2000). The seed can be stored in moist peat moss in ziplock bags for 2 to 3 years at 5 °C and maintain a high germination percentage (Finneseth et al., 1998a).
Container size is an important consideration in the production of pawpaw seedlings. Tall containers should be used to accommodate the developing taproot of pawpaw seedlings; for example, Rootrainers (Spencer-Lemaire Industries Limited, Edmonton, Alta., Canada) that are 5.1 × 6.35 × 25.4 cm (2 × 2.5 × 10 inches) with a volume of
737.4 cm³ (45 inch³), or similar sized containers, have been used to successfully produce seedlings (Pomper et al., 2002a, 2002b, 2002c). Seedling vigor in containers can be quite variable if pawpaw seed is collected from trees with differing genetic backgrounds. Seedlings grown in Roottrainers should be transplanted when the seedling has produced about 10 leaves, otherwise seedlings may set a terminal bud and stop growing for that season (author, unpublished). When growing pawpaw seedlings or transplanting them, use tall containers of 3.8 to 7.6 L (1 to 2 gal) volume, such as Treepots (Stuewe and Sons, Inc., Corvallis, Ore.), to accommodate the developing taproot. In a greenhouse experiment, stratified seed was sown into Pro-Mix BX potting medium (Premier, Red Hill, Pa.) containing Osmocote 14–14–14 (Scotts Co., Marysville, Ohio) at 1.85 kg·m⁻³ (3.118 lb/yard³) or 3.70 kg·m⁻³ (6.236 lb/yard³) in 3.8-L containers of three different heights (short, medium, and tall) (Table 1). Plants were destructively harvested 24 weeks after sowing. Although there was great variation in seedling vigor, which was likely a result of the mixed seed source, the seedlings had the greatest root dry weight in tall 3.8-L Treepots at either rate of fertilization (Table 1). Pawpaw seedlings grown in short containers developed root spiraling (Fig. 1). The larger root system of seedlings in the taller containers should improve field establishment. With optimal greenhouse conditions
and vigorous rootstock, rootstock can reach 7 to 10 mm (0.28 to 0.39 inch) in diameter in 3 months and be chip budded and scion growth forced to produce a large grafted tree [0.9 m (3 ft)] in 1 year (Layne 1996). However, less vigorous rootstock may require a 2-year cycle, 1 year to grow graftable rootstock and a second year for the scion to grow.
Through a series of greenhouse experiments, growth of pawpaw seedlings was examined in a range of potting substrates and fertilization regimes. A well-aerated potting substrate, such as Pro-Mix BX, with a high sphagnum peat moss component (>75% by volume), cation exchange capacity, and water holding capacity can be used effectively in pawpaw container production. Pomper et al. (2002a) found that pawpaw seedlings had the greatest plant height, leaf number, and dry weight (roots and shoots) when grown in either the potting substrate Pro-Mix BX or 1 sand:1 sphagnum peatmoss (vol/vol), as compared to 6 pine bark :1 sand or 4 pine bark :1 sand :1 sphagnum peatmoss potting substrates; these seedlings were grown in Rootrainers with each substrate and fertilized with 100 mg·L-1 N as Peters 20–20–20 water-soluble fertilizer plus soluble trace elements once per week. Seedling root dry weight was about 30% greater and shoot dry weight about 70% greater in Pro-Mix BX and
Table 1. Growth characteristics of pawpaw seedlings grown in a greenhouse in three different 3.8-L (1-gal) containers varying in height and at two levels of slow-release fertilizer [Osmocote 14-14-14 (14N-6.1P-11.6K)].
| Treatmentz | Leaf no. | Ht (cm) | Root dry wt (g) | Shoot dry wt (g) | Total dry wt (g) | Root to shoot ratio |
|---|---|---|---|---|---|---|
| Container size | ||||||
| 1 | 24.8 | 47.4 | 2.5b | 7.2 | 9.7 | 0.34 by |
| 2 | 26.6 | 46.5 | 2.6b | 7.8 | 10.4 | 0.39 b |
| 3 | 23.9 | 46.2 | 3.4a | 6.3 | 9.7 | 0.56 a |
| Fertilizer level | ||||||
| 1 | 24.0 | 48.4 | 3.1 | 6.6 | 9.7 | 0.49y |
| 2 | 26.2 | 45.0 | 2.6 | 7.6 | 10.2 | 0.37 |
| Analysis of variance P | ||||||
| Block | 0.76 | 0.43 | 0.03* | 0.75 | 0.06 | 0.01* |
| Container size | 0.49 | 0.94 | 0.02* | 0.20 | 0.74 | 0.01* |
| Fertilizer | 0.24 | 0.24 | 0.07 | 0.12 | 0.53 | 0.01* |
| Container size × fertilizer | 0.24 | 0.26 | 0.87 | 0.72 | 0.73 | 0.50 |
23.8 L at three different heights of 1) 20.32 × 12.70 × 16.51 cm (8.0 × 5.0 × 6.5 inches), 2) 15.875 × 17.78 × 13.335 cm (6.25 × 7.0 × 5.25 inches), and 3) 10.2 × 35.6 × 10.2 cm (4 × 14 × 4 inches), for container top diameter × height × bottom diameter. Fertilizer treatments were with Osmocote 14–14–14, 3 to 4 month release product (Scotts Co., Marysville, Ohio) at 1) 1.85 kg · m -3 (3.118 lb/yard3) or 2) 3.70 kg · m -3 (6.236 lb/yard3) incorporated into Pro-Mix BX (Premier Horticulture, Inc. Red Hill, Pa.) potting substrate; plants were destructively harvested at 24 weeks and growth characteristics examined; mean separation within container size by least significant difference (LSD), P < 0.05. n = 60; 1.0 g = 0.035 oz; 1.0 cm = 0.39 inches.
yMeans for fertilizer level were significantly different using an independent t test with a P < 0.05.*Significant P < 0.05
*Significant P < 0.05.