Kirk W. Pomper,1 Desmond R. Layne,2 and Eddie B. Reed3

ADDITIONAL INDEX WORDS. Asimina triloba, root:shoot ratio, kentucky banana

SUMMARY. Growth of pawpaw (Asimina triloba) seedlings in containers was examined in a factorial greenhouse experiment with four treatment levels of the slow-release fertilizer, Osmocote 14–14–14 (14N–6.1P–11.6K), incorporated in Pro-Mix BX potting substrate at 0, 0.13, 0.26 or 0.81 kg·m-3 (0, 0.22, 0.44, or 1.37 lb/yard3) and three treatment levels of liquid-feed fertilizer of Peters 20–20–20 (20N–8.7P–16.6K) water-soluble fertilizer at 0, 250, or 500 mg·L-1 (ppm). When plants were harvested 18 weeks after sowing, seedlings subjected to the highest rate of Osmocote 14–14–14 at 0.81 kg·m-3 and liquid-feed at 500 mg·L-1 had the greatest total biomass, about 3-fold greater than nonfertilized plants. In a separate greenhouse experiment, growth of seedlings was examined

Kentucky State University, Land Grant Program, 129 Atwood Research Facility, Frankfort, KY 40601-2355.

with Osmocote 14–14–14 as the sole fertilizer source at six treatment levels of: 0, 0.81, 2.22, 4.43, 8.86, or 17.7 kg·m-3 (0, 1.37, 3.74, 7.47, 14.9, or 29.9 lb/yard3). Early seedling growth was hastened in the 2.22 kg·m-3 treatment rate, but delayed in 17.7 kg·m-3 treatment rate, when compared to nonfertilized control plants. When seedlings were harvested 17 weeks after sowing, plants had the greatest shoot, root, and total dry weight with Osmocote 14–14–14 at a rate of 2.22 kg·m-3. Root:shoot ratio decreased from about 1.5 without Osmocote 14–14–14, to about 0.65 at rates of 2.22 kg·m-3 or greater. Based on the results of this study, the slow-release fertilizer, Osmocote 14–14–14, can be used effectively as a sole fertilizer source when incorporated into potting substrate at a rate of 2.22 kg·m-3 or at a reduced rate of 0.81 kg·m-3 when supplemented with weekly applications of liquid-feed fertilizer at a rate of 500 mg·L-1 of Peters 20–20–20, to enhance production of container-grown pawpaw seedlings.

The North American paw- paw has potential as a high- value tree-fruit crop or as a landscape component (Layne, 1996; Pomper et al., 1999). Historically, pawpaws have been difficult to propa- gate. Pawpaws develop a strong tap- root with a fragile root system, which can be easily damaged upon digging; therefore, most commercial nurseries in the United States propagate their own pawpaw trees in containers (un- published). Although this results in a significantly greater cost to the pur- chaser, trees usually have a strong, healthy root system for field establishment. Un- fortunately, high prices for seedling and grafted trees are limiting the develop- ment of a commercial pawpaw industry. Recent studies at Kentucky State Uni- versity have developed recommendations (e.g., potting substrate type, the use of tall containers to accommodate the seedling’s strong tap root, etc.) to- ward the successful container produc- tion of pawpaw (Finneseth et al., 1998a, 1998b; Jones et al., 1998; Layne, 1996; Pomper et al., 2002a, 2002b). How- ever, further improvements in cultural methods for container production of robust pawpaw seedlings for grafting, transplanting to orchards, or home land- scapes would be desirable.

There is little information available on the fertilization requirements of paw-

paw. Slow-release fertilizers are an efficient means to meet the nutritional needs of container-grown nursery crops. Osmocote is a popular encapsulated, slow-release fertilizer that provides a relatively constant supply of nutrients for plant absorption over a fixed time frame. Because fertilizers such as Osmocote release nutrients slowly, they eliminate the need for labor-intensive liquid fertilization that may lead to excessive nutrient leaching and contamination of ground water (Nelson, 1998). Osmocote, alone or in combination with water-soluble fertilizer, has been used to successfully produce many tree species in containers. For example, false weymouth pine (Pinus pseudostrobus) and douglas pine (Pinus douglasiana) trees grew well when fertilized using Osmocote 14–14–14 at a rate of 4 kg·m⁻³ (6.74 lb/yard³) (Mexal et al., 1995). River birch (Betula nigra), bald cypress (Taxodium distichum), sycamore (Platanus occidentalis), and weeping willow (Salix babylonica) have been grown successfully outdoors in containers with Osmocote 16–7–12 (16N–3.1P–10K) at a rate of 13 kg·m⁻³ (21.9 lb/yard³) (Ruter, 1994). Green ash (Fraxinus pennsylvanica) and red oak (Quercus rubra) have been grown successfully in containers with Osmocote 18–6–12 (18N–2.6P–10K) at a rate of 3.6 kg·m⁻³ (6 lb/yard³), supplemented with weekly applications of 200 mg·L⁻¹ N from 20–20–20 (20N–8.7P–16.6K) water soluble fertilizer (Arnold and Struve, 1989). Pomper et al. (2002b) found that pawpaw seedlings grew well in Pro-Mix BX potting substrate (Premier Horticulture, Inc., Red Hill, Pa.) with weekly liquid-feed applications of 500 mg·L⁻¹ Peters 20–20–20 water-soluble fertilizer plus soluble trace elements; however, further examination of specific nutritional requirements for container production of pawpaw seedlings could lead to enhanced growth of seedlings.

The objective of this study was to examine early growth and development of greenhouse-grown pawpaw seedlings with either slow-release fertilizer (Osmocote) as the sole fertilizer source or with Osmocote supplemented by water-soluble fertilizer, in an effort to determine the optimal fertilization for enhanced growth of pawpaw seedlings in containers.

Materials and methods PLANT MATERIAL. For all experi-