years under refrigeration with little loss in viability. Use of superabsorbent polyacrylamide copolymers (Terra-Sorb AG; Terra-Sorb Industrial Services Intl., Bradenton, Fla.) in place of sphagnum moss also has proven effective for pawpaw seed storage (Tim Boland, Beaumont Nursery, Michigan State Univ., personal communication).
Stratified or after-ripened seeds are planted either in a prepared seedbed or in containers. Scarification of the seedcoat is not necessary. If many seeds are to be planted, especially for container production, it is helpful to pregerminate them. This can be best accomplished by using seeding flats; moist, peat-based soil mix; cheesecloth; and bottom heat so soil is at 32°C (Jim Gilbert, Northwoods Wholesale Nursery, Molalla, Ore., personal communication). First, line the bottom of a seeding flat with moist soil mix, which then is covered with a layer of cheesecloth. Place a single layer of stratified seeds on the cheesecloth, and then cover it with another layer of cheesecloth. Next, place 1 to 3 cm of moist soil mix over the top layer of cheesecloth. Place flats over the bottom-heat source (heat mat, hot-water pipe under bench, etc.). Keep soil moist. The top layer of cheesecloth and soil mix can be easily lifted and replaced to check for germination, which should occur within 2 to 3 weeks. Pregerminated seeds can then be sown directly into containers (described below). Under ideal conditions, percent germination is ≈90%, but depending on seed source, stratification conditions, microbial contamination, etc., the percent germination may be 60% or less. Use of this procedure greatly reduces waste in terms of soil, containers, greenhouse space, etc., for those seeds that do not germinate.
Pawpaw seed germination is hypogeal and cotyledons remain within the seedcoat. Finneseth et al. (1996) postulated that the primary function of the cotyledons is absorption of food reserves from the ruminate endosperm for transfer to the developing seedling. Seeds should be sown to a 3-cm depth in a moist, well-drained soil or other medium that has good aeration (i.e., ProMix BX; Premier, Red Hill, Pa.) in containers that are 20 to 25 cm deep (i.e., 740-cm³ Rootrainers; Hummert, Earth City, Mo.). Following radicle emergence, taproot growth proceeds to the bottom of the container and lateral roots begin to develop. Typically, when soil is at 24 to 29C, the shoot does not emerge from the soil until 9 weeks following sowing (including time spent in pregermination). Shoot emergence is hastened 10 days by heating soil to 29 to 32C from sowing date on (Layne and Kwants, 1995). The optimal conditions for greenhouse production of robust seedlings include the following: day/night means of 27/24C; maximum light intensity 1000 μmol·m⁻²·s⁻¹ photosynthetic photon flux (greenhouse whitewashing recommended, especially during summer months); 16-h photoperiod (extended by high-pressure sodium lamps); fertilization 2× per week to runoff during shoot growth phase; and soil at 29 to 32C (Jones et al., 1995; Layne and Kwants, 1995). For seedlings with 2 to 12 unfolded leaves grown in 740-cm³ Rootrainers, 20N–20P–20K Peter’s water-soluble fertilizer plus trace elements (Scotts, Allentown, Pa.) at 250 mg·liter⁻¹ is optimal. Recent experiments at Kentucky State Univ. (KSU) have demonstrated that provision of root-zone heating (soil at 29–32C) from sowing date to the 12-leaf stage resulted in a 63% increase in total plant biomass and a 90% increase in lateral root biomass when compared to those seedlings grown at ambient soil temperature (24C) (Layne and Kwants, 1995). Once 12 or more unfolded leaves are attained, transplant seedlings to a larger container. They will continue to grow actively once “potted-on” if transplanted before they exhaust the existing soil volume; otherwise, they will set a terminal bud and stop shoot growth. We transplant into 40-cm-deep, 7.6-liter Treepots (Stuewe and Sons, Corvallis, Ore.) and then boost the fertilization rate to 500 mg·liter⁻¹ with 20N–20P–20K Peter’s water-soluble fertilizer plus trace elements. If root spiraling in containers is a problem, containers can be coated with a latex paint mixture containing copper compounds of low solubility (Arnold and Struve, 1993). The copper leaches locally from the paint and burns root tips, causing lateral branching farther back on the root and stimulating formation of a more fibrous root system. This paint formulation is commercially available as SpinOut (E.C. Geiger, Harleysville, Pa.), but it can be prepared by the user for lower cost using readily accessible materials.
In the past, recommendations for pawpaw seedling production included growing seedlings outside, in a seedbed under shade (Gould, 1939). Top growth of only 8 to 10 cm in the first year was common. Shade in the first year is essential (greenhouse or field), as full sunlight conditions are lethal, probably because of excessive leaf heating that results in photoinhibition or photooxidation of photosynthetic pigments (Krause, 1988). High light intensity in the greenhouse also causes leaf damage. By using photoperiod extension, light intensities not exceeding 50% of full sunlight, temperature regulation, soil warming and fertilization, we have produced pawpaw seedlings with up to 1.5 m of top growth in one season in the greenhouse. Trees of this size are ideal for field transplanting and have sufficient diameter for chip-budding.
(1939). Top growth of only 8 to 10 cm in the first year was common. Shade in the first year is essential (greenhouse or field), as full sunlight conditions are lethal, probably because of excessive leaf heating that results in photoinhibition or photooxidation of photosynthetic pigments (Krause, 1988). High light intensity in the greenhouse also causes leaf damage. By using photoperiod extension, light intensities not exceeding 50% of full sunlight, temperature regulation, soil warming and fertilization, we have produced pawpaw seedlings with up to 1.5 m of top growth in one season in the greenhouse. Trees of this size are ideal for field transplanting and have sufficient diameter for chip-budding.
Bare-root storage and/or shipping of pawpaw seedlings has not been very successful. Until reliable bare-root methods become available, storing and shipping trees should be in containers. Pawpaws can be successfully overwintered in refrigerated cold storage, as can most other container-grown nursery crops. If refrigerated cold-storage facilities are unavailable, pawpaws can be successfully overwintered in containers outside if adequate precautions are taken. A key to successful overwinter survival is that the root system not be allowed to freeze. Containers can be overwintered in insulated, heated cold frames provided root-zone temperature does not drop below -7circC (Tim Boland, personal communication).
At KSU (USDA hardiness zone 6), we have overwintered more than 2000 trees successfully on the ground outside without an additional heat source. This was accomplished in the following manner: 1) We acclimated seedlings in the greenhouse to a dormant condition by withholding fertilization, reducing watering, lowering greenhouse temperature, and removing supplemental illumination. 2) We defoliated dormant seedlings in the greenhouse and thoroughly watered immediately before taking them outdoors in the fall. Do not prune trees before overwintering. 3) We mowed the overwintering site and oriented rows in a north–south direction. 4) We transported trees in 7.6-liter pots to the overwintering site and laid them on the ground, six pots wide and 75 long. We placed the top of one pot just below the bottom of the pot above (north of it) and laid the stem on top of the pot above it. Since the south end of the row has a greater tendency to dry out, we laid down a row of “blank” pots (pots without plants but filled with moist soil). It is very helpful to place plants by group to make spring removal easy. 5) We spread rodenticide bait around the perimeter of the row every 60 to 90 cm. 6) We positioned cinder blocks every 60 to 90 cm along the sides and ends of the row. 7) We rolled microfoam (laminated polypropylene) horticultural insulation blanket (Ametek Microfoam Div., Chaddsford, Pa.) over the pots and left extra to drape over the sides and ends of the row. 8) We then rolled clear polyethylene (0.01 mm thick) over the microfoam, with extra draping over sides and ends of the row. 9) We used cinder blocks to secure the polyethylene as the microfoam was brought up close to the outside pots and flush with the soil for a tight fit. It is important that this row is well sealed on all sides and that there is no slack, otherwise wind may tear the polyethylene and unduly expose the plant material. Also, fold and secure any extra polyethylene under the cinder blocks. 10) We set up an inexpensive electric deer fence for winter protection. Using this procedure during Winter 1994–95, 95% of the trees survived and were suitable for spring planting. Under Kentucky winter conditions, soil/air temperature can be maintained just above freezing using this technique, even when outside air temperatures reach as low as -30circC (Bob McNeil, Univ. of Kentucky, personal communication). There was evidence of some rodent (field mice) activity in the soil of some pots, but no feeding damage was noted. Soil moisture retention in pots was excellent by spring. Microfoam insulation can be reused for several years if taken care of properly. Microfoam is white and transmits light to the plants under it. Once the chilling requirement has been met and temperature and light are adequate, budbreak may occur under the microfoam, which is undesirable, as these succulent shoots have a poorly developed cuticle and will quickly desiccate when brought out from under the insulation. Care should be taken to ensure that plants are brought out not only before budbreak but also after the last threat of a killing frost.