Plant trees in the field when they are still dormant. Although trees can be planted in the fall, this is a more risky practice than spring planting. To ensure success in fall planting, the trees must be hardened-off and acclimated properly (i.e., naturally defoliated). Plant them before the first killing frost. Spring-plant a dormant tree that has had its chilling requirement fulfilled at the time of local, native pawpaw budbreak (early April in Kentucky). Planting holes should match the existing containerized root system. For 7.6-liter containers, a handheld power auger with an 8-inch drill bit works nicely for drilling holes. Do not plant trees when soil is too dry or wet. Hole drilling in wet, clayish soils will result in glazing the hole walls, which may impede root penetration into the soil. Soil should be well drained, deep, fertile, and slightly acid (i.e., pH 5.5–7). Preplant soil tests are desirable to make necessary amendments. Plant trees such that the soil line of the pot is even with the soil line of the field. Water-in trees immediately after planting to ensure adequate root-to-soil contact. Seedlings must also receive adequate water in the year of establishment. Pawpaw trees establish and grow best when they are given shelter the first year in the field. This is accomplished by using tree shelters that are used in reforestation (i.e., 1-m TUBEX tree shelters; Treessentials Co., St. Paul). Tubes can be secured to steel reinforcing bars or pressure-treated wooden posts. Tree shelters provide several benefits in the year of establishment: 1) reduction of light intensity incident upon tree; 2) wind reduction; 3) water conservation; 4) protection from browsing animals; and 5) protection from herbicides. At least one nursery that sells container-grown pawpaw trees also sells tree shelters and strongly urges their customers to use them. This practice has significantly reduced the need to send replacement plants later (Ken Asmus, Oikos Tree Crops, Kalamazoo, Mich., personal communication). Tree tubes are not without problems, however. Mice sometimes are attracted to them as winter shelters; invariably they sever the sapling’s trunk to enlarge their nests (R.N. Peterson, personal communication). Occasionally, fall acclimation of trees grown in shelters may be delayed and tip dieback from cold injury may occur. Tree shelters may not be well suited in the Deep South of the United States. They have caused excessively high temperatures that are injurious to the plant (Dean Evert, Univ. of Georgia, personal communication). If seedlings or grafted trees to be planted are 1 m or taller, tree shelters are not needed.
Weed control is necessary, especially in the establishment year. Depending on site, soil erodibility, whether tree shelters are used, etc., herbicides can be used to control weeds. At KSU we have used wood chip mulch to a 15-cm depth and have achieved good weed control and water conservation in the year of mulching. This step is labor intensive, and cost is a factor if one does not have access to free or low-cost chips. In the Deep South, mulch breaks down too readily to be practical (Dean Evert, personal communication). Another alternative is weed mats (VisPore tree mats; Treessentials Co., St. Paul). Mats are 1 m², 0.06 mm thick, ultraviolet-stabilized black polyethylene, with 620 heat-molded microfunnels per 10 cm². Weed competition is restricted from the root zone and water conservation is greatly enhanced for up to 3 years. These tree mats are designed to be used along with tree shelters to conserve water and avoid supplemental irrigation.
Granular fertilizer can be broadcast in the spring. Fertigation with liquid fertilizer can be useful if drip irrigation is installed in the orchard. If weed mats are used, run the drip line and emitters under the mats. We do not currently have recommendations for feeding field-planted trees, but we fertilize with 20N–20P–20K Peter's water-soluble fertilizer plus trace elements at 500 mg·liter-1 once in May, June, and July during the active growth phase. Fertilizer recommendations based on foliar and soil analyses need to be developed. Recommended tree spacing at present is 5.5 m between rows and 2 m apart in the row. Row orientation should be north–south. spring during budbreak. If many roots are lost during digging, prune the shoot to bring it into balance with the existing root system. Containerized seedlings transplant with high success.
Transplanting trees from the wild usually is unsuccessful. Young trees dug from a thicket or grove are often root suckers with few, brittle roots that have very few root hairs. Due to the poorly developed root system and frequent absence of shelter following transplanting, transplanting shock is usually severe, resulting in the death of the root sucker. Transplanting seedlings is most successful when done in the
spring during budbreak. If many roots are lost during digging, prune the shoot to bring it into balance with the existing root system. Containerized seedlings transplant with high success.
Pawpaws are easily propagated by several cloning methods, such as grafting and budding. These techniques include whip-and-tongue, cleft, bark inlay, and chip budding. Chip budding is most successful when the seedling rootstock is at least pencil thick in diameter and actively growing. Winter-collected, dormant scion budwood should have had its chilling requirement fulfilled. Do not graft flower buds onto rootstock. For storage, dip the shoot bases of the budwood in paraffin, place in a labeled polyethylene bag, and heat-seal it. Under these conditions, budwood can be stored reliably in a refrigerator for up to 3 months. Do not allow budwood to dry out, but I do not recommend wetting paper towels or adding free water to the plastic bags, as this can lead to microbial overgrowth. Larger vegetative buds have a higher success rate than smaller buds. When performing chip budding, try to match the diameter of the budwood with that of the stock plant. It is best to place buds on the smooth side of the stock between the nodes. Since vegetative buds occur oppositely and alternately on the stock and branching occurs in one plane, budding on the side perpendicular to this plane reduces confusion as to which shoot is the grafted one. I recommend wrapping the graft with Parafilm M laboratory film (American National Can, Greenwich, Conn.) strips cut to 2 × 15 cm. Parafilm M is flexible, moldable, self-sealing, and moisture resistant. When stretched, it applies adequate pressure to stimulate callus production and it maintains good humidity for union formation. We wrap buds starting one wrap above the bud, proceeding over the bud to one wrap below, and then returning back up and tying off above the bud. Within 2 weeks, buds will begin expanding and may penetrate the Parafilm M or just enlarge under it. In the latter case, Parafilm M is easily removed by using a sharp budding knife and making a shallow incision along the length of the wrap on the side opposite the bud. Take care not to damage the scion bud in the process of removing Parafilm M. Once scion growth commences, we recommend cutting back the top of the stock to 30 to 60 cm in height, leaving six or more functional lower leaves, and rubbing off all competing buds on the rootstock. This procedure will reduce apical dominance and enable the bud to grow actively while maintaining photosynthesis to supply carbohydrates and nutrients for the developing scion. We have found that cutting the stock back to just above the bud reduces scion growth. Once the scion is 30 cm long or more, remove the lower leaves on the stock and cut the stock back to a height of 20 to 25 cm above the union. Under greenhouse conditions, we also have found that leaving the stock as described above is extremely helpful because the stub of the stock projecting above the union then provides a “stake” to secure the growing scion. Some scion varieties appear to grow more horizontally from the graft union than others and using this technique ensures uniform habit and development of a strong union and vertically oriented scion. We use masking tape to tie up the scion as it grows. Once the scion becomes woody, this tape is no longer needed. Starting with a large, healthy rootstock, up to 1.5 m of scion growth can be attained in as few as 3 months under the optimal greenhouse conditions described above. Whip grafting is successful on seedling rootstock material that is as small as 3 to 4 mm in diameter, provided the scion is of similar diameter (Jim Gilbert, Northwoods Wholesale Nursery, Molalla, Ore., personal communication).
Other vegetative (clonal) propagation techniques such as root cuttings, hard and softwood cuttings, and tissue culture have met with poor or marginal success in the past by others. Current efforts here at KSU have demonstrated that winter-harvested root cuttings will sucker profusely when grown in the greenhouse with bottom heat. We have had more success with large (>2 cm in diameter) cutting pieces than smaller cuttings. We are attempting to root these cuttings and to culture nodal explants. One commercial business is attempting to root pawpaw hardwood cuttings in the greenhouse (Mike Byrne, Propagation Technologies, Lawton, Mich., personal communication). We are also conducting experiments to clonally propagate pawpaws by softwood cuttings and mound layering in the field. Hickman (1985)