APS_JANUARY2024

J ournal of the A merican P omological S ociety

16

Materials and Methods Vigorous, two-year-old American elder berry plants, which had been produced using air-root pruning, were obtained from a com mercial nursery (Forest Keeling Nursery, Els berry, MO) on 1 Nov 2021. Plants arrived in polyethylene containers (8.78-L, 19.1 cm-tall, 27.9 cm-diameter at rim, 22.0 cm-diameter at the base) with roots in a pot-bound condition. The potting medium consisted of 8 pine bark: 4 perlite: 2 sphagnum peat moss: 1 vermicu lite: 1 sand (v/v) amended with 2 kg Osmo cote 13N-5.7P-10.8K, 1.5 kg Nitroform Blue Chip 38N-0P-0K (AgrEvo, Wilmington, DE), and 1.7 kg Micromax micronutrients (Scotts Co., Marysville, OH) per m 3 . Plants were placed in an outdoor nursery area at the Hor ticulture and Agroforestry Research Center, New Franklin, MO, where they naturally de foliated. On 28 Nov. 2021, plants were cov ered with a polyethylene foam blanket (Hum mert International, St. Louis, MO) and plastic sheeting for winter protection. On 4 Apr 2022, plants were temporarily un covered for pruning and repotting into larger containers using four treatments. Treatments included: 1) root pruning + transplanting into a larger container (RP + T); 2) root pruning + no transplanting into a larger container (RP + NT); 3) no root pruning + transplanting into a larger container (NRP + T); and 4) no root pruning + no transplanting into a larger con tainer (NRP + NT). For root pruning, a bat tery-operated reciprocating saw (Milwaukee Tools, Jackson, MS) was used to remove 2.5 cm around the perimeter of the root ball where the plant tissue had been in contact with the sides of the 8.8-L container. Also, the bottom 2.5 cm of the root ball, which contained cir cling roots as large as 0.8 cm-diameter, was pruned with the saw. After pruning, plants were either transplanted into larger 14.1-L containers (Classic 1600; Nursery Supplies, Chambersburg, PA) using the same potting medium described above or left in the origi nal containers. The dimensions of the 14.1-L containers were 26.7 cm-tall, 29.8 cm-wide at the top rim, and 26.7 cm-diameter at the

shoot growth (Fang et al. 2020). Container production also has disadvan tages including less space for root growth and retention of moisture and nutrients, which ul timately affects plant growth, flowering, and several physiological processes (i.e., photo synthesis, root respiration, etc.) (Ferree et al. 1999; Poorter et al. 2012). Moreover, when perennial plants are retained for successive seasons, roots become pot-bound with large canopies that are susceptible to damage dur ing periods of high winds if not adequately protected or secured in place (McClure BA, personal communication) Growth-regulating compounds, such as daminozide (Cahoon et al. 1977; Luckwill and Silva 1979; McGregor 1987), paclobutrazol (Curry 1988; Greene 1986), and prohexadi one-calcium (Byers and Yoder 1999; Elfving et al. 2003; Greene 1999; Kon et al. 2020) have been used to reduce vigorous shoot growth of perennial woody plants temporarily. Another strategy for controlling excessive shoot growth of red raspberry (Darnell et al. 2008), grape (Ferree et al. 1999; Lee 1997; Webster et al. 1997), apple (Campbell et al. 2022; Ferree 1992; Saure 2007; Schupp and Ferree 1987, 1988, 1990), and pear (Poni et al. 1992) has been root pruning. While this practice decreas es plant size, it can also affect nutrient uptake, water relations, photosynthesis, fruit set, and fruit size (Darnell et al. 2008; Geisler and Fer ree 1984; Schupp and Ferree 1988). Previous work with container-grown Amer ican elderberry has focused primarily on pol lination and disease management (Warmund and Mihail 2019; Warmund et al. 2019, 2022). Moreover, cultural information on maintain ing potted plants for fruit production in suc cessive growing seasons is limited. Nonethe less, shoot growth becomes vigorous without dormant pruning and roots become restricted in pot culture (McClure BA, unpublished data). Thus, a study was conducted on vigor ous two-year-old American elderberry plants to determine the effects of root pruning and transplanting into larger containers on subse quent growth and fruiting.