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Journal of the American Pomological Society 78(1): 15-23 2024 Root Pruning and Increasing Container Size of Pot-Bound American Elderberry Plants M ichele R. W armund 1 , E lijah J. P oehlman , and M ark R. E llersieck Additional index words: Sambucus nigra subsp. canadensis , fruit quality, fruit yield, plant growth Abstract American elderberry fruit is typically grown in the field at relatively low plant densities due to its vigorous growth habit. To investigate the potential for container production, two-year-old American elderberry plants that had pot-bound roots were subjected to the following treatments in 2022: 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 container (NRP + NT). In 2023, young plants were obtained to duplicate the experiment. In 2022 and 2023, the RP + NT treatment reduced shoot growth by 38.8% and 25.5% respectively, compared with the NRP + T treatment. Also, the NRP + T plants consistently had the greatest shoot dry weight among treat ments. Root dry weight of RP plants was reduced by 30.5 to 34.0% compared with the NRP plants. The T treatment mitigated the effect of root pruning on root dry weight. In 2022, none of the plants produced fruit, which may be attributed to adverse weather conditions. In 2023, all plants produced fruit, except those re ceiving the RP + NT treatment. Although the NRP +NT plants produced the greatest yield among treatments, further refinements in the container-based system are needed to increase fruit yield.

American elderberry [ Sambucus nigra L. ssp. canadensis (L.) Bolli] is a medium-sized, herbaceous shrub with an extensive root sys tem (Charlebois et al. 2010). On these shrubs, drupes are produced in compound umbels (Zomlefer 1994). Historically, the fruit was used for folk medicine, dyes, and for culi nary purposes (Moerman 1998; Stang 1990). More recently, elderberry flowers are used as a flavorant in beverages and fruit is used as a health supplement, a food and drink ingredi ent, and for liqueur and wine (Charlebois et al. 2010; McGowan 2017). Since the late 1990s, there has been a re surging interest in elderberry cultivation, particularly in the midwestern United States where producers are seeking niche crops (Byers 2010). Several high-yielding selec tions have been acquired from the wild and re leased for planting (Byers and Thomas 2011; Gasic et al. 2020; Warmund et al. 2016). For commercial field-production, typical plant

spacings are 1.2 m in the row and 3.0 to 3.7 m between rows (2272 to 2777 plants/ha) with mature plants producing 4483 to 8967 kg∙ha -1 of fruit (Byers et al. 2014). However, as the elderberry industry evolves, there is interest in alternative production systems, such as con tainerized plants grown in a soilless substrate, to increase fruit yield for enhanced profitabil ity (Uhlig JW, personal communication). In addition to boosting fruit yield, contain er-based systems provide several benefits in production efficiency. Some of these attributes include the precise control of plant nutrition, automated irrigation and fertigation systems, and use of lower quantities of pesticides with less off-target movement (Li and Bi 2019). These advantages may be particularly impor tant for problematic sites with soils that are in fertile or saline, contaminated with unwanted chemicals, or infested with insects, diseases, or weeds. Also, container production allows adjustments in spacing as plants develop more

Division of Plant Sciences and Technology, University of Missouri, Columbia, MO 65211. 1 Corresponding author. E-mail: warmundm@missouri.edu Root Pruning and Increasing Container Size of Pot-Bound American Elderberry Plants