2017 ASHS Annual Conference

Carya cordiformis K. Koch (bitternut hickory) offers traits favored for trees used in urban forests and other managed landscapes. Due to similar growth habits, hardiness, and durability, bitternut hickory could replace Fraxinus L. (ash). Hickories nonetheless often are overlooked due to claims of resistance to successful transplanting, a supposed result of dominant taproots and minimal development of lateral roots. Root pruning is used to modify coarse root systems and to remove circling roots in container-grown plants. The tolerance of and responses to root pruning of many taxa, including the ideal timing and extent of root removal, are unknown. How container-grown seedlings of bitternut hickory respond to root pruning during production has not been described. We examined the effects of the timing and extent of root pruning on root morphology and shoot development of 1-year-old seedlings of bitternut hickory. Our hypothesis was that moderate root pruning during dormancy would increase fibrous (<2 mm diameter) first-order lateral roots and coarse roots arising from the pruning wound (taproot branches) without restricting the development of the shoot. The distal one-third (moderate) or two-thirds (severe) of the taproot of seedlings was removed either when plants were dormant or when buds were swollen; these two times of pruning were compared because the annual initiation of root growth occurs before bud break. Stem height, caliper, first-order lateral roots, taproot branches, and shoot and root dry weights were quantified 183 days after roots were pruned. Moderate pruning did not reduce the weight of the shoot or the root system in either dormant plants or the plants with swollen buds. Moderate and severe pruning led to one to three and to zero to three taproot branches, respectively. Root pruning did not increase the mean number of first-order lateral roots, regardless of the timing and extent of pruning. Severe taproot pruning when buds were swollen reduced caliper and height of shoots, number of first-order lateral roots, as well as the shoot and root dry weight. These effects did not manifest for plants treated when dormant. We conclude that moderate root pruning during dormancy or when buds are swollen, as well as severe root pruning during dormancy, induces taproot branches of one-year-old seedlings of bitternut hickory. Over time, these taproot branches may lead to desirable architecture of container-grown plants.