Hickories are deciduous woody perennials of the genus Carya (Elias, 1980). Most are stately trees of medium to large size, exhibiting grand ornamental features that justify their use as shade trees. All are recognized for their large, protuberant taproots (Thompson and Grauke, 1991). The characteristic taproot and minimal root branching of the hickories are often associated by nursery growers with the slow initial development of seedlings and a resistance to successful transplanting. For example, Dirr (2009) attributed the 60% mortality of transplanted seedling liners of Carya ovata to their carrot-like taproot. Because of this issue, the extent of production and horticultural application of hickories in the nursery trade remain minimal.
Harris et al. (2001) suggested that root pruning of Quercus rubra L. (red oak), which presumably results in a more branched or fibrous root system, may facilitate transplanting and container production. Root pruning can affect the number of lateral roots and subsequently the likelihood of transplant success (Schultz and Thompson, 1990). Some evidence suggests that increasing the number of lateral roots by removing parts of the taproot can result in an increase in tree height (Woodroof and Woodroof, 1934). Zhang et al. (2015) observed no difference in the height or caliper in seedlings of Carya illinoinensis (Wangenh.) K. Koch after exposure to root-pruning treatments; however, they found root pruning promotes the growth of first-order lateral roots. The type and size of lateral roots may also govern seedling establishment after transplanting. Coarse lateral roots arising from pruning sites as a result of undercutting in the field, sometimes referred to as wound roots or taproot branches, are considered important for posttransplant establishment and survival (Schultz and Thompson, 1990). Initiation of lateral roots is stimulated by auxin and other growth regulators (Esau, 1965). Plant growth regulators have been used with other species that perform poorly after transplanting, such as Q. rubra and Liriodendron tulipifera (L.). Crunkilton et al. (1994) exposed the roots of seedlings of Q. rubra to IBA by incorporating granules of IBA-infused clay into the growing medium and described increases in root length, shoot diameter, shoot dry weight, and leaf area. Carlson (1974) observed an increase in the number of lateral roots by 24 times in seedlings of Q. rubra after exposure to indole-3-acetic acid (IAA) solution. Kelly and Moser (1983) found root regeneration could be enhanced by soaking the roots of seedlings of L. tulipifera in solutions of IBA before transplanting.
Little information exists on the morphology of seedlings of Carya spp., other than C. illinoinensis, and on effects of root pruning on seedling shoot and root morphology of hickories. Often, investigations on the effects of root pruning with coarsely rooted species focus on seedlings past their first year of growth (McCraw and Smith, 1998) without considering modification of seedling root morphology through root pruning shortly after seed germination. Furthermore, there has not been any investigation of the effects IBA may have on lateral root branching of any Carya species. Our objectives were to quantify the number of fibrous first-order lateral roots, branching of the taproot, and shoot development of five species of hickory, as well as to determine the effects of root pruning and auxin on root morphology and shoot growth.
Carlson, W.C. 1974 Root initiation induced by root pruning in northern red oak. Forest Res. Rev., Ohio Agr. Res. Dev. Ctr., Wooster
Crunkilton, D.D., Garrett, H.E. & Pallardy, S.G. 1994 Growth and ectomycorrhizal development of northern red oak seedlings treated with IBA HortScience 29 771 773
Dirr, M. 2009 Manual of woody landscape plants: Their identification, ornamental characteristics, culture, propagation and uses. 6th ed. Stipes, Pub., Champaign, IL
Elias, T.S. 1980 The complete trees of North America: Field guide and natural history. Van Nostrand Reinhold Company & Times Mirror Magazines Inc., New York, NY
Esau, K. 1965 Plant anatomy. 2nd ed. John Wiley & Sons, New York, NY
Harris, J.R., Niemiera, A., Fanelli, J. & Wright, R. 2001 Root pruning pin oak liners affects growth and root morphology HortTechnology 11 49 52
Kelly, R.J. & Moser, B.C. 1983 Root regeneration of Liriodendron tulipifera in response to auxin, stem pruning, and environmental conditions J. Amer. Soc. Hort. Sci. 108 1085 1090
Ouedraogo, F.B., Brorsen, B.W., Biermacher, J.T. & Rohla, C.T. 2018 Alternate methods of transplanting pecan trees HortTechnology 28 795 798
Prager, C.M. & Lumis, G.P. 1983 IBA and some IBA-synergist increases of root regeneration of landscape-size and seedling trees J. Arboric. 9 117 123
Thompson, T.E. & Grauke, L.J. 1991 Pecans and other hickories (Carya), p. 839–904. In: J.N. Moore and J.R. Ballington (eds.). Genetic resources of temperate fruit and nut crops. Int. Soc. Hort. Sci, Wageningen, The Netherlands
Vandevender, J.C. 2014 Propagation protocol for production of field-grown Carya ovata (Mill.) K. Koch plants (1-0). 22 Feb. 2016. <https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/wvpmcot12151.pdf>
Wood, B.W. 2003 Propagation of major tree nut crops cultivated in North America, p. 21–53. In: D. Fulbright (ed.). Nut tree culture in North America. Vol. 1. Northern Nut Growers Assoc., Inc. Pub., Northfield, OH
Zhang, R., Peng, F.R., Yan, P., Cao, F., Liu, Z.Z., Le, D.L. & Tan, P.P. 2015 Effects of root pruning on germinated pecan seedlings HortScience 50 1549 1552