River Birch (Betula nigra) Growth and Root Extension from Three Production Systems during Landscape Establishment

Twelve harvested river birch from each of three production systems (field, below-ground fabric container (FC) and pot-in-pot (PiP)) were set in above ground rows heeled in with wood chips for two months, then transplanted in June 2012. Half of the trees from each treatment were root pruned to remove any visible defects and 2” thick root mats were removed from PiP trees. In Nov. 2013 trees were air-spaded to expose roots in a 3’ wide x 1’ deep trench the length of the tree row.  We counted the number of roots at 3’, 5’, and 7’ transects and measured the extension of the longest roots in each direction. All trees were able to establish new roots, with a mean north-south spread of 11.7’ for PiP, 12.1’ for FC and 13.3’ for field-grown trees, which corresponded with total stem caliper increases of 2.0, 2.7 and 3.7” respectively. These differences were correlated with initial tree size (field-grown trees were largest and PiP smallest at transplanting). The strong radial distribution of structural roots observed in field-grown trees is purported to be the best structure for long term tree health. Circling roots in unpruned PiP rootballs were still prominent and enlarged, which would potentially restrict growth in future years; however, they did not prevent successful extension of new roots into the surrounding soil during landscape establishment. Root pruning to remove defects in PiP trees at transplanting was successful and no additional defects were observed at the termination of this experiment. The FC trees generally had good root structure but some new roots grew inward or tangentially to the original rootball.