Optimising root pruning responses in mature Platanus x acerifolia trees

Pruning is a critical cultural practice in arboriculture. A long history of research and experience has informed current best practices on the timing and methods used during branch pruning. But comparatively little is known about root pruning. To improve our understanding, three roots from each of 32 mature Platanus x acerifolia (Aiton) Wild trees were exposed via air spade and severed. Root pruning was undertaken in October 2020 and April 2021, representing Southern Hemisphere spring and autumn pruning timings, respectively. Roots were pruned internodally or at a junction (to a side root). Together, these effects resulted in four treatments: I-S (internodal pruning in spring), I-A (internodal pruning in autumn), J-S (junction pruning in spring), and J-A (junction pruning in autumn). Three years after these treatments were imposed, pruned roots were re-exposed with the air spade, excised, and regrowth and discolouration responses were measured. Most new roots grew within 10 mm of the pruning cut, around the traumatic cambium and in callus tissue formed in this zone. The number of new roots produced following root pruning was affected only by position (internode, junction), where roots pruned internodally produced an average of 26.93 new roots in contrast to the junction roots’ average of 19.44. The percentage of the cut roots' cross-sectional area (CSA) recovered by new root regrowth was significantly affected by position, and the interaction between position and season. Roots pruned internodally produced 2.5 times more of the cut root's CSA than the junction roots. The greatest amount of the cut roots’ CSA was regrown by roots pruned internodally in spring, which recovered 17.93 %. When the severed roots were dissected longitudinally through the central axis, internal wood discolouration following root pruning was significantly affected by season but not position. Roots cut in spring had significantly more discolouration than roots cut in autumn. Overall, the least amount of wood discolouration was seen when roots were cut back to a side root in autumn (J-A). In this experiment, optimising the response of London plane trees to root pruning depended on whether the desired outcome was to maximise or minimise root regrowth, which could be achieved by pruning roots internodally in autumn (I-A), or to a side root in autumn (J-A), respectively.