Root Distribution of ‘Brightwell’ and ‘Premier’ Rabbiteye Blueberries as Influenced by Pecan Shell Mulch

Abstract

Pecan ( Carya illinoinensis[Wangenh.] K. Koch) shell waste lacks effectual, economic disposal. If shells could be repurposed as mulch, then growers may be able to treat shell byproduct as a resource. In 2016, root distribution and growth of ‘Brightwell’ and ‘Premier’ rabbiteye blueberries ( Vaccinium virgatumAiton syn. V. ashei Reade) was examined using the Horhizotron™. Each HorhizotronTM had four wedge-shaped quadrants filled with 10 cm of an amended 80% pine bark and 20% sand (by volume) substrate, then 7.6 cm of “fresh” pecan shells (FPS), “aged” pecan shells (APS), pine bark nuggets (PB), or an unamended 80% pine bark and 20% sand substrate (PBS). Growth was determined weekly by measuring the horizontal root length (HRL) and root depth (RD) of the five longest roots on either side of a quadrant. Roots that grew into the substrate and mulch treatment layers were not measured separately. ‘Premier’ HRL showed roots in FPS grew a shorter distance across the quadrant profile than roots in PBS, but had similar HRL with APS and PB. In ‘Brightwell’, both shell treatments had shorter HRL across the quadrant than the roots in PB and PBS. RD measurements for ‘Premier’ showed roots generally initiated at the same depth for FPS, APS and PB, though the roots in PBS had shallower growth than the roots in PB and FPS. ‘Brightwell’ RD showed roots initiated more into the upper portions of the quadrant profile in APS and PBS than in FPS or PB. Root system architecture was reflected in root dry weight (RDW). For both cultivars, substrate layer RDW was similar across all treatments, but mulch layer RDW varied. Though APS had a higher mulch layer RDW than the PB treatment in ‘Premier’, differences in RDW within the mulch layer did not impact total root dry weight (mulch layer RDW + substrate layer RDW). In ‘Brightwell’, APS had a higher RDW than FPS and PB, though PBS was similar to both APS and FPS. Unlike ‘Premier’, total RDW in ‘Brightwell’ was impacted by differences in mulch layer RDW, as the quadrants that contained FPS and PB had a lower total root dry weight than the quadrants containing APS and PBS. These results indicated that root growth in pecan shells, as compared with root growth within and below pine bark, was not hindered.