Suitability of Pecan Shell Mulch for Weed Control and Rabbiteye Blueberry Establishment

Abstract

Waste disposal for pecan (Carya illinoinensis) shells lack effectual, economic methodologies. If shell waste could be repurposed as a mulch, then pecan growers may have an opportunity to treat shell byproduct as a resource by supplying rabbiteye blueberry (Vaccinium virgatum) growers with an alternative to pine bark. Three studies were conducted to determine the effect of pecan shell mulch on weed control and rabbiteye blueberry establishment. In Feb. 2016, an on-farm planting of ‘Krewer’ rabbiteye blueberries was installed in Auburn, AL to determine the efficacy of seven treatments: “fresh” pecan shells, “aged” pecan shells, or pine bark mini-nuggets at 7.6 cm or 15.2 cm depths, and a no mulch (bare ground) treatment with no weed control except mowing. All mulch treatments had a lower weed density than no mulch, though, as the season progressed the 7.6 cm mulches resulted in higher weed density than the 15.2 cm mulches. From May¬–Aug. 2016, all mulches had higher soil moisture than no mulch; however, in Sept. 2016, 15.2 cm aged shells had a higher soil moisture than no mulch and 7.6 cm pine bark. Soil and soil-mulch interface temperatures were generally higher in the shell mulches than pine bark; however, plant size was only reduced in no mulch compared to 7.6 cm pine bark. Regarding weed control and transplant survival, pecan shells performed similarly to pine bark as a mulch during the first year of plant establishment. Root growth of rabbiteye blueberry cultivars ‘Brightwell’ and ‘Premier’ was examined using the Horhizotron™, a technology that nondestructively measures horizontal root growth. The Horhizotron™ was constructed from eight panels of glass that fastened into an aluminum base, forming four wedge-shaped quadrants around the original root ball. Each quadrant was filled with 10 cm of an amended 4:1 pine bark:sand (v/v) substrate. 7.6 cm of “fresh” pecan shells, “aged” pecan shells, pine bark mini nuggets, or an unamended 4:1 pine:sand substrate control was then randomly applied to each of the four quadrants. Root growth rates were determined weekly by measuring the length and depth of the five longest roots on either side of a quadrant. Horizontal root length (HRL) in ‘Premier’ showed that roots tended to initiate further from the original root ball into the quadrant profile in the control, pine bark, and aged shells than in the fresh shells. HRL in ‘Brightwell’ showed that roots in the control and pine bark grew further from the original root ball than in either shell mulch. In ‘Premier’ roots were more concentrated in the upper portions of the quadrant profile in the control than either fresh shells or pine bark, though aged shells were similar. Root depth in ‘Brightwell’ showed that roots in control and aged shells grew more in the upper portion of the quadrant profile than in pine bark and fresh shells. The location of root growth in the quadrant profiles was reflected in root dry weight (RDW). For both cultivars, RDW within the substrate layer was similar in all treatments, but mulch layer RDW varied. In ‘Premier’ mulch layer RDW was lower in pine bark than the remaining treatments; however, total RDW was similar across all treatments. In ‘Brightwell’ mulch RDW was lower in pine bark and fresh shells than in the aged shells, while control was similar to both shell mulches. Those differences impacted total RDW, as the quadrants that contained pine bark and fresh shells had a lower total RDW. These results indicated that as compared to pine bark, root growth in pecan shells was not hindered. The third experiment was conducted to determine the level of weed control that could be obtained by using pecan shell mulch on crabgrass (Digitaria sanguinalis), common ragweed (Ambrosia artemisiifolia), or spotted spurge (Euphorbia maculata); three problematic weed species in the southeastern United States Pine bark mulch was also included in the evaluation to provide a commercial standard for control. Results from the first experimental run showed that the weed germination by mulch depth interaction influenced weed counts of each weed species, and all mulch treatments, regardless of depth, resulted in complete control of each weed species. Data from the second and third experimental runs varied, and no distinctive trend in weed suppression was observed, though the smaller particle size of pecan shells may have been more favorable for weed seed germination for spotted spurge. Further evaluation of weed control of these weed species is recommended.