Herbicide options for managing competitive vegetation during the establishment of Pinus radiata and Pseudotsuga menziesii var. menziesii in Southland, New Zealand

Abstract

Background: Control of weeds is important for survival, growth and uniformity of planted conifers, such as Pinus radiata D.Don and Pseudotsuga mensiesii var. menziesii (Mirb.) Franco. Despite pressure to find environmentally benign herbicides for these species, little research has identified non-residual herbicides that are cost-effective and result in minimal growth loss compared to those used operationally. The objective of this study was to compare efficacy and cost of current operational practice to a range of alternative herbicide treatments for the establishment of these two species using data from two trials located at the same site. Methods: Treatments for each species were applied in both the first and second year following planting and included weed-free controls, operational practice using terbuthylazine and hexazinone (applied as both spot treatment and broadcast application) and a range of treatments consisting of combinations of clopyralid, triclopyr and haloxyfop (CTH), applied with or without oversowing with less competitive grasses and annual herbaceous species. Application of glyphosate during winter was included within the Pseudotsuga mensiesii trial as a first year treatment only and as a second year treatment following spot application of CTH in the first year. Results: For the Pinus radiata trial, broadcast application of CTH in spring of years 1 and 2 was most effective, with mean stand volume (m 3 ha −1 ), at 2.5 years, exceeding the weed-free control and operational treatment by 8 and 11%, respectively, although these differences were not statistically significant at the 5% level. Use of the operational treatment as a spot in the first year, followed by broadcast application of CTH in the second year, was moderately effective when used with or without oversowing. For the Pseudotsuga mensiesii trial, the four most effective treatments were the operational treatment, broadcast application of CTH during year 1 (CTH-Dec), broadcast application of CTH during spring of years 1 and 2, and broadcast application of glyphosate during winter (winter glyphosate). Although not significantly different, at 2.5 years mean stand volumes in these treatments were on average 24, 29, 33 and 35% lower than the weed-free control, respectively. Both CTH-Dec and winter glyphosate were less expensive than the operational treatment.