Assessment of herbicides for selectively controlling broom (Cytisus scoparius) growing with radiata pine (Pinus radiata) in New Zealand

Abstract

Background: Broom (Cytisus scoparius (L.) Link) is an invasive weed in many commercial radiata pine (Pinus radiata D.Don) plantations throughout New Zealand. A variety of herbicides are available to selectively control young established broom plants during the first year after planting pines, but there is little published information describing which are safest for pines and most effective as the broom grows older. Methods: A pot experiment was conducted to assess the efficacy of six selective herbicide treatments applied at three rates (recommended (medium) rate, half (low) rate and double (high) rate) for controlling broom of various ages (3, 6, 9 and 12 months old). Five treatments involving registered herbicides were compared with fluroxypyr, which is not registered for use in radiata pine or on broom. As broom is a major competitor in plantation forestry, the adverse effects of these herbicides on 12-month-old radiata pine were also assessed. Results: The high rate of each treatment killed all the 3-, 6-, and 9-month-old broom plants, but only the high rate of hexazinone or the clopyralid/triclopyr mixture killed the 12-month-old broom plants. In addition, the medium rate of clopyralid/triclopyr, terbuthylazine or clopyralid/triclopyr/picloram/aminopyralid killed all 3-, 6-, and 9-month-old broom plants. The remaining treatments were not completely effective (i.e. median injury rating was less than 10). All of the registered herbicides caused minimal damage to radiata pine, but the damage caused by fluroxypyr to radiata pine was considered too great to recommend operational use of this chemical on its own. Conclusions: Mixtures based on clopyralid with triclopyr gave the best selective control of potted broom plants up to 12 months old. Such mixtures will also preserve grass ground covers. Although fluroxypyr is unsuitable for use by itself, it could be assessed in further work for use with clopyralid.