Amidosulfuron: a potential alternative herbicide to asulam for the control of bracken ( <i>Pteridium aquilinum</i> ) in woodlands

Abstract Bracken (Pteridium aquilinum (L) Kuhn) is a widespread, vigorous fern which is found throughout the world. While it is native to UK woodlands, it can also outcompete and kill seedlings when mature trees are removed and can harbor disease carrying insects. It is therefore often considered a nuisance weed that needs to be managed. However, non-chemical methods are not always practical or effective, and the herbicide asulam, which was widely utilized to control bracken, is no longer approved for use in Europe. We therefore tested the efficacy and tree tolerance of the herbicide amidosulfuron, in combination with different spray adjuvants, as possible future alternatives for the use of asulam to control problematic bracken infestations in woodland. Although further research is required to confirm crop tolerance, we found that where there are no viable non-chemical options, and where crop trees are otherwise likely to die or be severely supressed, applications of 0.023 kg a.i. ha−1 amidosulfuron [e.g. as 0.03 kg ha−1 Squire Ultra® (75% w/w amidosulfuron)] plus Mixture B NF® adjuvant at 2% of final spray volume, may be a potentially suitable future replacement for the use of asulam to control bracken in recently planted stands of coast redwood [Sequoia sempervirens (D. Don) Endl.], Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], grand fir (Abies grandis Lindl.), Japanese red cedar [Cryptomeria japonica (L.f.) D. Don], Macedonian pine (Pinus peuce Griseb.), Norway spruce [Picea abies (L.) H. Karst.], oak (Quercus robur L.), Scots pine (Pinus sylvestris L.), silver birch (Betula pendula Roth), Sitka spruce [Picea sitchensis (Bong.) Carr.], cherry (Prunus avium L.), sycamore (Acer pseudoplatanus L.), and western red cedar (Thuja plicata Donn). However, it is recommended that small scale user trials are always undertaken to test crop safety in local conditions before embarking on any large scale treatment program. Currently amidosulfuron is not approved for use in forestry situations, and there is some uncertainty over the future availability of any amidosulfuron products in the UK beyond 2028.