Effect of herbicides applied pre- and post-emergence on forestry weeds grown from seed

Abstract

Weeds growing from seed can cause severe problems in forest nurseries and in woodland establishment by competing for resources with the young trees, leading to reduced growth and survival. Herbicides approved for use on new plantings of farm forestry and forest nurseries were usually developed originally for use in agricultural crops. As a result, information on the susceptibility of weeds that may be a problem in forestry situations rather than the rotational cropping systems used in agriculture is limited. In this series of glasshouse experiments the efficacy of residual and foliar-acting herbicides primarily used on agricultural crops was tested on 14 species of weeds. Cardamine hirsuta, Epilobium ciliatum, Poa annua, Senecio vulgaris, and Spergula arvensis were all effectively controlled by diphenamid, lenacil, metamitron, metazachlor and napropamide when applied immediately after sowing. These species were also controlled when sown into soil sprayed 1 month earlier, indicating persistent phytotoxicity of the herbicides. Cirsium vulgare, E. ciliatum, Senecio jacobaea, Trifolium repens and Urtica dioica grown from seed were well controlled by low dose pre-emergence applications of atrazine, clopyralid+cyanazine, metazachlor and simazine. Ranunculus repens was effectively controlled by pendimethalin and propyzamide. Cyanazine applied early post-emergence also controlled all these species, but their susceptibility to other herbicides varied. Pre-emergence control of the perennial weed species Agrostis stolonifera, Rubus fruticosus and Rumex obtusifolius grown from seed was given by low doses of atrazine, clopyralid+cyanazine, metazachlor and propyzamide. Isoxaben, napropamide and pendimethalin were effective on A. stolonifera and R. obtusifolius. Early post-emergence applications of atrazine and cyanazine controlled all these species; other herbicides varied in efficacy. E. ciliatum, R. repens, S. jacobaea and U. dioica as small seedlings were well controlled by recommended doses of amidosulfuron, pyridate and tribenuron-methyl. T. repens was generally unaffected even at the earliest growth stage. The doses of residual herbicides required for weed control were relatively low, indicating the greater susceptibility of weeds grown in containers compared with field conditions, probably due to restricted rooting depth. The results provide guidance on which herbicides may be worth testing in field conditions for the control of these problem weeds, however diphenamid is no longer commercially available and cyanazine is currently not being supported by the EC Pesticide Review Programme.