Effect of foliage-applied imazapyr and glyphosate on common forest weed species and Norway spruce

Abstract

The phytotoxic effects of imazapyr, applied at varying doses and times, were tested in field experiments on common forest plant species in south-eastern Norway. Glyphosate was used as a reference herbicide. Imazapyr, applied in July or August, gave excellent overall control of weed and brush species. Doses of 100–150 g a.e. ha −1 of imazapyr had an equivalent effect to the recommended dose of glyphosate (1000 g a.e. ha −1. Grasses and broad-leved species showed approximately equal susceptibility to imazapyr, but when mixed vegetation was treated, low broad-leaved species re-invaded the treated areas faster than grasses. When Norway spruce was treated with imazapyr during the dormant period, aerial shoot growth was stopped for one to three growing seasons depending on the rate of application; however, there were no visible symptoms of injury. When the imazapyr treatment was applied outside the dormant period, shoot tips were killed. Preplanting applications of imazapyr at 100–1600 g a.e. ha −1 in summer had no adverse effect on Norway spruce planted the following spring. The experiments have shown that imazapyr is an interesting alternative to glyphosate in preplanting situations.