Season of application affects herbicide efficacy in Pinus radiata plantations in the southern Cape region of South Africa

SYNOPSIS Three herbicides (glyphosate, hexazinone and metsulfuron methyl) at three rates were tested on indigenous herbaceous shrubs (macchia) in a one-year-old virgin Pinus radiata stand. A mechanical ring weeding of 1 m radius was included as a control treatment. Herbicides were applied over-the-top to uncovered P. radiata. These treatments were tested for each of four seasonal applications. Tree height and diameter growth were measured to determine efficacy of the treatments. Two floristic surveys indicated satisfactory weed suppression. Summer was the best season to apply herbicides. Glyphosate and hexazinone were significantly better than metsulfuron methyl. Hexazinone had no detrimental effect on P. radiata and improved their height growth, whereas glyphosate scorched the trees, but improved their diameter growth. Optimum application rates of these herbicides are 2 000 g and 1500 g active ingredient (a.i.) hexazinone and 1 500 g a.i. glyphosate per hectare.