Improved control of Taraxacum officinale with 2,4-D in the field from ammonium sulphate additions

Taraxacum officinale G.H. Weber ex Wiggers (dandelion) is a common weed of cool-season turfgrass with 2,4-D as the most commonly used herbicide for its control. Hard water is known to antagonise 2,4-D dimethylamine herbicide efficacy in greenhouse studies, and this antagonism is mitigated by adding the water conditioning agent ammonium sulphate (AMS) to spray mixtures. As greenhouse herbicide efficacy trials do not always provide similar control to field studies, this experiment was undertaken to determine how hard water and AMS influences 2,4-D dimethylamine control of T. officinale in established lawns. Hard water was prepared to achieve 0, 275, 450 and 675 mg CaCO3 L-1. The 2,4-D was applied at each hardness level with and without AMS at 20 g L-1 in eight separate field trials across three sites and two seasons. Regardless of water hardness level, 2,4-D treatments with AMS added reduced T. officinale cover at 52 weeks after application (WAA) compared to those without AMS. Similarly, fewer T. officinale plants were recorded when adding AMS to 2,4-D. Taraxacum officinale cover at 52 WAA was higher when applied in water with a hardness of 450 mg CaCO3 L-1 or more compared to applications in distilled water (0 mg CaCO3 L-1). When hard water antagonism occurred, the addition of AMS to the spray mixture reduced the antagonism. Based on this field experiment and consistent with greenhouse experiments, applicators should mix AMS at 20 g L-1 when applying 2,4-D dimethylamine, especially in hard water.