Herbicides are an important tool in managing weeds in turf and agricultural production. One of the earliest selective herbicides, 2,4-D, is a weak acid herbicide used to control broadleaf weeds. Water-quality parameters, such as pH and hardness, influence the efficacy of weak acid herbicides. Greenhouse experiments were conducted to evaluate how varying water hardness level, spray solution storage time, and adjuvant inclusion affected broadleaf weed control by 2,4-D dimethylamine. The first experiment evaluated a range of water-hardness levels (from 0 to 600 mg calcium carbonate [CaCO₃] L^–1) on efficacy of 2,4-D dimethylamine applied at 1.60 kg ae ha^–1 for dandelion and horseweed control. A second experiment evaluated dandelion control from spray solutions prepared 0, 1, 4, 24, and 72 h before application. Dandelion and horseweed control by 2,4-D dimethylamine was reduced when the CaCO₃ level in water was at least 422 or at least 390 mg L^–1, respectively. Hard-water antagonism was overcome by the addition of 20 g L^–1 ammonium sulfate (AMS) into the mixture. When AMS was included in spray mixtures, no differences were observed at 600 mg CaCO₃ L^–1, compared with distilled water. Spray solution storage time did not influence dandelion control, regardless of water-hardness level or adjuvant inclusion. To prevent antagonism, applicators should use a water-conditioning agent such as AMS when applying 2,4-D dimethylamine in hard water.

Nomenclature: 2,4-dichlorophenoxyacetic acid (2,4-D); 2,4-D dimethylamine; dandelion, Taraxacum officinale F. H. Wigg. TAROF; horseweed, Conyza canadensis (L.), Cronq. ERICA