Response of Common Garden Annuals to Sublethal Rates of 2,4-D and Dicamba with or without Glyphosate

An experiment was conducted in 2017 and 2018 to determine the sensitivity of common garden annuals to sublethal rates of 2,4-dichlorophenoxyacetic acid (2,4-D) and dicamba with or without glyphosate. Sublethal rates corresponding to 1/10×, 1/100×, and 1/300× of the full labeled rate (1×) of 2,4-D (1.0 lb/acre), 2,4-D plus glyphosate (1.0 lb/acre plus 1.0 lb/acre), dicamba (0.5 lb/acre), and dicamba plus glyphosate (0.5 lb/acre plus 1.0 lb/acre) were applied to ‘Prelude’ wax begonia ( Begonia × semperflorens-cultorum ), ‘Wizard’ coleus ( Solenostemon scutellarioides ), ‘Pinto’ zonal geranium ( Pelargonium × hortorum ), ‘Dazzler’ impatiens ( Impatiens walleriana ), ‘Bonanza’ french marigold ( Tagetes patula ), ‘Hurrah’ petunia ( Petunia hybrida ), ‘Titan’ madagascar periwinkle ( Catharanthus roseus ), and ‘Double Zahara’ zinnia ( Zinnia marylandica ). Visible injury, plant height, number of flowers, and dry weight were recorded at specific time intervals after treatment. When averaged across all annual plant species, the 1/10× rate of 2,4-D plus glyphosate resulted in 51% injury 28 days after treatment, whereas the 1/10× rate of dicamba plus glyphosate resulted in 43% injury. Treatments causing the greatest injury also resulted in the greatest reduction of dry weight, height, and flower production. Coleus was the most sensitive species in the study; dry weight was reduced by 16% and 18% compared with the nontreated controls from 1/300× rates of 2,4-D plus glyphosate and dicamba plus glyphosate, respectively. French marigold and zonal geranium had greater sensitivity to treatments containing 2,4-D, but coleus and zinnia had greater sensitivity to treatments containing dicamba. Petunia exhibited a high tolerance to 2,4-D or dicamba applied alone (>6% injury) but was highly sensitive when glyphosate was added to 2,4-D and dicamba (