Annual Flower Injury from Sublethal Rates of Dicamba, 2,4-D, and Premixed 2,4-D + Mecoprop + Dicamba

Greenhouse experiments were conducted to compare visible injury from sub- lethal rates of 2,4-D, dicamba, and a premixed product of 2,4-D + mecoprop + dicamba for eight annual flowers and to describe herbicide injury symptoms for these annual spe- cies. Herbicides were applied at rates 0.05×, 0.1×, and 0.2× of their highest labeled rate for turfgrass to simulate spray drift conditions. Visible injury varied between species, herbicide rate, and time after herbicide application. Alyssum (Lobularia maritima Desv.) showed the greatest initial injury and ageratum (Ageratum houstonianum Mill.) showed the greatest injury at 4 weeks after treatment. Symptom severity increased as herbicide rate increased, with the greatest injury from the premixed product, followed by 2,4-D, and then dicamba. The eight species varied in their degree of visible injury and flower produc- tion to dicamba, 2,4-D, and the premixed product. Reduced flowering was most obvious for prolific flowering species such as alyssum. Impatiens (Impatiens wallerana L.), salvia (Salvia splendens Sello), and snapdragon (Antirrhinum majus L.) produced more flowers in response to sublethal dicamba rates compared to the untreated plant. All rates of 2,4-D generally reduced flowering compared to untreated plants, except the lowest rate of 2,4-D for geranium (Pelargonium xhortorum Bailey) and snapdragon. Dahlia (Dahlia hortensis Cav.) sprayed with dicamba at the highest rate produced three times as many stems as plants untreated or those sprayed with 2,4-D. Overall order of species susceptibility to sublethal rates of dicamba, 2,4-D, or the premixed product from most susceptible to least susceptible was ageratum > alyssum > marigold (Tagetes erecta L.) > dahlia > geranium = salvia = snapdragon = impatiens. Differences in overall susceptibility to the plant growth regulator herbicides evaluated should provide useful information to horticulturalists designing annual flower beds and borders and lawn care applicators. Herbicide spray drift is a concern for ap- plicators, product manufacturers, and regula- tory agencies. The spray drift task force was created in 1990 to develop a generic database in response to spray drift data requirements set forth by the Environmental Protection Agency (EPA, 1999). Representatives from 40 chemical companies developed a database from studies on the atomization, transport and deposition of sprays from aerial, ground, and orchard airblast applications, which were used to validate spray drift deposition models such as AgDRIFT (Teske et al., 1997). Hewitt (2000) reported that even though spray drift can be modeled based on tank-mix physical properties, atomizer type, and operation, air flow and other parameters, the effect of a given amount of active ingredient is product- and species-specific. Marrs and Frost (1997) reported that most effects to nontarget plants downwind from MCPA and mecoprop, two plant growth regulator herbicides, were confined to an 8-m zone and that a few species showed increased performance at the 2- to 4-m downwind zone, which they attributed to the hormonal effect on growth processes. They also suggested that an 8-m buffer zone should be implemented to adequately protect susceptible habitats from the most deleterious impacts on community processes. Similarly, Hatterman-Valenti et al. (1995a) suggested a 1.5-m buffer area for pos- temergence broadleaf herbicide applications to turfgrass to reduce nontarget plant injury. Un- fortunately, maintaining a nonsprayed buffer area during postemergence broadleaf herbicide applications to turfgrass may not be practical, especially for small residential lawns heavily infested with broadleaf weeds. Plant growth regulator herbicides such as 2,4-D and dicamba have been used extensively for broadleaf weed control in grass crops. Marth and Mitchell in 1944 reported on use of 2,4-D to control broadleaf weeds in turfgrass. As early as 1957, injury to garden plants from 2,4-D drift was reported (Dabbs and Forsberg, 1957). In 1963 injury symptoms to trees from 2,4-D spray drift was described (Phipps, 1963). However, information on injury symptoms to annual flowers from spray drift of 2,4-D and other plant growth regulator herbicides is limited. Hatterman-Valenti et al. (1995b) compared the susceptibility of several annual bedding plants to low concentrations of triclopyr and 2,4-D, and reported that petunia (Petunia xhyb-