Annual flower injury from sublethal rates of dicamba, 2,4-D, and premixed 2,4-D + mecoprop plus dicamba

Greenhouse experiments were conducted to compare visible injury from sublethal 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 species. Herbicides were applied at rates 0.05x, 0.1x, and 0.2x 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 production 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.