Using Growing Degree Days to Schedule Florasulam Applications for Dandelion (Taraxacum officinale) Flower Suppression.

Dandelion (Taraxacum officinale) is a deeply taprooted perennial broadleaf. It is one of the top problem weeds for lawn care professionals in the Midwest. Dandelion is particularly obvious to the homeowner in early spring during peak flowering. Defendor (florasulam) is a new active ingredient to the turf industry. The unique result from Defendor applications is that it prevents dandelion plants from flowering. The objectives of this study were 1) Determine how early an application of Defendor can be made and still provide optimum flower suppression, 2) Determine what dandelion growing degree day an application of Defendor provided optimum flower suppression, and 3) Determine at what dandelion growing degree day will an application of Defendor cease to prevent dandelion flowering. The experiment was conducted at two locations in 2013 in West Lafayette and Indianapolis, IN and again in 2014 in West Lafayette, IN. Herbicides were applied in 815 L/ha (87 gpa) water with a CO₂ pressurized sprayer at 207 kPa (30 psi).  Bloom reduction of dandelion was rated as percent reduction in the number of blooms per plot.  Additionally, using digital image analysis, we quantified the percent of the images containing yellow pixels (dandelion blooms) with ImageJ.  All data were analyzed using SAS (SAS institute, Inc.). Means were separated using Fisher’s protected least significant difference when F tests were significant at α=0.05. This data suggest that applications of Defendor should be applied at or before the accumulated growing degree days reach 70. All applications prior to 70 accumulated growing degree days provided similar results for dandelion flower suppression.