Field pansy (Viola rafinesquii) is a winter annual that can germinate in either the fall or spring, and is sometimes called "Johnny-jump-up" because of its rapid spring development. The leaves of field pansy are mostly smooth and without hairs. The leaf margins have small notches that become more easily detectable as the plant matures (Figure 1). The leaves are mostly round or oval, but become more narrow and linear up the flowering stem (Figure 2). Another distinguishing feature is the presence of stipules that can be as much as 1-inch in length, which occur along the flowering stem where the leaf bases join the stem. Field pansy has attractive pale yellow to purple flowers which consist of 5 petals and 5 sepals. The petals are most often blue to purple, often with dark purple lines within and in the "throat" of the flower, the colors fade from blue or purple to white or sometimes yellow (Figures 3 and 4). The sepals are much smaller in size and inconspicuous when compared to the petals, but are light green in color and hairless. Field pansy will usually only reach 5 or 6 inches in height when fully mature but can form mats throughout no-till crop production fields where dense infestations exist.
Field pansy has received more attention in recent years because it is one species that is not controlled well by "standard" burndown applications of glyphosate in the spring. Even burndown applications of glyphosate plus 2,4-D have not provided acceptable levels of field pansy control in many no-till corn and soybean fields. Weed scientists at Kansas State University have conducted some research on the control of this species in recent years. Their research, along with other work done on the control of this species, has shown that fall applications, especially fall applications of herbicides with residual activity, should provide good control of field pansy. Research has also shown that even a single glyphosate application in the fall will provide better control of this species than the same amount of glyphosate applied in the spring.
If fall applications are not made and this weed is present in the spring, the addition of dicamba (Banvel, Clarity, Distinct) to a glyphosate burndown should provide much better control of field pansy than standard burndown applications of glyphosate plus 2,4-D, although a longer replant interval will be required when dicamba has been applied. Some researchers have found that even as little as 2 fluid ounces of Clarity plus glyphosate plus 2, 4-D will enhance field pansy control dramatically. Another option other than adding dicamba to the burndown is to add a preplant herbicide to the glyphosate plus 2, 4-D, one that has both contact and residual activity on field pansy. In corn, products that we know will enhance the burndown and residual control of field pansy include atrazine, Balance, and any Callisto-containing product (Lumax, Lexar, etc.). In soybean, products that contain FirstRate (Authority First, Sonic, Gangster) and Harmony GT (Basis, Envive, Resolve Q, etc.) will also enhance the burndown and residual control of field pansy compared to applications of glyphosate plus 2, 4-D alone.
REVISED: October 2, 2015