Written by Rutendo Nyamusamba, former SDSU Extension Agronomy-Crops Field Specialist.
An erect plant growing to between one and three feet, curlycup gumweed (gumweed) is easy to notice in the pastures from July to September, its flowering period. The plant branches at the top with each branch producing an individual bright yellow flower of about one inch diameter (Figure 1). Gumweed is found on rangelands, pastures, disturbed sites and in ditches all through South Dakota. For example, given an opportunity gumweed will form nearly pure stands in overgrazed pastures. Curly Cup Gumweed has no relative feed value and in some cases is toxic to livestock and sticks to animal hair and wool, therefore the immediate action after noticing the flowering plant, is to consider available control options. The word out is that, farmers can spray high rates of herbicide when they notice it at its flowering stage, but based on research done at SDSU, once the plant is flowering, herbicide control is too late and fall herbicide control of gumweed is ineffective.
Figure 1. (Above) Curlycup Gumweed (Grindelia squarrosa) flower.
Gumweed, a member of the sunflower family (Asteraceae), is also known as gum plant, rosin weed or tarweed. It is a biennial or short-lived perennial plant that reproduces by seed. Its leaves have saw-toothed edges and alternate along the stem (Figure 2). The flower head has characteristic curved bracts (cups) (Figure 3) which exude a large amount of a sticky, gummy resin. Other parts of the plant also exude this resin.
Figure 2. (Above) Curlycup Gumweed (Grindelia squarrosa) leaves and their arrangement on the stem.
Figure 3. (Above) Curlycup Gumweed (Grindelia squarrosa) flower head.
Good grazing management is crucial to the control of gumweed. Timing for herbicide control of gumweed is best in early June but can be delayed to mid- or late June, which allows the control of both new seedlings and second-year plants. Two quarts per acre of 2,4-D ester 4L are effective for early control until mid-June while metsulfuron products (such as Escort at one ounce per acre) may be more effective for late applications (after mid-June but prior to flowering).
For further help with gumweed control or other pasture weed problems refer to SDSU Extension's 2013 Weed Control Guide for Pasture and Range; or contact your regional Extension Office or the Extension Agronomy-Field Specialist nearest you.