Are you referring to burdock, a weed which has leaves similar to rhubarb, and purplish flowers that turn into stickers or burrs? If so, it is a biennial weed. It grows vegetatively the first year, and does not flower and produce seeds until the second year. So if you spray it with a broadleaf herbicide such as 2,4-D in it's first year of growth, you may be able to prevent it from developing seeds. Because it has a fleshy taproot, it often will resprout, so repeated applications may be necessary. Also remember that seeds lying dormant in the soil can sprout in succeeding years, so you may have to continue herbicide treatments for several years.