Mare's-tail is a common weed also known as horseweed. The plant is an annual that can appear in the spring or fall, reaching heights of 6 feet, with leaves that increase in size the farther down the plant you go. Some varieties of mare's-tail are resistant to glyphosate, one of the most common herbicides. This resistance is especially prevalent in the central United States.
Pull up any existing mare's-tail weeds in your yard by hand. Get up as much of the root system as possible. If the weeds are mature and do not come up easily, use a hand spade to dig them out.
Discard the weeds in the trash. Do not compost the weeds, because you may spread dormant seeds with the compost in the future.
Till the soil with a garden tiller at a depth of 12 inches. Tillage is the most effective method of mare's-tail control.
Apply a metribuzin herbicide to any regrowth that appears the following spring. Follow application instructions on the package. Apply the herbicide when the mare's-tail is 4 inches tall or less. Wait the instructed time for the herbicide to take effect, then till the soil again.