Thistles thrive in many types of climates and soil conditions. All thistles share a few similar characteristics, including sharp spines and invasive root systems. The tall thistle is one type of biennial thistle. This plant requires two years to mature and reproduce. Suitably named, tall thistles reach a height of 10 feet in many areas. These invasive weeds often pose problems in pastures and landscapes, creating an eyesore and damaging nearby plants and grasses.
Examine your yard or pasture early in the spring to locate small thistles. Look for rosettes on the surface of the soil. This early growth signals the presence of thistles before they mature and reproduce.
Discourage single plants by cutting through the stem beneath the surface of the soil. Use a sharp shovel to separate the top of the plant from its root system. Use this method in areas containing small amounts of tall thistles.
Remove existing tall thistles before they reproduce and scatter seeds. Cut the plants with a mower in late spring to remove maturing flower buds before they go to seed. Cutting at this stage kills many tall thistles. Avoid cutting the thistles too early, as these hardy weeds tend to grow back and continue to mature.
Apply an herbicide to remove tall thistles from your yard. Purchase an herbicide labeled for use on tall thistles. Apply the herbicide to the area with tall thistles late in the fall or early in the spring, before flower buds form on stems. Apply when temperatures are moderate, between 60 and 85 degrees F. Follow the package instructions when applying herbicides.