There are 9 species of thistle known to exist in Florida. Most thistles are biennial plants. They begin as a rosette. The thistle remains in this stage for the first year. In year two, the thistle sends up a stalk and flowers, produces seeds and dies. Thistles bolt (send up the stalk) January through July and flower from April through August. Each plant can produce up to 4,000 seeds.
The key to controlling thistles is to keep the plant from flowering and producing seeds.
Mechanical or hand removal is effective but may not be efficient. To hand remove, you can simply cut off the entire rosette just below the soil surface or cut off the bolt before it flowers. This is tedious if you have a large amount of thistle on your property. Mowing is a way to mechanically cut the bolt and stop seed production but is less effective because not all will be bolting and flowering at the same time.
Herbicides are often more flexible and less time consuming. However, timing is extremely important. Thistles in the rosette stage are the most susceptible to herbicides but are hardest to see in the field since they lay flat on the ground. Once the thistles bolt, they are harder to kill with herbicides. Using 2,4-D, thistles are easily controlled in the rosette stage but are only 40% controlled at flowering. Remember that 2,4-D is an Organo-auxin herbicide and therefore anyone who applies this herbicide must follow the Florida Organo-auxin Herbicide Rule.
Scouting fields for thistles January through March can save time and money since timing of treatment is so important. Herbicide applications during the rosette stage are more effective. If thistles bolt, cut the stalks off before they flower. The goal is to stop seed production and therefore stop the spread of thistles. Most thistles only live for 2 years so just by stopping seed production you can decrease the number of thistles on your property.
For more information: Thistle Control in Pastures