Thistle is an opportunistic and invasive plant. Once established, it spreads quickly and may choke out other plants in the area. There are hundreds of varieties of thistle in the United States. Bull thistle is virtually the only variety that grows in wetlands. These plants can reach 8 feet in height and make access to the area difficult. Since wetland areas are protected throughout most of the United States, care must be taken to minimize damage when spraying thistle in wetlands.
Cut the bull thistle down to ground level with lopping shears. Bull thistle should be cut in early summer before the flowers develop. The stems of the plant are hollow at this time, making it easier to cut down. Furthermore, the weed has yet to produce seeds and the plant is less likely to grow back when cut down at this time. Cutting the plants first minimizes regrowth and limits the amount of herbicide necessary.
Spray the bull thistle with a low-toxicity, vinegar-based broad-leaf herbicide prescribed for use on thistle when the weeds grow back to a few inches in height. These herbicides are best for use in the wetlands because they pose the least threat to its delicate ecosystem. Spray on a day with low wind and no rain forecast for the next 48 hours to minimize the likelihood that the spray will spread to other plants. Follow the manufacturer's instructions for application rates.
Re-spray any thistle that grows back.
Plant the thistle-prone area with thick ground cover vegetation native to the wetlands in that area. This will prevent bull thistle from establishing itself in the area again and reduce the likelihood that you will have to re-spray.