Thistle is an aggressive, opportunistic plant that is easily identified by its purple blossoms and sharp spines. They often invade gardens and planting beds and, once established, they can be difficult to get rid of. If allowed to grow unchecked around the roots of juniper, thistle will multiply and rob the juniper bush or tree of essential nutrients and water.
Cut the thistle back to ground level with pruning shears. Be sure to wear gloves when you do this; thistle's spines are sharp and some people are allergic to the chemicals they emit.
Apply a broad-leaf herbicide prescribed for use on thistle when the thistle regrows to a few inches. Broad-leaf herbicides are generally safe to use around trees and shrubs. But you should still limit the likelihood that the herbicide will come into contact with the juniper. Granular herbicides are ideal because they offer the least risk of spreading to other plants. If you use a spray herbicide, apply directly onto the thistle on a windless day when the spray is unlikely to spread. Follow the manufacturer's instructions for application.
Reapply the herbicide if the thistle comes back. After a few applications, the thistle should be gone for good.