Boneset (Eupatorium perfoliatum) should be pruned during the early spring while it is still dormant or during the late fall after it has finished blooming. To prune a boneset, first remove any dead or damaged stems, cutting them back to their base or to healthy wood. Next, cut back any remaining stems by about one-third to one-half of their length to encourage bushier growth and more blooms. Make clean, angled cuts with sharp pruning shears. Additionally, if you wish to control the size and shape of your boneset, you can pinch back new growth or remove some stems entirely.