Commonly known as wormwood, Artemisia "Powis Castle" grows 3 feet tall and 2 feet wide in U.S. Department of Agriculture zones 6 through 9. The plant's flower production is unreliable. When this perennial blooms, it makes hardly visible yellow blossoms from August to October. Powis Castle is grown for its silver foliage that releases a pleasant scent. This herbaceous plant develops quickly and becomes untidy. Make pruning cuts at the right places to control its shape and size.
Soak the pruning shears in a bucket filled with nine parts water and one part bleach for 20 minutes. It sanitises the blades, preventing pathogens from transferring to the artemisia plant.
Find the leaf bud that breaks open farthest from each branch's tip in late winter to early spring. Starting with that bud, count three to four live buds toward the stem's tip. Make the pruning cut 1/4 inch above the last bud you count. Slant the shears to snip the twigs at 45 degrees. Locating the live buds first guarantees you preserve live wood so the shrub grows again. Do this step just as the foliage begins to unfurl to reduce the artemisia's height and spread before the plant begins to grow vigorously.
Prune spindly and spread-out Powis Castle stems in the summer. Remove one-third to one-half of the branch's length if you can take that much out without cutting into dead wood. When trimming, retain several live buds on each stem.
Do not cut Powis Castle to the ground.