An SFGate article suggests that pruning should be done on a silver mound plant, also known as Artemisia schmidtiana, after the flowers fade in the summer season. Pinch back shrubs in April or May to shape them.
To prune a silver mound plant, take one-third to one-half of the branch off with pruning shears to encourage new growth the following season. When pruning, also remove any dead or broken branches. The shrub is deer resistant and is hardy in USDA Hardiness Zones 3 to 8. The silver mound plant is valued for its pink, blue or lavender blooms and its silvery foliage.