Ask Mr. Smarty Plants

Artemisia, like many other perennials do have a general lifespan and will become less vigorous as each year passes. You will also notice that the plant will get woodier as it ages. It is important to divide many perennials every 3-4 years so they keep producing vigorous new shoots. With Artemisia, take a sharp spade and slice through the clump removing the older center portions. Keep the younger, more vigorous parts of the plant from the outer perimeter of the clump.

Julie Ryan in her book "Perennial Gardens for Texas" says that 'Silver Mound' (a cultivar of non-native A. schmidtiana) is very fine-leaved and creates soft mounds of misty, cloud-like foliage. It is not usually invasive. Seldom over a foot in height, it makes an excellent border or rock garden plant. The middle of the clump tends to die out, but well-drained soil, allowing ample room for each plant, and trimming plants back before they bloom help prevent this.
Artemisia are not set back by the heat of late summer except in high humidity: the combination makes them prone to rot.  She also writes that it likes well-drained soil and does better in poor, sandy soils than rich ones.

So, for next year, prune them somewhat hard again in the early spring and perhaps divide them and replant the more vigorous sections at that time too.