I have that cultivar of yarrow and it is indeed attractive. I cut it back when the flowers begin to wilt and die, cutting the stems back by half their length just above a set of leaves. Trim the yarrow back by half after each subsequent flowering. Yarrow usually flowers twice but the second time does not always occur all at once. Therefore, cutting back individual stems helps keep the plant tidy. Prune the entire plant in late fall or early winter after it dies back naturally.