The best time to divide yarrow is in fall or very early spring. Achillea 'Moonshine' which is just coming into bloom now in my garden, is best divided in fall since it blooms so early in spring. And, it's a clumper that gets woody over time, so it must be divided to keep it vigorous. Most other yarrows are slow spreaders and bloom somewhat later. I divide those in early spring by just digging into the side of the plant and lifting what I want.