Yarrow

Leave established yarrow off the watering schedule

Once the plants have a season of roots on them, water only in prolonged drought. Keep yarrow in full sun on lean, well drained ground rather than in a bed that is irrigated on a timer. Yarrow is drought tolerant and is short lived wherever the soil stays wet, because constantly moist ground brings on root rot and favors powdery mildew.

Divide the clump in fall, once the bloom is finished

Lift the whole clump in fall, pull or cut it into sections that each carry roots and shoots, throw away the old woody center, and replant the vigorous outer pieces at the same depth. Time the work for four to six weeks before your ground freezes so the divisions root in before winter; in cold zones that means early fall, not late. Water the divisions in and expect to repeat this every two to three years. Yarrow dies out from the middle as it ages, and once the crown is hollow the plant blooms less and carries smaller heads. Minnesota Extension puts spring- and summer-blooming perennials in the fall division window, and Clemson calls for dividing yarrow every two to three years.

Deadhead spent flowers to extend bloom and stop the seeding

As the first flush of flowers browns, cut the spent heads back to the next set of leaves or down to the basal foliage. Heads left standing set seed, and common yarrow seeds itself around aggressively on top of spreading by rhizome. Cutting them also prolongs the bloom on most cultivars.

Check for powdery mildew in shade and for spittlebug foam

Through summer look over the leaves for a white powdery coating and the stems for frothy spit. Open crowded clumps up by dividing or by cutting back whatever is shading them, so sun and air reach the foliage, and wash spittlebug foam off with a strong jet of water. Powdery mildew on yarrow is worse in shade and crowding, so sun and air movement are the real control. The spittlebug's foam is what shields it from UV and from predators, so knocking the foam off does the work a spray would.

Cut flowers for drying when the heads feel firm

Cut the stems once every flower cluster has reached full size and color and the heads feel firm to the touch. Bundle the stems and hang them upside down in a warm, dry, well ventilated place such as an attic or shed for two to three weeks, until thoroughly dry. Heads cut before they firm up collapse and shatter as they dry, and yarrow only holds its color as an everlasting if it goes into dry moving air at peak.