Prune the flower clusters at the stalk’s base with a pair of pruning shears. Wait until the pollen on the flowers is visible and at least half of the flower clusters are open. According to the University of Maryland Cooperative Extension Service, the best time to harvest the flowers is early in the morning after the dew has dried but before the day starts to heat up.
Pull off any of the leaves that will be submerged when the cut yarrow is in the vase.
Place the bottom of the yarrow's stem under running water. Snip off the stem’s bottom inch and place it immediately into a vase filled with room-temperature water.
Place the flowers in a cool spot out of direct sunlight. Place the yarrow flowers and vase in the refrigerator at night when no one is appreciating them.
Change the yarrow's water once every three days.