Gardeners grow yarrow (Achillea), a perennial flowering herb, for aesthetic and medicinal purposes. The time for trimming yarrow varies by purpose. Pruning to deadhead the flowers may continue throughout the growing season. Preparing yarrow for winter may include a one-time cut nearly to the ground.
Encourage New Growth
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Yarrow flowers may be pruned to the first node as they open. This will break apical dominance and encourage flowering from other nodes. Hand pruners produce a sharp, clean cut. Yarrow flowers may reach a height of 3 feet and attract butterflies to the flowerbed.
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Deadheading
Fragrant, fern-like foliage produces abundant blooms if the yarrow plant is well-sited in full sun. Deadheading—removal of spent flowers to encourage new blooms—involves pinching the old blooms from the stems or using hand pruners to cut them off.
Renewal Pruning
In mid-summer, when flowering has ended and the yarrow plant looks ragged, renewal pruning may be appropriate. Removing several inches of the stem, again above a node, may result in more blooms from the yarrow flower. When frost is imminent, southern gardeners may trim to a few inches from the ground for appearance of the flowerbed. Those in more northern areas may wait until spring, before the yarrow blooms. If cut to the ground, yarrow will first appear in spring as a dainty, frilly ground cover before sending up shoots.