Yarrow (Achillea millefolium) is a tough perennial often chosen for its drought tolerance and longevity. Without regular intervention, however, it can become unruly or aggressive in the garden. Strategic pruning is necessary for maintaining the plant’s tidy appearance, promoting overall health, and preventing unwanted spreading through aggressive self-seeding. Different pruning methods are applied throughout the growing season to achieve specific outcomes, from encouraging fresh blooms to preparing the plant for dormancy.
Maintenance Pruning for Repeat Flowering
During the active growth season (late spring through early fall), the primary pruning technique is focused on removing spent flower heads. This action redirects the plant’s energy away from seed production and channels it back into vegetative growth, which results in subsequent flushes of flowers. Removing the dried, faded umbels improves the overall aesthetic of the yarrow patch.
To deadhead correctly, locate the stem of the spent flower cluster and trace it down to the nearest set of healthy leaves or a visible lateral bud. Make a clean cut with sharp, sterilized shears just above this new growth point. This precise cut encourages the development of side shoots, which will then produce new flowers, extending the bloom period well into the autumn. Allowing the plant to set seed can lead to a vigorous spread of seedlings that may not retain the desirable characteristics of the parent cultivar.
Corrective Pruning: Managing Height and Flopping
Yarrow is prone to becoming “floppy,” especially when grown in overly rich soil or in partial shade where the stems stretch out seeking light. This leggy growth can cause the entire clump to splay outward. This intervention, often referred to as the “Chelsea chop,” is performed in late spring or early summer, typically around late May or early June, before the main flowering period is completely finished.
The corrective prune involves shearing the entire yarrow plant back by a significant amount, usually between one-third and one-half of its current height. This sacrifices the first wave of blooms but forces the plant to divert resources into creating new, shorter, and sturdier lateral branches. The resulting second flush of flowers will appear a few weeks later than the original blooms, but they will be borne on compact, robust stems. This technique is particularly effective for managing vigorous cultivars that naturally grow to three feet or more.
Preparing Yarrow for Winter
This annual hard cutback should be performed either in late fall, after the foliage has died back following the first heavy frost, or in very early spring before new growth emerges. The timing often depends on the gardener’s preference for winter tidiness versus providing shelter for beneficial insects and small wildlife.
Using shears, cut all the stems down to approximately 2 to 6 inches above the soil line, leaving only the basal rosette of leaves. This practice helps to remove potential overwintering sites for fungal spores and insect pests, contributing to a healthier start for the plant. The removal of old material also allows sunlight to warm the soil around the crown in the spring, which encourages vigorous new growth to begin promptly.