Plant problems

Why Are My Achillea Leaves Curling?

Q: How do I deadhead and prune achillea?

Correct deadheading and pruning of achillea (yarrow) maintains a tidy appearance, extends the flowering season, and keeps the plants compact and productive in UK gardens. Deadheading during the flowering season: as individual flower heads within the flat-headed, corymb-type inflorescences of achillea begin to fade and turn brown, the whole stem bearing the faded flower head can be cut back to the base (to a pair of leaves or a lateral shoot lower down the stem); cutting back to a lateral shoot lower on the stem encourages the development of a smaller, secondary flower head from the lateral shoot, extending the overall flowering period; leaving faded flower heads in place reduces the likelihood of further flowering from that stem; with regular and prompt deadheading, achillea can maintain a continuous succession of fresh flower heads throughout the summer from June to September. Leaving seed heads for winter interest and wildlife: the flat, dried, golden-brown or fawn seed heads of achillea (particularly 'Coronation Gold' and other tall cultivars) are very ornamental when left on the plant through autumn and into winter; they dry naturally on the stem and hold their shape well; they are also used by small finches and other seed-eating birds; the dried stems and seed heads provide habitat for overwintering insects; if winter seed-head interest is desired, do not deadhead in autumn; simply cut down the dead stems in late winter or early spring. The Chelsea Chop and cutting back: cutting the stems back by approximately one-third to one-half in late May to early June (the Chelsea Chop) produces a delayed but shorter-stemmed, more self-supporting, later flush of flowers; cutting back all the stems in June after the first main flowering flush to about 15 to 20 cm above soil level often stimulates a fresh flush of new growth and a second, smaller flowering in late summer; the overall autumn foliage rosette can be left in place over winter and cut back in spring.

Achillea (yarrow) is a tough, sun-loving, free-draining perennial widely grown in UK gardens. The finely divided, feathery, aromatic leaves curl, turn grey-white, or become sparse most often from powdery mildew in dry late summer conditions; from waterlogging and root rot in heavy, poorly draining UK clay soils (achillea, like lavender, hates wet roots); from aphid infestations on developing shoot tips; or from stem eelworm in rare cases.

Powdery mildew

The most commonly encountered foliar disease of achillea in UK gardens. White, fine, powdery coating on the surfaces of leaves and stems; affected portions of the feathery leaves curl slightly; the normal grey-green foliage appears pale, bleached, or white. Most prevalent in hot, dry, late summer conditions with poor air circulation. Paradoxically most common in very thin, poor, dry soil in a hot, exposed position.

What to do

Grow in average fertility, well-drained but not bone-dry soil; a very poor, thin, drought-prone soil in full sun can paradoxically worsen mildew in late summer; water during prolonged dry spells in July to September to maintain some root moisture; ensure good air circulation around plants by not planting too densely; cut back and dispose of the most severely mildewed stems and leaves; divide the clump in spring to allow better air circulation; sulphur-based fungicide sprays are approved in the UK for powdery mildew on ornamental plants.

Waterlogging and root rot

Achillea is adapted to freely draining, often thin, poor, well-drained soils and is quite intolerant of persistently wet, heavy, poorly draining clay soils. In a very heavy clay soil that holds water persistently, the fibrous roots develop crown and root rot; the plant loses vigour progressively, the normally feathery foliage becomes sparse and limp, and the plant may die back in winter and fail to re-emerge in spring.

What to do

On heavy, poorly draining UK clay soils, improve drainage by incorporating large quantities of coarse horticultural grit into the planting hole; or grow in a raised bed or gravel garden where drainage is naturally better; avoid planting in low-lying, wet spots; achillea thrives in conditions that would suit lavender, thyme, or rosemary in terms of drainage and sun exposure; these are the conditions to aim for.

Aphid infestations

Aphids attack developing shoot tips and flower stems in spring and early summer; feeding causes the young developing leaves at the shoot tip and small leaflets on the developing flower stem to curl and distort. Generally less severe and damaging on achillea than on more susceptible plants such as roses or lupins, but can cause visible distortion of the young foliage and reduce the appearance of flower stems.

What to do

Squash or rub off aphid colonies on shoot tips by hand; knock off with a jet of water; tolerate for a few days to allow natural predators (ladybirds, lacewing larvae) to colonise; heavy infestations that persist after natural predators have had time to build up can be treated with insecticidal soap spray applied to coat the aphids directly.

Stem eelworm

Ditylenchus dipsaci (stem and bulb nematode) can infect a range of garden perennials including achillea. Infected plants show distorted, stunted, swollen, abnormally fleshy or discoloured stems and leaves; growth is distorted and abnormal-looking; heavily infected plants are severely stunted with brownish or bleached discolouration of the affected tissue. Much less commonly encountered than powdery mildew or waterlogging but persistent when present.

What to do

No chemical treatment is available to amateur UK gardeners for stem eelworm; remove and dispose of affected plants (do not compost); do not plant susceptible species (including many bulbs, strawberries, and other perennials as well as achillea) in the affected area for several years; grow in a different area of the garden with fresh, clean soil or fresh container compost.

Frequently asked questions

Why are my achillea leaves curling?

Achillea leaves curl most commonly because of powdery mildew (white fine powdery coating on leaf surfaces and stems; leaves curl slightly; foliage appears pale bleached or white; most prevalent in hot dry late summer conditions with poor air circulation; grow in average fertility well-drained soil; water in prolonged dry spells July to September to maintain some root moisture; good air circulation; cut back and dispose of most severely mildewed stems; sulphur-based fungicide approved for UK ornamental plants), waterlogging and root rot (adapted to freely draining often thin poor soils; quite intolerant of persistently wet heavy poorly draining clay soils; plant loses vigour; foliage becomes sparse and limp; improve drainage with large quantities of coarse horticultural grit; or grow in raised bed or gravel garden; conditions that suit lavender thyme or rosemary are the target), aphid infestations on developing shoot tips (aphids cause young leaves and leaflets at shoot tip and on flower stem to curl and distort; squash or rub off by hand; knock off with water jet; insecticidal soap if needed), or stem eelworm (distorted stunted swollen abnormally fleshy or discoloured stems and leaves; remove and dispose of affected plants; do not compost; do not plant susceptible species in affected area for several years; no chemical treatment available to amateur UK gardeners).

How do I grow achillea successfully in UK gardens?

Well-drained soil: grows best in freely draining average to poor-fertility soil; adapted to dry thin sunny calcareous soils of natural habitat; in very fertile rich heavily composted UK garden soils produces lush soft tall floppy growth; performs particularly well in a gravel garden a sunny dry bank or a well-drained south-facing border. Full sun: requires full sun for most of the day; in shade growth becomes drawn lax and floppy; flowering significantly reduced; less silvery compact aromatic foliage. Division and renewal: benefits from division every 2 to 3 years to maintain vigour and flowering quality; after 2 to 3 years without division centre of clump typically becomes woody bare and unproductive; divide in spring as new growth emerges; split clump into sections and replant outer more vigorous divisions while discarding the congested woody centre. Winter management: most cultivars die back to a low rosette of ground-level leaves in winter; leave on the plant for crown protection; cut back dead stems and upper foliage in late autumn or spring; leave the ground-level winter rosette in place until spring; avoid cutting completely to bare soil in autumn.

Why is my achillea flopping over?

Most common reasons: too much soil fertility and too little sun. Excessive soil fertility: achillea adapted to relatively poor thin infertile soils; in very rich fertile heavily composted UK garden border produces lush excessively tall soft heavy flower stems that flop under their own weight especially after heavy rain; do not enrich soil around achillea with compost or fertiliser; grow in average to poor-fertility well-drained position. Insufficient sun: in partial shade produces drawn weak tall lax stems that flop; move to a sunnier position. Solutions for established flopping clump: divide and replant in sunnier poorer-soil position; install grow-through support or link stakes in May when stems about 30 to 40 cm tall; the Chelsea Chop (cut back stems by one-third to one-half in late May to early June) produces a delayed but shorter-stemmed more self-supporting later flush of flowers; choose a compact cultivar such as 'Moonshine' (45 to 60 cm) rather than very tall 'Coronation Gold' (90 to 120 cm) or 'Gold Plate' (up to 150 cm).

How do I deadhead and prune achillea?

Deadheading during flowering: as individual flower heads fade, cut the whole stem bearing the faded flower head back to a pair of leaves or a lateral shoot lower down the stem; cutting back to a lateral shoot encourages a smaller secondary flower head extending the overall flowering period; with regular prompt deadheading achillea can maintain a continuous succession of fresh flower heads throughout summer from June to September. Leaving seed heads: flat dried golden-brown or fawn seed heads of 'Coronation Gold' and other tall cultivars very ornamental through autumn and winter; also used by seed-eating birds; provides habitat for overwintering insects; if winter interest desired do not deadhead in autumn; cut down dead stems in late winter or early spring. Chelsea Chop and cutting back: cut stems back by one-third to one-half in late May to early June for delayed shorter-stemmed more self-supporting later flush; cutting back all stems in June after first main flowering to about 15 to 20 cm above soil level often stimulates a fresh flush of new growth and a second smaller flowering in late summer.