Why Are My Athyrium Fronds Curling?

Drought stress

Drought is the most common cause of athyrium frond curl. The finely divided, soft fronds have a large combined surface area and dry out rapidly in low-moisture conditions. In dry soil the pinnae curl inward along their length, the frond tips curl downward, and in severe cases the entire frond curls and browns from the tip down in a process that can progress to the whole frond within a week. Athyrium is more sensitive to drought than dryopteris and should not be planted in positions without reliable soil moisture. The ornamental cultivars of Athyrium niponicum are particularly susceptible.

What to do

• Water athyrium regularly during dry periods, keeping the soil consistently moist from spring through summer. The deciduous nature of athyrium means all its photosynthesis is concentrated in the growing season, and drought stress during this period affects the following year's plant vigour.
• Apply a generous mulch of leafmould or garden compost around athyrium each spring. Leafmould that replicates the woodland-floor leaf-litter layer of its natural habitat is ideal.
• Plant athyrium in positions where the soil remains naturally moist: under deciduous trees, beside north-facing walls, in sheltered borders, or near water features. Avoid south-facing positions or positions exposed to drying winds.

Slug damage

Slugs are a significant and very common problem for athyrium. The soft, finely divided fronds are extremely attractive to slugs and are much more vulnerable than the tougher fronds of dryopteris or polystichum. Slug feeding causes the pinnae to develop holes and curl around the damage, and in severe cases the entire frond is reduced to a skeleton of rachis and midrib. The emerging croziers in spring are the most vulnerable stage: slug damage at the crozier stage causes the entire frond to emerge distorted and curled. The moist, shaded conditions that athyrium requires are also ideal slug habitat, making this a persistent and difficult-to-avoid problem.

What to do

• Apply biological nematode slug control (Phasmarhabditis hermaphrodita) around athyrium in early spring when soil temperatures reach 5 degrees Celsius. This is the single most effective intervention for protecting the vulnerable emerging croziers.
• Apply a ring of sharp horticultural grit around each plant. The fine texture of the grit is particularly effective against the smaller slug species that cause the most frond damage.
• Hand-collect slugs from around and between athyrium fronds in the evening during warm, wet spring weather. Regular evening checks are the most reliable manual control.
• Apply a second nematode treatment in early autumn to reduce the overwintering slug population around the crown.

Late frost damage

Athyrium is deciduous and does not emerge until late spring, which means it naturally avoids many of the late frosts that damage earlier-emerging ferns. However, in a warm spring that stimulates early emergence followed by a late cold snap, the unfurling croziers can be damaged by frost, causing them to curl, blacken, and collapse. Athyrium niponicum and its cultivars, emerging from Japanese mountain woodland habitats, are generally well-adapted to late frosts and recover quickly.

What to do

• Cover emerging athyrium croziers with horticultural fleece when late frost is forecast after they have begun to emerge. The low, spreading growth of most athyrium cultivars makes fleece protection straightforward.
• Leave the dead fronds from the previous year in place through winter: they provide a degree of frost insulation to the crown and delay emergence by a week or two in cold springs.
• New croziers emerge from the crown within two to three weeks of frost damage in established plants. The rhizome is rarely damaged by spring frost alone.

Too much sun

Athyrium is a shade fern and performs poorly in full or half-day sun. In too much sun the fronds curl inward and the finely divided pinnae develop bleached, scorched patches. The delicate structure of athyrium fronds makes them particularly susceptible to sun scorch compared to tougher fern genera. Even Athyrium niponicum 'Pictum', which produces its best silver colouring in slightly brighter light than the fully shaded native species, suffers frond damage in direct afternoon sun.

What to do

• Plant athyrium in dappled shade under deciduous trees, or in partial shade on the north or east-facing side of structures. Morning sun with afternoon shade is acceptable for most athyrium species in moist soil.
• For Athyrium niponicum 'Pictum', a position with bright indirect light rather than deep shade produces the best colour. Avoid both full shade and full sun: the ideal is the bright, dappled light of an open woodland canopy.

Alkaline soil

Athyrium prefers slightly acidic to neutral soil and performs poorly in alkaline or chalky conditions. In alkaline soil the fronds develop chlorotic yellowing between the veins, the tips curl and brown, and overall growth is significantly reduced. The native Athyrium filix-femina tolerates a wider pH range than many ornamental ferns, but even the common lady fern grows less vigorously in alkaline conditions than in its preferred moderately acidic soil.

What to do

• Test soil pH before planting athyrium in a new area. A pH above 7.5 will cause problems. Lower pH by incorporating acidic organic matter such as ericaceous compost or pine bark.
• In chalk gardens, grow athyrium in raised beds or containers with acidic, humus-rich, ericaceous compost mixed with leafmould and sharp sand.
• Apply chelated iron sequestrants to the soil around athyrium in alkaline conditions to temporarily correct iron chlorosis.

Frequently asked questions

Why are my athyrium fronds curling?

Athyrium fronds curl most often from drought stress or slug damage. The finely divided, soft fronds of athyrium are particularly susceptible to moisture stress, causing the pinnae to curl inward and the frond tips to curl downward in dry conditions. Slugs feed on the fresh young fronds in spring and through summer, causing the pinnae to curl around the feeding damage and develop holes. Both problems are common for athyrium grown in UK gardens.

Is Athyrium niponicum 'Pictum' (Japanese painted fern) difficult to grow?

Athyrium niponicum 'Pictum', the Japanese painted fern, is no more difficult to grow than the native lady fern but requires the same conditions: consistently moist, humus-rich, slightly acidic soil in dappled shade. It is less vigorous than the native Athyrium filix-femina and grows more slowly, which makes it more susceptible to slug damage and drought stress in the early years after planting. In appropriate conditions it is reliably hardy and long-lived in UK gardens, producing its distinctive silver, green, and burgundy fronds each summer.

What conditions does athyrium need?

Athyrium thrives in moist, humus-rich, slightly acidic to neutral, well-drained soil in dappled shade or partial shade. It is more moisture-demanding than dryopteris and performs poorly in dry, sunny positions. Soil enriched with leafmould or well-rotted garden compost is ideal. Athyrium filix-femina, the common lady fern, is a native British species that naturalises in moist woodland, hedgebanks, and streamside habitats, indicating the range of conditions it tolerates. The ornamental cultivars and species such as Athyrium niponicum 'Pictum' require similar conditions but are generally less vigorous than the native species.

Does athyrium die back in winter?

Yes, athyrium is deciduous and dies back completely to the crown in autumn. Unlike dryopteris, which is semi-evergreen and retains some fronds through a mild winter, athyrium fronds yellow and collapse entirely as the temperature drops in autumn. The rhizome or crown overwinters underground and produces new croziers in spring, typically emerging in April. The dead fronds can be removed in autumn or left to provide some winter protection to the crown.