Why Are My Asplenium Fronds Curling?

Drought stress

Drought causes the strap-shaped fronds of hart's tongue fern to curl inward along their length and develop brown, papery margins, progressing from the frond tips downward as the stress intensifies. Despite being more drought-tolerant than many other ferns once established, asplenium scolopendrium still requires consistently moist, humus-rich soil to produce its characteristic glossy, deep green fronds. Plants in sandy or chalky soil without adequate moisture, or in the dry root-shadow of large conifers or buildings, are most susceptible. Established plants can tolerate short dry spells but will curl noticeably if the soil remains dry for more than one to two weeks in summer.

What to do

• Water asplenium during dry periods, particularly in its first two years in the garden before the root system is fully established. Once established, the plant is more resilient but still benefits from irrigation in a dry UK summer.
• Apply a mulch of garden compost or leafmould around the plants each spring. Hart's tongue fern in its natural habitat grows in leaf-litter-enriched soil in woodland, and this organic mulch replicates those conditions.
• Plant asplenium in positions naturally shielded from the worst of summer sun and drying winds, under trees or beside north-facing walls where the soil remains cooler and retains moisture more effectively.

Frost and cold wind damage

Although asplenium scolopendrium is hardy across most of the UK, severe winter frost and cold, drying winds cause the evergreen fronds to curl, turn yellow-brown, and collapse. The fronds are damaged by the combination of freezing temperatures and low humidity: the evergreen fronds continue to transpire through winter but cannot replace moisture when the soil is frozen, causing them to desiccate and curl. This is most common in exposed, windy positions or during a particularly severe continental frost event. The crown of the plant is rarely damaged and produces fresh new fronds in spring even after severe frond damage.

What to do

• Plant asplenium in sheltered positions protected from cold winds, particularly north and east winds in winter. A position beside a south or west-facing wall, or under a dense evergreen canopy, provides significant wind protection.
• Wrap vulnerable plants with horticultural fleece in severe frost warnings. The compact crown and relatively small size of most asplenium specimens makes fleece protection straightforward.
• Remove frost-damaged fronds in early spring when the new croziers become visible. The new fronds will replace them quickly and are best revealed by cutting away the damaged old foliage.

Slug damage

Slugs feed on the young, emerging fronds of asplenium in spring and on the soft tissue of established fronds. The characteristic holes and ragged margins from slug feeding cause the affected areas of the broad frond to curl around the damage. The glossy surface of hart's tongue fronds is less attractive to slugs than the softer fronds of athyrium or matteuccia, making slug damage generally less severe, but in a wet spring in a slug-prone garden, asplenium can suffer significant frond damage.

What to do

• Apply biological nematode slug control (Phasmarhabditis hermaphrodita) around asplenium in spring when soil temperatures reach 5 degrees Celsius.
• Apply a ring of sharp grit around the crown of each plant. The grit deters slugs from crossing to the young emerging fronds, which are the most vulnerable stage.
• Hand-collect slugs from around asplenium in the evening during wet spring weather. Regular checks are the most reliable manual intervention.

Too much sun

Asplenium scolopendrium is a shade fern and performs poorly in full, exposed sun. In full sun the broad fronds curl inward, develop bleached or scorched patches and the characteristic deep glossy green colour fades to a yellowish tone. The simple, undivided frond structure makes sun scorch particularly visible on hart's tongue fern compared to finely divided ferns where scorching is more distributed. Morning sun with afternoon shade is generally acceptable, particularly in moist soil.

What to do

• Relocate asplenium to a shaded position in dappled shade or full shade. Unlike many ferns, hart's tongue fern actually tolerates quite deep shade and is one of the best ferns for north-facing aspects and heavy shade under trees.
• East-facing positions that receive gentle morning sun but shade from midday onwards are ideal in most UK gardens and produce healthy, glossy fronds.

Poor drainage

While asplenium scolopendrium requires moist soil, it does not tolerate waterlogging. In persistently waterlogged or compacted, airless soil, the crown deteriorates, the fronds yellow and curl, and the plant gradually declines. This is most common when asplenium is planted in heavy, unimproved clay that becomes saturated in winter. The contrast with the well-drained, calcareous soils of its natural woodland habitat is significant: good drainage combined with consistent moisture is the key requirement.

What to do

• Improve drainage before planting asplenium in heavy clay by incorporating coarse grit and garden compost into the planting hole and surrounding area.
• Plant asplenium slightly proud of the surrounding soil level to prevent water pooling around the crown in heavy rain.
• If the garden soil is persistently waterlogged, grow asplenium in raised beds or containers with excellent drainage.

Frequently asked questions

Why are my asplenium fronds curling?

Asplenium fronds curl most often from drought stress or frost damage. Asplenium scolopendrium, the hart's tongue fern, is evergreen and its strap-shaped fronds curl inward along their length and develop brown, papery edges when the soil dries out or when the plant is exposed to cold drying winds in winter. Frost causes the fronds to collapse and curl in severe winters, though the plant typically regenerates from the crown in spring. The smaller spleenwort species such as Asplenium trichomanes are less prone to drought but may curl if dislodged from their preferred crevice habitat.

Is asplenium (hart's tongue fern) evergreen?

Yes, Asplenium scolopendrium, the hart's tongue fern, is fully evergreen and retains its glossy, strap-shaped fronds year-round. The old fronds persist until the new fronds emerge in spring, at which point the oldest fronds can be removed. In severe winters the fronds may suffer cold damage and curl or brown, but the crown survives and produces new growth in spring. The smaller asplenium species such as Asplenium trichomanes (maidenhair spleenwort) and Asplenium ruta-muraria (wall-rue) are also evergreen and very cold-hardy.

What conditions does asplenium need?

Asplenium scolopendrium thrives in moist, humus-rich, alkaline to neutral soil in deep shade or dappled shade. Unlike many ferns, the hart's tongue fern actually performs better in alkaline conditions and is commonly found growing wild on limestone walls, in chalk woodland, and on the mortar of old stone walls. It tolerates drier conditions than many other ferns once established. The smaller spleenwort species prefer to grow in the crevices of walls, rocks, and paving, where they are self-sustaining in natural moisture from the substrate.

Why does my asplenium have brown frond edges?

Brown edges on asplenium fronds most commonly indicate drought stress, cold wind damage, or frost damage. In dry conditions the strap-shaped fronds of hart's tongue fern develop brown, papery margins as the outermost tissue is the first to desiccate. Cold, drying winter winds cause similar browning on the frond edges, particularly on plants in exposed positions. The distinctive row of brown, linear sori on the underside of the fronds are completely normal and are the spore-producing structures of the plant, not a sign of disease.