Why Are My Polypodium Fronds Curling?

Drought stress

Drought causes polypodium fronds to curl along their length and develop brown, papery tips, progressing from the frond apex toward the base as the stress intensifies. Although polypodium is more drought-tolerant than most garden ferns, it still requires some consistent moisture through its active growing period from late summer to spring. Plants in very exposed, sunny positions or in substrates that drain extremely rapidly, such as vertical wall crevices, are most susceptible. Established plants growing in moist, leafmould-rich crevices are significantly more resilient than recently planted specimens with shallow root systems.

What to do

• Water polypodium during dry periods from late summer through spring, when the plant is in active growth. Avoid regular watering in June and July when the plant is dormant, as this can promote waterlogging around the dormant rhizome.
• Tuck leafmould or well-rotted garden compost around the rhizome in autumn to improve the moisture retention of the substrate and provide additional organic matter for the growing season.
• If polypodium is established on a wall or rock surface, the natural moisture from the substrate is often sufficient in UK conditions, but supplementary watering during an unusually dry autumn may be needed in its first year.

Waterlogging

Waterlogging is a more serious problem for polypodium than for most moisture-tolerant ferns, because its summer dormancy period coincides with the time when it is least able to cope with saturated, anaerobic soil conditions. A polypodium whose rhizome sits in waterlogged soil in summer, when the plant is not actively transpiring, is at risk of rhizome rot. This causes the plant to fail to produce its characteristic late-summer flush of new fronds, or to produce weak, distorted, curling fronds that quickly deteriorate.

What to do

• Plant polypodium in free-draining substrates: wall crevices, gaps between rocks, raised beds with excellent drainage, or on top of boulders with leafmould tucked around the rhizome. This replicates the naturally fast-draining conditions of its wild habitat.
• Avoid planting polypodium in heavy clay or in low-lying positions where water accumulates in summer. If the garden soil is poorly drained, grow polypodium on a raised structure: a dry-stone wall, a large boulder, or a raised, free-draining bed.
• Do not mulch heavily around dormant polypodium in summer with moisture-retaining materials. A light, open mulch of coarse bark or gravel is preferable to dense compost in summer.

Too much sun

Polypodium tolerates more sun than most ferns, particularly in the cool, moist west of the UK where it grows wild in exposed positions on rocky outcrops. In warmer, drier lowland conditions, however, full afternoon sun causes frond curl and browning. The ideal position in lowland UK gardens is dappled shade or morning sun with afternoon shade, combined with a substrate that drains freely but retains some moisture.

What to do

• In lowland gardens, plant polypodium on the north or east-facing side of walls or rocks, where it receives gentler morning light and is shaded from the hottest afternoon sun.
• In cooler, wetter upland or western gardens, polypodium can tolerate more sun exposure and may even thrive in relatively open, bright conditions, as the higher rainfall and lower temperatures reduce the impact of direct light.

Wrong soil type

Polypodium grows naturally on substrates very different from conventional garden soil: rocky surfaces, old mortar, mossy tree bark, and peaty, acidic substrates in upland areas. In rich, dense garden soil it often grows poorly compared to other ferns, as the moisture-retentive, fertile conditions do not suit its preference for free-draining, lower-fertility substrates. Fronds may curl and yellow in overly rich, wet conditions even when the plant appears well-positioned for light levels.

What to do

• Grow polypodium in a substrate more similar to its natural habitat: a mixture of leafmould, coarse grit, and a small amount of garden compost tucked into wall crevices or between rocks is ideal.
• Avoid planting polypodium in heavily amended, fertile beds designed for moisture-loving ferns. It is better suited to a dry-stone wall garden, a rock garden, or the base of a large boulder than to a conventional shaded border.

Frost damage

Polypodium is generally frost-hardy but the fronds produced in late summer and early autumn are soft and frost-sensitive when newly emerged. A sharp early frost in October, before the fronds have fully hardened, can cause the fresh growth to curl and brown. Once fully hardened, the winter fronds are resilient to typical UK frost. The rhizome itself is very frost-hardy and survives even significant frost penetration into the substrate.

What to do

• In frost-prone areas, delay the emergence of new fronds by withholding water in late summer, which slows the plant's transition from dormancy. The hardened old fronds that persist from spring provide adequate winter interest even if the new fronds are frost-damaged.
• Cover newly emerged fronds with fleece when frost is forecast in early autumn if the fronds are visibly still expanding and not yet fully hardened.

Frequently asked questions

Why are my polypodium fronds curling?

Polypodium fronds curl most often from drought stress or from waterlogging during the summer dormancy period. Polypodium vulgare, the common polypody, is an unusual fern that produces new fronds in late summer and autumn, remains evergreen through winter and spring, and then enters summer dormancy in June and July when the old fronds die back. Drought during the growing season (late summer to spring) causes the fronds to curl and brown. Waterlogging in summer, when the rhizome is dormant and less able to tolerate saturated soil, causes the rhizome to deteriorate.

When does polypodium produce new fronds?

Polypodium vulgare is unusual among UK ferns in producing its new fronds in late summer and autumn, from August to October, rather than in spring like most other ferns. The fronds remain green and active through winter and spring, providing valuable winter interest, and then die back in June and July when the fern enters summer dormancy. This reverse cycle means that summer is when polypodium looks its worst and winter is when it looks its best, which is the opposite of most garden perennials. Do not be alarmed when polypodium dies back in June: this is normal behaviour.

Where does polypodium grow best?

Polypodium vulgare grows best on free-draining, rocky, or wall-top substrates in partial shade or dappled shade, replicating its natural habitat on rock faces, old walls, the surface of boulders, and on the branches of old trees in the west of the UK. It is one of the few ferns that thrives in relatively dry conditions and performs better on well-drained, slightly acidic to neutral substrates than in the moist, rich soil preferred by most other garden ferns. In gardens, it is excellent planted in the crevices of dry stone walls, on top of large rocks, or in raised, free-draining beds.

Is polypodium evergreen?

Polypodium vulgare is semi-evergreen, retaining its fronds from autumn through to early summer. The fronds produced in late summer and autumn persist through the winter and spring, providing excellent year-round foliage interest in the garden. They die back naturally in June and July when the fern enters summer dormancy, at which point the planting area may look bare until the new fronds emerge again in late summer. Mulching the dormant area in summer with garden compost can help retain moisture and suppress weeds during this period.