Polypodium glycyrrhiza (Licorice Fern)
Polypodium glycyrrhiza
Table of Contents
Introduction & Discovery
Polypodium glycyrrhiza, known universally as the licorice fern, is the defining epiphytic fern of the Pacific Northwest temperate rainforest and one of the most culturally and ecologically significant ferns of indigenous North America. Its species epithet, coined by Daniel Cady Eaton in 1856 from the Greek glykys (sweet) and rhiza (root), acknowledges a property that had been known to coastal First Nations for millennia: the rhizome tastes distinctly of licorice, a sweetness derived from the saponin osladin and related compounds polypodosides A, B, and C, the same class of molecules that give European P. cambricum and P. vulgare their historical reputation as natural sweeteners. For Haida, Tlingit, Tsimshian, Nuu-chah-nulth, Makah, and Coast Salish peoples, the rhizome was a prized winter-collected delicacy, a cough medicine, and a spiritual food tied to specific harvesting protocols. Botanically, the species is a diploid (2n=74) member of the Polypodium vulgare complex whose nearest relatives are scattered across the northern Pacific rim, and its amphi-Pacific distribution (reaching Japan and Kamchatka in small populations) makes it a biogeographical puzzle still actively studied by pteridologists. Ecologically it is a winter-growing, summer-dormant epiphyte most commonly found cascading from the mossy north-facing boughs of bigleaf maple (Acer macrophyllum), where its pendant green fronds emerge with the October rains, photosynthesise through the cool wet months, and retreat into bare rhizome by July. For gardeners, this phenology is both its charm and its challenge: where Pacific maritime conditions prevail, the licorice fern offers year-round interest peaking when everything else sleeps; outside that narrow climatic zone, it struggles. This profile explores the species at the depth it deserves, from its fine cytological structure and rainforest ecology to its ethnobotanical history and the practical details of growing it indoors or mounted on bark.
Discovery & Naming
The licorice fern was known to Pacific Northwest First Nations for millennia before any European botanical description, and indigenous knowledge provided the first systematic understanding of its distribution, phenology, and uses. European botanical contact began with the Spanish Malaspina Expedition (1791) to the Pacific coast, whose naturalists collected but did not formally describe the species, and accelerated with the Vancouver Expedition (1792-1795) naturalist Archibald Menzies, who collected ferns including licorice fern during the British survey of the Pacific coast. Menzies' specimens reached the British Museum but remained unnamed for decades. Various early North American floras through the early 1800s confused the species with European P. vulgare, a persistent taxonomic problem that would not be fully resolved until the 20th century. Daniel Cady Eaton (1834-1895), the pre-eminent American pteridologist of his era, formally described the species as Polypodium glycyrrhiza in 1856, basing the name on the sweetness of the rhizome and on collections from Oregon and California. The type locality is generally given as coastal Oregon. Through the later 19th and early 20th centuries, the species was variously treated as a subspecies of P. vulgare, as Polypodium falcatum, and under several other synonyms, before Lloyd and Lang in the 1970s and Haufler and Windham in the 1980s-90s definitively established its status as an independent diploid species (2n=74) within the vulgare complex. Molecular phylogenetic studies by Christopher Haufler, Michael Windham, Kathleen Pryer, and colleagues at Duke and the University of Kansas through the 1990s and 2000s placed P. glycyrrhiza within a well-defined Pacific Northwest clade and elucidated its role as a parent in several hybrid species including tetraploid P. calirhiza. Ethnobotanical documentation by Nancy Turner beginning in the 1970s and continuing through the present has created the most thorough anthropological record of the species' cultural significance among any North American fern.
Native Range & Distribution Map
Distribution map showing the native range of Polypodium glycyrrhiza.
Biology & Frond Morphology
Polypodium glycyrrhiza belongs to the genus Polypodium in the family Polypodiaceae, producing simple-pinnate evergreen (winter-green, summer-deciduous in drier portions of range), lanceolate fronds 15-50 cm long and 4-12 cm wide, deeply pinnatifid with 10-25 alternate pinna pairs cut nearly to the rachis. pinnae are linear-oblong, 2-6 cm long, entire to finely serrate at the apex, with a pointed tip and slightly narrowed base. stipe is straw-yellow to greenish, typically one-third to one-half the length of the blade, glabrous to sparsely scaly at the base. rachis is green and grooved on the upper surface. texture is thin-herbaceous and distinctly softer than most other polypodium species, reflecting adaptation to cool humid conditions rather than drought. fronds arise singly along a creeping rhizome rather than in tight clusters. fronds that unfurl from coiled fiddleheads (croziers). Like all ferns, it reproduces via spores borne on the underside of fertile fronds rather than flowers and seeds, and its life cycle alternates between a dominant sporophyte (the visible plant) and a small, short-lived gametophyte stage.
Reproduction & Propagation
Propagation of Polypodium glycyrrhiza can be achieved through several methods:
- Spores: Collect ripe spores from the underside of fertile fronds, sow on sterilised peat or peat/perlite mix. Do not cover. Keep humid and in bright indirect light. Prothalli (gametophytes) typically develop in 4–12 weeks, and young sporophytes appear after a further 2–6 months.
- Division: Mature clumps with multiple crowns or creeping rhizomes can be divided in spring as new fronds emerge.
- Rhizome cuttings / offsets: Epiphytic genera (Davallia, Polypodium, Phlebodium) can be propagated from 5–10 cm rhizome segments with at least one frond and visible roots.
Cultivation & Substrate
Successful cultivation of Polypodium glycyrrhiza depends on matching three conditions to its natural habitat: consistent moisture without waterlogging, a humus-rich yet well-drained substrate, and the correct light level for its frond type — whether dappled woodland shade, bright filtered light, or, for a handful of rock ferns, direct sun.
Substrate: Loose, highly organic, moisture-retentive but fast-draining epiphytic mix with a slightly acidic to neutral pH 5.5-6.5 50% coarse conifer bark (Douglas fir or cedar, 1-2 cm pieces), 20% coir or well-rotted leaf mould, 15% pumice or coarse perlite (4-6 mm), 10% composted leaf litter (preferably bigleaf maple), 5% horticultural charcoal Heavy peat-based potting soils, clay loams, ordinary garden soil, any mix that compacts over time, calcareous grit or dolomite additives that raise pH, fine silica sand that reduces aeration Critical; the substrate must drain completely within 30-60 seconds of watering, with no standing water retained in the pot base For authentic epiphytic presentation, mount rhizome directly to bigleaf maple bark, cork bark, or tree fern slab with soft twine over a sphagnum-moss pack, omitting the substrate mix entirely Every 2-3 years, replace the upper third of the mix in autumn to restore organic content without disturbing the rhizome Living moss (Hypnum, Plagiothecium, or Isothecium) or fresh fallen maple leaves improve humidity retention and mimic the natural canopy-soil environment
Water: Rainwater or tap
Light: Shade to dappled shade in the wild (typically 15-40% of full sun intensity), with a strong preference for the filtered light beneath deciduous canopies such as bigleaf maple (Acer macrophyllum) or the perpetually diffuse light of coastal fog forests. In cultivation, tolerates deeper shade than European P. vulgare but performs best with morning sun and afternoon shade in cooler maritime climates. Direct midday sun causes rapid frond scorching and accelerated summer dormancy. The species is photosynthetically active primarily during the cool, low-light winter months, which distinguishes its light requirements from most temperate ferns.
Humidity: 50-85%
Common Mistakes to Avoid
The most frequent and usually fatal mistake is treating Polypodium glycyrrhiza like a conventional evergreen fern and watering it year-round. Summer dormancy is not a failure mode but the species' natural state, and watering a dormant rhizome in a warm greenhouse reliably produces rot within weeks. A second common error is burying the rhizome in substrate; the rhizome must rest on the surface with only feeder roots descending. Third is placing the plant in high-light or warm positions, either on sunny windowsills or in heated living rooms, which produces yellowing fronds, accelerated dormancy, and generally poor growth. Fourth is using municipal tap water with high chloramine or calcium, which burns the thin fronds and alters epiphytic substrate pH over time; rainwater or RO water is strongly preferred. Fifth is attempting cultivation in continental climates with hot summers and cold dry winters without a dedicated cool greenhouse: the species cannot adapt to a phenology reversed from its native one. Sixth is propagating by dividing too small a rhizome section (less than 5 cm with only one frond); small divisions often rot before establishing. Seventh is moving a summer-dormant plant to a warmer location hoping to keep it evergreen; this confuses the physiology and weakens the rhizome. Finally, overfeeding during dormancy causes rhizome scald; always withhold fertiliser from May through August.
Seasonal Considerations
Autumn (September-November): the wake-up season. Resume regular watering in early to mid-September as temperatures drop and natural rainfall returns. New rhizome tips push out within 2-3 weeks and the first fronds unfurl through October. Begin monthly quarter-strength feeding. Inspect for summer damage to rhizome and remove any blackened tissue. Reduce interior temperatures if possible to 10-15°C at night to trigger strongest frond production. Winter (December-February): peak growing season. Maintain steady moisture, never allowing substrate to dry more than the top 2 cm. Fronds reach full size and begin forming sori by February. Keep temperatures between 4 and 18°C; protect from heated indoor air if grown indoors by placing near cool windows or in unheated porches. Continue monthly light feeding. Outdoor plantings need no special protection in USDA zones 7-9 but benefit from a light covering of fallen maple leaves in zone 6-7 cold snaps. Spring (March-May): spore production peaks. Sori mature through April and release spores through May. Continue regular watering but begin gradually reducing fertiliser by mid-April. Repot or divide if needed in April-early May before dormancy. Summer (June-August): dormancy. Fronds yellow and drop progressively through June and July. Once all fronds have senesced, reduce watering to minimal maintenance level (just enough to keep the rhizome from shrivelling, typically once every 10-14 days in containers). Do not feed. Move containers to the coolest, shadiest available location. Protect outdoor plantings from prolonged heat waves by occasional deep soaking of the root zone. Prepare for autumn flush by top-dressing substrate with fresh leaf mould in late August.
Diseases & Pests
Common issues affecting Polypodium glycyrrhiza in cultivation:
- Root/rhizome rot: Caused by waterlogged substrate, compacted soil, or overwatering in cool weather. Ensure the growing medium is well-aerated and never let pots sit in standing water for prolonged periods.
- Fungal leaf spot & Botrytis: Brown or grey blotches appear in stagnant, overly humid conditions. Improve air circulation, remove affected fronds, and avoid wetting foliage late in the day.
- Scale insects & mealybugs: The most common fern pests, hiding on stipes and frond undersides. Wipe off with a cotton swab dipped in diluted isopropyl alcohol, or treat with horticultural soap. Many chemical pesticides scorch fern fronds — always test on one frond first.
- Spider mites: Fine webbing and stippled fronds, common in dry indoor air. Raise humidity and rinse fronds regularly with tepid water.
- Frond browning (tip burn): Caused by dry air, direct hot sun, fluoridated or chlorinated tap water (especially in Nephrolepis, Calathea-loving filmy ferns), or soluble-salt build-up from fertiliser. Flush the pot with rainwater and reduce feeding.
Indoor Growing & Terrariums
Polypodium glycyrrhiza can be grown indoors as a houseplant or terrarium subject when its humidity and light requirements are met.
Indoor Setup
- Light: Bright indirect light — an east- or north-facing window, or 30–60 cm under an LED grow light (10–12 hours/day). Most ferns scorch in direct midday sun.
- Humidity: 50–80%. Group plants, stand pots on a pebble-and-water tray, or run a humidifier; misting alone rarely raises ambient humidity enough.
- Temperature: 16–24°C (60–75°F) for most indoor species; avoid cold drafts and hot radiators.
- Substrate: Peat-free potting mix with added perlite and orchid bark for drainage; epiphytic genera (Platycerium, Davallia) grow best mounted on bark or in a bark-heavy orchid mix.
- Water: Keep consistently moist but never waterlogged. Let the top 1–2 cm of substrate dry slightly between waterings in winter.
- Air circulation: A gentle fan discourages fungal leaf spot without drying out the fronds.
Landscape & Garden Use
Polypodium glycyrrhiza can be used in the garden wherever its hardiness and habitat preferences are matched. Ferns are classic choices for shaded borders, woodland gardens, stumperies, rockeries, stream-side plantings, and ground-cover under trees.
Landscape Tips
- Companions: Hostas, Astilbe, Heuchera, Tiarella, Epimedium, hellebores, snowdrops and other shade-tolerant perennials are classic partners.
- Soil preparation: Incorporate leaf mould or composted bark to improve moisture retention and mimic a forest floor.
- Mulching: A 3–5 cm mulch of chipped bark or leaf litter protects the rhizomes, conserves moisture, and slowly releases nutrients.
- Watering: Establish new plantings with regular deep watering during the first growing season; most hardy ferns need only occasional supplemental irrigation once established.
Conservation & Collector Notes
Globally secure (IUCN Least Concern, G5 NatureServe) with vast populations distributed across the Pacific Northwest rainforest biome. State-level assessments vary: California ranks the species as S4 (apparently secure) with southernmost populations in Monterey County considered locally vulnerable; Oregon and Washington list it as S5 (secure); British Columbia lists it as S5; Alaska as S4. No federal protections apply. Primary conservation concerns are climate-change driven rather than direct: warming summers extend dormancy stress, reduced summer fog frequency threatens canopy epiphyte populations, and large-scale logging of mature bigleaf maple forests removes the most productive epiphytic habitat. Traditional harvesting by First Nations remains sustainable at the scale practised and is protected under various treaty rights; commercial foraging for the rhizome (for craft sweeteners and herbal medicine) is largely unregulated but currently has negligible population-level impact. Long-term monitoring of epiphytic populations is coordinated by regional botanical gardens including the UBC Botanical Garden and the University of Washington Rare Plant Care and Conservation program, with citizen-science observations contributing to iNaturalist and the Consortium of Pacific Northwest Herbaria.
Collector Notes
For pteridology enthusiasts and rare-plant collectors, Polypodium glycyrrhiza is a subtly rewarding subject that repays close observation. Seek ethically sourced material from specialty nurseries in the Pacific Northwest (Fancy Fronds Nursery, Far Reaches Farm, Gossler Farms, and the occasional offerings from UBC Botanical Garden plant sales) rather than collecting from the wild, which damages populations and may violate local regulations. Provenance matters: plants originating from coastal Oregon/Washington populations adapt poorly to Mediterranean California climates, while inland Cascade populations are slightly more heat-tolerant. Specialty forms worth seeking include the 'Longicaudatum' cultivar with elongated pinna tips, the compact 'Malahatense' from Vancouver Island with shorter, denser fronds, and occasional crested or bifurcated forms found in the wild that propagate true from division. Collectors interested in the broader Polypodium vulgare complex often grow P. glycyrrhiza alongside P. amorphum, P. hesperium, and P. calirhiza to observe subtle cytological and morphological differences; the group offers genuine taxonomic puzzles still being resolved by molecular studies. Quality mounted specimens on genuine bigleaf-maple bark slabs are occasionally offered and make striking display pieces. Voucher specimens should be pressed in August when rhizome structure is clearest, with careful preservation of the scaly rhizome architecture alongside a mature frond. Digital photography of rhizome scales, sori, and pinna tips aids later identification. The species is a joy to watch through its full year of growth-dormancy cycles, and committed collectors often keep multiple accessions from different provenances for comparative study.
Ethnobotany & Cultural Significance
Polypodium glycyrrhiza occupies a place of honour in the ethnobotanical traditions of the coastal First Nations of the Pacific Northwest, where the sweet rhizome has been harvested, eaten, and medicinally employed for thousands of years. Haida peoples of Haida Gwaii collected the rhizome in late autumn and winter, scraping away the scales and chewing the fresh green cortex as a sweet food and cough suppressant; the Haida name k'aay refers specifically to the licorice fern rhizome. Tlingit families gathered it during winter storms, roasting or chewing it fresh and using decoctions for sore throats and respiratory congestion. Nuu-chah-nulth peoples of the west coast of Vancouver Island called it tłiłtłiichap or similar cognates, and the sweet rhizome served as a natural children's candy as well as a medicine for colds and coughs. Makah, Quileute, and other Olympic Peninsula peoples used the rhizome as a breath-freshener and mild stimulant before long journeys. Coast Salish communities (Lummi, Squamish, Musqueam, Cowichan, and others) harvested licorice fern from mossed maple branches and rocky bluffs, the rhizomes used as cough medicine, as a sweetener for bitter medicines, and as a food for children and the elderly who had difficulty with tougher fare. Traditional protocols emphasised harvesting only in the wet season when rhizomes were plump, leaving the growing tip intact for regeneration, and never taking from the same stand two years in a row. Modern ethnobotanical studies (Turner, Kuhnlein, Ignace and colleagues) document continuing use. The active sweet compound is the saponin osladin (also present in European P. cambricum and P. vulgare) together with polypodosides A-C, the last isolated and characterised from P. glycyrrhiza specifically by Kim, Kinghorn and colleagues in 1988-1989; polypodoside A is reported to be approximately 600 times sweeter than sucrose on a molar basis. Pharmacological interest continues in the anti-inflammatory and expectorant properties.
Frequently Asked Questions
Does the rhizome really taste like licorice, and is it safe to taste?
Yes to both. The taste is a genuine sweet-anise flavour from the saponin osladin and polypodosides A-C, distinctly reminiscent of licorice or fennel. Tasting a small scraped fragment of your own cultivated rhizome is safe and is indeed one of the traditional pleasures of growing this species. Indigenous peoples consumed it regularly without ill effect for millennia. The sweetness is most pronounced in autumn and winter when rhizomes are plump; summer rhizomes taste flatter. Avoid tasting wild specimens whose identity you are not 100% certain of, and never consume large quantities.
Why do the fronds yellow and drop every summer? Is my plant dying?
No, your plant is doing exactly what it should. Polypodium glycyrrhiza is a winter-growing fern with built-in summer dormancy inherited from its Pacific Northwest maritime climate, where summers are dry. Yellowing and frond loss from June through August is normal and healthy. The rhizome remains alive, stores energy, and will produce fresh fronds when temperatures drop and moisture returns in September-October. Resist the urge to water heavily or move to a warmer spot during dormancy; both interventions actually harm the plant.
Can I grow it epiphytically on a tree branch like in the wild?
Yes, and this produces the most beautiful and authentic presentation. The best host trees mimic native bigleaf maple or Oregon white oak with smooth, moss-friendly bark and slightly basic chemistry. Attach small rhizome divisions to a shaded lower limb using soft cotton twine over a pack of damp sphagnum moss. Mist daily for the first two months and maintain humidity above 60%. Establishment takes two full winter seasons. In climates with dry summers or cold winters below -10°C, this technique works only with specimens moved indoors or protected during the extreme seasons.
Is Polypodium glycyrrhiza really the same as the sweet European polypody P. cambricum?
No but close. They are distinct diploid species (both 2n=74) within the Polypodium vulgare complex, separated by millions of years of Pacific-Atlantic isolation. They share the sweet rhizome chemistry (osladin saponin) and similar pinnatifid morphology, but P. cambricum has a Mediterranean winter-wet distribution and narrower, firmer fronds, while P. glycyrrhiza is Pacific maritime with softer, more translucent fronds. Hobbyists often grow both to appreciate the transoceanic convergence.
Can I use the rhizome as a sweetener for cooking or tea?
Indigenous peoples traditionally chewed fresh rhizome as a natural breath-sweetener and added decoctions to bitter medicines. Small quantities as a novelty herbal tea flavouring or licorice-flavour candy are reasonable from cultivated material. It is not a practical kitchen sweetener because the saponin sweetness is unstable when heated and the rhizome yields are small. Never harvest wild rhizomes except under indigenous traditional protocols in your region.
Why does my indoor plant do well in winter but fail every summer?
Typical summer home temperatures (22-28°C) force the plant into stressed dormancy. During dormancy, the rhizome is vulnerable to rot if kept wet and warm. Move the plant to the coolest room in the house during summer (a basement, north porch, or spare bedroom ideally below 20°C), reduce watering dramatically, and let it rest. It will recover in autumn. Without cool summers the species rarely lasts more than two or three years indoors.
Is it legal to collect from the wild for my garden?
Collection from public lands (national forests, state parks, provincial parks) is prohibited without a permit in all US and Canadian Pacific Northwest jurisdictions. First Nations reserve lands have their own protocols that must be respected. Private-land collection requires owner permission. Beyond legality, wild collection damages epiphytic communities that take decades to develop. Always purchase from reputable specialty nurseries or propagate from ethically obtained divisions.
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Quick Reference Summary: Polypodium glycyrrhiza
Golden Rule: Match moisture, light and humidity to each fern’s natural habitat — woodland ferns need shade and humus, rock ferns need drainage, filmy ferns need constant humidity.
Polypodium glycyrrhiza, the licorice fern, is the defining epiphytic fern of the Pacific Northwest temperate rainforest, cascading from mossed bigleaf maple branches from southeast Alaska to Monterey, California, with disjunct populations reaching Kamchatka and northern Japan. This diploid member of the Polypodium vulgare complex (2n=74) is famous for its sweet rhizome, flavoured by the saponin osladin and the polypodosides A-C, compounds that made it a prized winter food and cough medicine of Haida, Tlingit, Nuu-chah-nulth, Makah, and Coast Salish peoples for thousands of years. Botanically distinguished by thin-herbaceous, lanceolate pinnatifid fronds with acute pinna tips, creeping scaly rhizomes, and a strikingly inverted phenology (winter-growing, summer-dormant), the species matches its maritime climate niche with notable specificity. Formally described by Daniel Cady Eaton in 1856 and separated from European Polypodium vulgare only through 20th-century cytological and molecular work by Lloyd, Lang, Haufler, Windham, and Pryer, it now stands as a biogeographical puzzle whose amphi-Pacific distribution invites continuing study. In cultivation it rewards gardeners who can replicate its cool, wet-winter, dry-summer climate (USDA zones 6-9, ideally 7-8) with elegant winter greenery and the occasional traditional pleasure of tasting the genuinely sweet licorice-flavoured rhizome. Best grown mounted on bark or in loose organic substrate on north-facing mossed surfaces, it combines ecological intrigue, ethnobotanical depth, and quiet horticultural beauty in a single species that belongs in every serious fern collection.