Selaginella uncinata (Peacock Spikemoss, Blue Spikemoss, Hooked Spikemoss, Rainbow Moss, Peacock Moss)
Selaginella uncinata
Table of Contents
Introduction & Discovery
Selaginella uncinata is among the most visually striking members of the genus Selaginella and one of the most photographed plants in any shaded terrarium or conservatory, celebrated worldwide for the vivid peacock-blue iridescence of its foliage. The common names Peacock Spikemoss, Blue Spikemoss, and Rainbow Moss all derive from this notable structural coloration, which is produced not by pigment but by thin-film interference within the specialised cuticle of the upper microphyll surface. Under moderate to deep shade the leaves appear a luminous teal to peacock blue when viewed at low angles, while under brighter light they shift toward an ordinary olive green. The species is native to subtropical southern and south-central China and adjacent parts of northern Vietnam, where it grows as a trailing ground-cover of the shaded forest understorey, on moist shaded cliff faces, along small stream banks, and among mossy outcrops in limestone and sandstone terrain. It was introduced to European horticulture in the nineteenth century and has since become one of the classic plants of the Victorian ferney and Wardian case, as well as a contemporary staple of terrarium culture, paludarium planting, and dart-frog vivaria. Taxonomically the plant belongs to Selaginellaceae, the single family of the order Selaginellales, a lineage of lycophytes that diverged from other vascular plants before the origin of true ferns. It is not a fern but a lycophyte, and displays the diagnostic features of the group: single-veined microphylls in four ranks along the stem, a minute membranous ligule on the adaxial base of each leaf, leafless rhizophores that emerge from the ventral side of the stem and produce anchoring roots on contact with substrate, and heterosporous strobili bearing both large megaspores and small microspores, a reproductive mode shared within living pteridophytes only with Isoetes. The specific epithet uncinata, from the Latin uncinus for a small hook, refers to the recurved apical tips of the lateral microphylls, which in fresh material visibly hook outward from the leaf plane. The physical basis of the iridescence in this species and in the related Selaginella willdenowii has been investigated in detail by plant physicists and remains a benchmark example of structural color in plants.
Discovery & Naming
The genus Selaginella was established by the French botanist Palisot de Beauvois in 1804 and formally circumscribed in his Prodrome des cinquième et sixième familles de l'Aetheogamie of 1805, as a segregate from the Linnaean Lycopodium intended to accommodate the heterosporous ligulate species. Selaginella uncinata was described from Chinese material in the mid nineteenth century and attributed to the pteridological literature of the period; the specific epithet uncinata derives from the Latin uncinus (small hook) and refers to the recurved apical tips of the lateral microphylls. The species entered European horticulture by the 1860s and is prominent in Victorian glasshouse and fernery literature of the 1870s and 1880s, where it is celebrated as Peacock Moss or Rainbow Moss for its blue iridescence. The iridescent structural coloration of the foliage drew scientific attention in the twentieth century; detailed optical and ultrastructural studies using transmission electron microscopy established that the blue reflection results from thin-film interference in a layered cuticle on the adaxial surface of the lateral microphylls. Influential physics-literature treatments of plant iridescence in the 2010s included detailed analyses of Selaginella willdenowii and S. uncinata as benchmark botanical photonic systems, with work by Matthew Thomas, Heather Whitney, Silvia Vignolini and others on the structural basis of iridescence in plants. Twentieth-century pteridological monographs by Alston, Jermy, and more recently Zhang and colleagues in the Flora of China placed S. uncinata in the section Ascendentes or Stachygynandrum of Selaginella. Molecular phylogenies of the genus in the twenty-first century have confirmed its placement within an East Asian clade including S. moellendorffii and the model heterosporous S. tamariscina.
Native Range & Distribution Map
Distribution map showing the native range of Selaginella uncinata.
Biology & Frond Morphology
Selaginella uncinata belongs to the genus Selaginella in the family Selaginellaceae, producing evergreen, prostrate, trailing lycophyte producing long creeping leafy stems rather than true fronds. main axes extend 30 to 60 centimetres or more, branching pseudo-dichotomously into flattened frond-like sprays that may climb short distances up rough bark or rock faces via hooked lateral microphylls. the species is the premier structural-colour lycophyte of horticulture: foliage of shaded plants displays a vivid blue-green iridescence produced by thin-film interference in the cuticle of the upper leaf surface, shifting toward matt olive-green as light intensity increases. microphylls are arranged in four ranks on each branch: two lateral ranks of larger oblong-ovate leaves with acuminate tips that curve into distinctive apical hooks, and two median ranks of smaller appressed acuminate leaves. each microphyll bears a minute membranous ligule on its adaxial base, diagnostic of selaginellaceae. strobili are small, quadrangular, and terminate short erect lateral branchlets. 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 Selaginella uncinata 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 Selaginella uncinata 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: A light, acidic, organically-rich yet free-draining medium. A reliable blend is two parts peat or coco coir, one part fine orchid bark or composted pine bark, one part horticultural perlite, and a small handful of charcoal granules per litre. Sphagnum moss may be added as a surface layer to conserve moisture and provide a pleasing natural appearance in terrarium plantings. Sterilised substrate is preferred for terrarium use. Slightly acidic, pH 5.0 to 6.5. The species tolerates pH 4.5 in humus-rich substrate but becomes progressively chlorotic above pH 7.0. Avoid alkaline or calcareous mixes. Where garden soil is alkaline, amend with acidic peat substitute, composted pine needles, or elemental sulphur to achieve the target range. Free drainage with steady moisture retention. The substrate must never become anoxic or waterlogged around the creeping stems and rhizophores, yet must never dry through. A drainage layer of 2 to 3 centimetres of horticultural grit or LECA at the base of terrarium containers is essential, and a perlite component of approximately 20 percent provides internal pore space within the mix. Low to moderate. An oligotrophic plant of the forest understorey, S. uncinata resents salt-rich substrates and responds poorly to standard-strength liquid fertiliser. A quarter-strength balanced liquid feed once a month during the growing season is ample for potted plants. Outdoor mats are essentially self-sustaining on the nutrient release from canopy leaf litter. High. The species thrives on a substrate dominated by partially decomposed organic material such as peat, coco coir, sieved leaf mould, or composted bark fines, reflecting the humus-rich forest-floor habitat of its native range. Annual top-dressing with a thin layer of fresh fine leaf mould or fine composted bark sustains vigour and helps to maintain soil moisture.
Water: Rainwater or soft tap
Light: Moderate to deep shade, with indirect bright light at the brightest end of the spectrum, is essential to develop and maintain the species' famous blue iridescence. Under 70 to 90 percent shade the foliage displays its full structural-colour shift, appearing vivid peacock blue to teal when viewed at low angles. Under brighter diffuse light the iridescence weakens and the foliage takes on a matt olive-green cast. Under direct sun the structural colour is destroyed within days and the microphylls bleach to papery yellow-brown. For indoor cultivation a position 1 to 2 metres inside an east-facing window, a north-facing window, or under low-intensity LED grow lighting on a 12 to 14 hour photoperiod suits best, providing roughly 80 to 180 micromoles per square metre per second of photosynthetically active radiation. In terrarium culture, pair with other deeply shaded understorey plants and avoid the bright top layer where taller foliage receives most of the light.
Humidity: See species profile
Common Mistakes to Avoid
The single most common fatal error is allowing the substrate or ambient air to dry through: the mat browns irreversibly within 24 to 48 hours. A second frequent mistake is siting the plant where it receives direct sun, which scorches foliage within hours and destroys the iridescent coloration. A third, specific to this species, is expecting the iridescent blue to develop under any lighting: structural colour requires deep bright shade and is strongest at moderate light intensity. Under very bright conditions the foliage turns matt green; under very deep shade the plant grows sparsely and loses vigour. A fourth mistake is using hard mineral-rich tap water, which accumulates as carbonate deposits on the cuticle, dulling the structural colour and eventually causing foliar scorch. Switch to rainwater or filtered water. A fifth is over-feeding; salt build-up from standard-strength fertiliser causes rapid stem-tip browning. A sixth is planting into compacted poorly-drained clay or into a pot without a drainage layer, which causes waterlogging and basal rot. A seventh is attempting open-room indoor culture in dry heated homes below 50 percent humidity, which produces slow decline without any acute event; a terrarium or humidifier is the practical answer. An eighth is planting the species into sensitive native moist woodland in regions where it may naturalise unwanted; responsible cultivation contains the plant.
Seasonal Considerations
Spring (March to May): active growth resumes as days lengthen. Repot pot-bound indoor specimens, renew substrate in terraria, and top-dress outdoor plantings with leaf mould. Begin quarter-strength liquid feeding once a month. Take stem-tip cuttings for propagation; rooting rates are highest now. Move summer-outdoor specimens to shaded verandahs as frosts end in mild zones. Summer (June to August): peak growth in warm humid conditions. Maintain constant moisture indoors and outdoors, mist daily in dry air, and shield from harsh afternoon sun. Iridescence is typically at its most vivid during this season. Do not transplant during heatwaves. Autumn (September to November): growth slows modestly as daylength shortens. Reduce feeding; stop altogether by late autumn. Clear fallen leaves that smother the mat. Bring outdoor summer specimens back indoors before night temperatures drop below 10 degrees Celsius. Move plants away from cold windows. Winter (December to February): the most challenging indoor season. Maintain humidity with a humidifier, pebble tray, or terrarium enclosure. Reduce watering but never allow drying. Keep well away from heating vents and cold glass. Supplement light with a low-intensity grow lamp if the winter photoperiod is very short. Outdoor plantings in zone 7 and marginal zone 8 benefit from a loose insulating cover of leaf litter or bracken.
Diseases & Pests
Common issues affecting Selaginella uncinata 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
Selaginella uncinata 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
Selaginella uncinata 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
Selaginella uncinata is widespread and common throughout its native range in southern China and adjacent northern Vietnam, and is not evaluated as threatened under IUCN Red List criteria or in regional red-data treatments. Within the extensive subtropical forest ecosystems where it grows it forms locally abundant populations on suitable moist shaded substrate and shows no evidence of global decline. Habitat loss through deforestation, agricultural conversion, and urban expansion across lowland southern China reduces suitable habitat locally, but the species is ecologically flexible and persists in secondary forest, bamboo thickets, and the margins of cultivated land so long as shade and moisture persist. It is widely cultivated throughout the subtropical and temperate world and ex situ conservation is comprehensively assured by its presence in botanical gardens, commercial nurseries, and private collections. The species has escaped cultivation in several regions including the southeastern United States, Japan, and some Pacific islands, where it naturalises in moist shaded woodland. In the eastern United States it is recognised as an introduced species with some invasive tendency, particularly in Alabama, Georgia, and Florida, but regulatory concern is mild compared to Selaginella kraussiana in New Zealand. Responsible cultivation outside the native range avoids planting directly into sensitive natural habitats and confines the plant to contained beds, terraria, or glasshouses.
Collector Notes
For the specialist Selaginella collector S. uncinata is a benchmark iridescent species that merits comparison with other structurally coloured plants. The species typically grows as a single clonal form in the horticultural trade, with little published cultivar diversification; reports of named selections such as Prima Donna or Lucida circulate in specialist terrarium communities but are not widely standardised. Wild-provenance material from different Chinese localities is rarely offered commercially but would be scientifically valuable for comparative study of iridescence, cold tolerance, and growth rate. A good collector's display places S. uncinata alongside S. willdenowii, the two principal iridescent spikemosses, and ideally extends to the erect Peacock Begonia (Begonia pavonina) and the aquatic Peacock Fern (a trade name for some forms of Microsorum pteropus although not iridescent), all under identical lighting, to demonstrate the diversity of structural coloration in the plant kingdom. Because of the species' ease of vegetative propagation, material is readily exchanged between collectors. Labelling, provenance documentation, and clean culture practices are especially valuable because the species tends to be loosely labelled at retail. Photographic documentation of iridescence under standardised lighting and angle is the hobbyist's scientific contribution of choice: published ecological literature on the phenomenon is substantial but collector-submitted images with metadata (light intensity, angle, camera exposure) supplement it usefully.
Ethnobotany & Cultural Significance
Selaginella uncinata has a documented but modest role in traditional Chinese ethnobotany, where several Selaginella species, including this one alongside the famous resurrection plant Selaginella tamariscina, appear under names including 翠云草 (cui yun cao, roughly green cloud grass) in regional folk medicine as cooling agents, diuretics, and topical applications for minor skin complaints and cuts. These uses are recorded in regional ethnomedical surveys but are not part of the classical materia medica of traditional Chinese medicine and are not codified in official Chinese pharmacopoeial standards. Phytochemical studies of S. uncinata have identified the characteristic biflavonoid compounds of the genus, including amentoflavone and robustaflavone, along with members of the selaginellin family of dimeric phenolic compounds; some of these show antioxidant and cytotoxic activity in laboratory assays but none is in modern clinical use. The primary cultural role of the species in the contemporary world is horticultural: introduced to European gardens in the nineteenth century, it has been a staple of glasshouse and terrarium culture for more than 150 years, features in classical Victorian horticultural literature, and is a frequent subject of botanical photography because of the striking iridescence. Scientific interest in the physics of that iridescence has produced a substantial literature on thin-film interference and photonic structures in plants, with S. uncinata and the related S. willdenowii serving as widely cited model systems.
Frequently Asked Questions
Why is my Peacock Spikemoss green instead of blue?
The blue iridescence of Selaginella uncinata is a structural colour produced by thin-film interference in the cuticle of the lateral microphylls, and is fully expressed only under moderate to deep bright shade. Under stronger light, including direct sun or a bright windowsill, the cuticle structure is disrupted and the foliage reverts to ordinary green. To restore the blue colour, move the plant to a darker position (a north-facing window, deep shade 2 metres from a south window, or a shaded corner of a terrarium), reduce exposure to direct sun entirely, and wait 3 to 6 weeks for new growth to develop under the new light conditions. The old matt-green foliage will not recover its colour; only newly emerged microphylls will display fresh iridescence.
Is tap water really unsuitable for this plant?
Hard mineral-rich tap water is the single largest chemical problem for Selaginella uncinata. Calcium carbonate and magnesium salts accumulate on the cuticle and in the substrate, dulling the iridescence, raising the pH into a range the species dislikes, and eventually causing stem-tip scorch. Where tap water is soft (below approximately 60 milligrams per litre of dissolved solids) direct use is acceptable. Elsewhere switch to rainwater, distilled water, reverse-osmosis water, or cooled boiled water for watering and misting. Flushing the substrate periodically with clean soft water also helps to prevent salt build-up from cumulative irrigation with harder water.
Can I plant Peacock Spikemoss in my outdoor shade garden?
Within USDA zones 8 to 10 and in mild-winter temperate regions with reliable summer rainfall or irrigation, yes. Choose a shaded moist position beneath tree ferns or large-leaved shrubs, on acidic organic-rich soil, and water through summer dry spells. Iridescence will develop under deep shade. In colder zones the species is unreliable outdoors and is better confined to a conservatory or terrarium. In some regions, particularly the southeastern United States, the plant has naturalised in moist deciduous woodland and should not be planted adjacent to sensitive native habitat; confine it to contained beds or pots where escape is unlikely.
How do I stop my terrarium Selaginella from taking over?
In a well-lit humid terrarium Selaginella uncinata can expand vigorously and smother smaller companion plants. Manage its spread by regular trimming with sterilised scissors back to the desired outline; discard trimmings rather than leaving them on the substrate, as even small fragments readily root. Physical barriers such as submerged plastic edging or a ring of dense hardscape help to define planting zones. When replanting a terrarium, move the spikemoss to a contained pocket or pot sunk in the substrate to prevent runaway spread. Avoid feeding to excess, since fertilisation accelerates the mat's expansion disproportionately.
Are the iridescent pigments harmful to my dart frogs or other vivarium inhabitants?
No. The blue iridescence of Selaginella uncinata is a structural phenomenon produced by thin-film interference in layered cuticle, not a pigment, and the plant contains no compounds known to be toxic to amphibians or small reptiles at any realistic exposure. The species is widely used in dart-frog vivariums, gecko terrariums, and other amphibian enclosures without documented adverse effects. Standard vivarium hygiene applies: avoid fertilisers during animal residency, rinse new cuttings thoroughly before introduction, and monitor for decaying material that could support pathogens. The plant's high humidity preference aligns well with most amphibian vivarium conditions.
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Quick Reference Summary: Selaginella uncinata
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.
Selaginella uncinata, Peacock or Blue Spikemoss, is a trailing subtropical lycophyte native to southern and south-central China and adjacent northern Vietnam, celebrated worldwide for the vivid blue iridescence of its foliage, produced by thin-film interference in the cuticle of the upper microphyll surface. Belonging to Selaginellaceae and not to the true ferns, it displays all the diagnostic lycophyte features of single-veined microphylls in four ranks, a minute membranous ligule on each leaf, rhizophores in place of true primary roots at stem nodes, and heterosporous strobili producing both large megaspores and small microspores. Culturally the species requires consistent substrate moisture, high humidity above 65 percent, soft water, moderate to deep bright shade, and temperatures of 16 to 26 degrees Celsius; iridescence develops only under deep shade and is lost under bright or direct light. It is hardy in USDA zones 7 to 10 outdoors and is a premier plant for shaded terraria, paludaria, vivariums, and glasshouse plantings in cooler climates. Propagation is very easy by division, stem cuttings, or natural layering. The species has escaped cultivation locally in the southeastern United States and some subtropical regions and should be confined to contained plantings where escape into sensitive habitat is possible.