Habitat and Ecology
​Selaginella siamensis is a unique species of spike moss (lycophyte) native to Indochina (Thailand, Laos, Cambodia, and Vietnam) and Southern China (Yunnan). It is a terrestrial plant, often found on rather dry ground with rocks in open forests and forest margins, typically at elevations between 800 and 1,800 metres. This species is adapted to a monsoon climate with a distinct seasonal life cycle: the above-ground stems often die back completely during the dry season, surviving as an underground rhizome, before reviving with the onset of the wet season.
​Description
​This is an evergreen perennial (when cultivated without a dry rest) that has a distinctive and highly ornamental growth pattern. The main stems can be long and creeping, with sections ascending or growing prostrate. It is noted for its unusual growth habit where the slender stems branch strictly dichotomously (forking equally), creating a symmetrical, feathery appearance. The minute, scale-like leaves (microphylls) are dimorphic.
​The most sought-after feature is the strong iridescence of the foliage. The leaves, especially when young and viewed in low, filtered light, shimmer with a metallic blue, purple, or electric teal colour. This iridescence is thought to be an adaptation to low light levels in its native habitat. The plant typically reaches about 25 cm in height in cultivation.
​Cultivation
​Selaginella siamensis is a delicate, moisture-loving plant best grown in a terrarium, vivarium, or humid cabinet in the UK to maintain its high humidity requirement. It requires bright, indirect light or shade; filtered light is essential to enhance the iridescent blue colouring, but direct sun must be avoided. The growing medium should be loose, airy, and very well-draining (e.g., pure sphagnum moss, fine bark, and coir) to mimic the shallow pockets of debris on rocks. The soil must be kept consistently moist during active growth, but you must take care to avoid constant saturation to prevent root rot.
​Optimal Daytime Temperature: 22-28  (72-82 F)
​Optimal Nighttime Temperature: Not below 18 C (64 F).
​High humidity (80% or more) is crucial, and tepid water should be used. For seasonal growers, a cool, significantly drier period in winter can trigger its natural dormancy, but this is not necessary for continuous home growth.
​Unique Features
​This species is a highly desirable collector's must-have due to its intense, metallic blue/purple iridescence—it is considered one of the most colourful species in the genus. The symmetrical, dichotomous branching creates a beautiful, unique architectural form. Its evolutionary adaptation to a dry season (shedding leaves and surviving as a rhizome) also makes it fascinating, though in stable indoor humidity, it is usually evergreen.
​Specifications
​Plant Size: established plants in 5 or 7 cm pots
​Plant Family: Selaginellaceae
​Plant Passport: A Selaginella siamensis B 140084 C [user input] D GB