Overview
The Dense Greenhood (Pterostylis spissa) is a distinctive, multi-flowered terrestrial orchid native to the magnificent karri and jarrah forests of Western Australia’s high-rainfall south coast. Distinguished by its tightly packed, upright floral spike carrying numerous translucent emerald-green and white hooded flowers, this winter-to-spring bloomer is a master of mechanical insect entrapment.
Biology and Physical Characteristics
Pterostylis spissa produces a dense basal rosette of three to six soft, bright green ovate leaves surrounding the stem base. The flowering stem rises to 40 centimeters and bears three to ten densely crowded, upright hooded flowers. The galea is formed by the union of the dorsal sepal and petals, creating a translucent protective helmet with fine dark green longitudinal veins. The lateral sepals are erect, loosely clasping the galea, while the interior irritable labellum is oblong, equipped with sensitive tactile hairs.
Behavior and Ecology
Flowering from August to November, the Dense Greenhood utilizes a spring-loaded trapdoor mechanism to capture tiny male fungus gnats. Lured by chemical secretions inside the flower, a gnat lands on the labellum; upon tactile contact, the irritable hinge instantly snaps inward, sealing the insect inside the floral chamber against the column. In its scramble towards the only visible light source at the top of the hood, the insect brushes against the viscidium and pollinia, ensuring pollen transport to the next flower.
Conservation and Human Impact
Classified as Least Concern, Pterostylis spissa enjoys high levels of protection within Walpole-Nornalup National Park, Mount Frankland, and Warren National Park. The moist microclimates provided by intact old-growth karri forest canopies are critical for its survival, buffering colonies against desiccating summer conditions.
Interesting Factoid
The specific Latin epithet ‘spissa’ means ‘dense’ or ‘crowded,’ referring to the uniquely tight, congested arrangement of flowers along its upright greenhood stem.
Last updated: October 2026