The geology of Annet is of Hercynian granite overlain with raised beach deposits. On the southern end there is a "thick bed of soil, part sand and part soil". The island is low-lying with a top height of 18 m and the coast consists of boulder storm beaches. The effects of wind exposure, salt spray and lack of topography, restricts diversity and only 53 species of vascular plants have been recorded. The north of the island is dominated by a well developed, thick, thrift ("Armeria maritima") turf whilst the southern part is dominated by dense stands of bracken ("Pteridium aquilinum"), bramble ("Rubus fruticosus") and bluebells ("Hyacinthoides non-scripta"), with some sand sedge ("Carex arenaria") and Yorkshire fog ("Holcus lanatus"). Thickets of tree mallow ("Lavatera arborea") have developed at the back of some of the boulder beaches. There were scattered colonies of shore dock ("Rumex rupestris") until a storm in 1982 swept away some of the boulder beaches. One colony remains in the corner of a relatively sheltered beach in the south of the island at a freshwater seepage. Shore dock is one of the primary reasons for the selection of the Isles of Scilly as a Special Area of Conservation (SAC). Annet is the ″British stronghold″ for the lichen "Roccella fuciformis".