The site is particularly noteworthy for the prominent granite cliffs and tors but it also supports maritime heathland, maritime grassland and scrub habitats together with populations of a number of rare plant and lichen species. Peninnis Head is significant for quaternary geomorphology. It demonstrates spectacular granite cliff topography with excellent examples of tors (rock outcrops formed by weathering), weathering forms and associated head deposits, from the glacial age. It is also significant in lying to the south of the glacial limit in the Isles of Scilly and provides comparison with glaciated bedrock areas in the north of the islands.
The thin skeletal and podzolic soils that overlie the granite together with the extreme exposure have led to the development of wind pruned “waved” maritime heath dominated by Heather Calluna vulgaris, Bell heather Erica cinerea and Western gorse Ulex gallii. In places the open heathland is being invaded by Gorse Ulex europaeus, Bracken Pteridium aquilinum and Honeysuckle Lonicera periclymenum.
Areas of more species-rich maritime grassland occur along the coastal margin where Thrift Armeria maritima, Yorkshire-fog Holcus lanatus, Sorrel Rumex acetosa, Buck’s-horn plantain Plantago coronopus, Sea beet Beta maritima, Common scurvygrass Cochlearia officinalis and Red fescue Festuca rubra are common. This short turf sward is also of importance for the occurrence of the nationally rare Early meadowgrass Poa infirma and the nationally scarce Western clover Trifolium occidentale.
The extreme oceanic conditions experienced at Peninnis Head have also encouraged the development of a rich lichen flora on cliff, tor and heathland habitats. Ramalina siliquosa occurs extensively and Roccella fucoides and Teloschistes flavicans are two particularly rare species that occur here.
Peninnis Head is a Geological Conservation Review site which lies within the Isles of Scilly Area of Outstanding Natural Beauty and Isles of Scilly Heritage Coast. and it is a SSSI.
Management of this site requires all year round care. As the intention of this management is to maintain and restore important habitats. Several different courses of action are required; grazing, bracken rolling/cutting, cutting and clearing European gorse, maintaining firebreaks and paths. Natural England surveys the land to see what condition the different habitats are like in that particular SSSI and suggests ways of how the site can be continuously improved..
In 2009 Peninnis lowland heath habitat was surveyed and found to be in an unfavourable but recovering condition due to the minimal amount of management. The results showed that appropriate management, particulary grazing was needed for the recovery to continue.
In 2010 a survey of the earth heritage at Peninnis Head was undertaken. The results showed that this was in a favourable condition but the re-introduction of grazing was needed to help uncover existing and undiscovered geological formations.