Hense Moor includes some of the best remaining examples of lowland mixed valley bog in Devon, and is typical of this habitat in South Western Britain.

Within an altitude range of between 150m and 210m, the valley supports a mosaic of different habitats. These are based on a variety of different soils derived from Keuper Marl on the valley bottoms, Greensand on the sides and Clay-with flints on the valley tops. Peat has formed where the drainage has been most impeded. Around the valley groundwater seeps from the Greensand and several streams arise in and flow through the site.

At the junction between the Greensand and Clay-with-flints moss-dominated springs occur, Bog-mosses include Sphagnum papillosum and S.tenellum.   The site as a whole supports a diverse moss flora, some sixty species having been recorded. Associated herbs are Bog Asphodel (Narthecium ossifragum), Bog St.John's Wort (Hypericum elodes), Pale Butterwort (Pinguicula lusitanica), Great Sundew (Drosera anglica) and Bog Pimpernel (Anagallis tenella).

Where wet heath grades into these boggy areas, Purple Moor-grass (Molinia caerulea) and Cross-leaved Heath (Erica tetralix) become more abundant, together with a variety of other species, including Fir Clubmoss (Lycopodium selago), Round-leaved Sundew (Drosera rotundifolia), Long-leaved Sundew (D.intermedia), Tawny Sedge (Carex hostiana), Carnation-grass (C. panicea), Heath Spotted Orchid (Dactylorhiza maculata) and Lousewort (Pedicularis sylvatica). On more freely drained areas the wet heath has Purple Moor-grass and Dwarf Furze (Ulex gallii) as the codominant species.

In some areas Rushes, in particular Hard Rush (Juncus inflexus) become more abundant. Some areas of this acid marshy grassland contain Marsh Helleborine (Epipactis palustris), along with Star Sedge (Carex echinata) and Meadowsweet (Filipendula ulmaria). Greater Tussock Sedge (Carex panicuata) forms well-developed stands in some areas near the main stream.

Adding to the habitat diversity of the site are several wooded streams. Under a mixed canopy of Alder (Alnus glutinosa), Goat Willow (Salix caprea), Birch (Betula spp.) and Pedunculate Oak (Quercus robur), the damp and shaded ground flora contains Golden Saxifrage (Chrysoplenium oppositifolium), Ramsons (Allium ursinum) and Enchanter's Nightshade (Circaea lutetiana).

On the well-drained steeper valley sides and tops dry heath has developed, dominated by Dwarf Furze. This mixes in with areas dominated by Bracken (Pteridium aquilinum) and scrub, most of which consists of Common Gorse (Ulex europaeus) and Bramble (Rubus fruticosus agg.). In some places on the flatter valley tops semi-improved grassland occurs. Main sward components are Yorkshire Fog (Holcus lanatus) and Sweet Vernal-grass (Anthoxanthum odoratum).

The site's wide variety of habitats in turn supports a rich invertebrate fauna including of particular note the Raft Spider (Dolomedes fimbriatus). Reptiles and amphibians are present in good numbers.