Lesnes Abbey Wood is approximately 88 hectares (217 acres) in size. The wood is made up of a large complex of ancient and secondary woodland, with adjacent areas of heathland and acid grassland. Sweet chestnut (Castanea sativa) and sessile oak (Quercus oak) dominate older woodland, the extent of the latter being particularly unusual in London. Birch (Betula spp.) and oak woodland on former heathland provides further structural variation.
The woodland supports a rich flora, including London rarities spurge laurel (Daphne laureola), southern woodrush (Luzula forsteri), thin-spiked wood-sedge (Carex strigosa) and wild daffodil (Narcissus pseudonarcissus), the latter at perhaps its only native site in the capital. In early spring the wood has a amazing display of native wild daffodils.
There are small but significant areas of heath and acid grassland contain heather (Calluna vulgaris), with lesser chickweed (Stellaria pallida), little mouse-ear (Cerastium semidecandrum), subterranean clover (Trifolium subterraneum) and the nationally scarce lesser calamint (Clinopodium calamintha).
The walls of the ruined abbey support further regionally uncommon plants, including rue-leaved saxifrage (Saxifraga tridactylites). The avifauna includes all three British woodpeckers, nuthatch and treecreeper. Reptiles include slow-worm and common lizard. The site also appears important for bats, including rare species. The site has over 900 species of invertebrate, more than 40 birds including redwing and fieldfare, over 70 recorded species of fungi, nearly 300 species of plant and 12 species of mammal. The site also includes the Abbey Wood Geological Site of Special Scientific Interest. Dr Paddy Cocker was of the opinion the site was also of SSSI quality for its plants and animals.