Billsmoor Park and Grasslees Wood is the name given to a Site of Special Scientific Interest (SSSI) in Northumberland, North East England, designated in 1954.

[1][2] Both park and wood are listed as an increasingly uncommon examples of an alder and oak woodlands, their scarcity arising out of clearfelling and over-grazing which prevents new growth from being established.

[2] Wooded areas of Billsmoor Park support alder (Alnus glutinosa) with some hazel (Corylus avellana) on lower lying wet soils, and an undergrowth of soft rush (Juncus effusus), tufted hair grass (Deschampsia cespitosa), wood-sedge (Carex sylvatica), greater-tussock sedge (C. paniculata) and pendulous sedge (C. pendula).

In very wet areas of the site, mire vegetation arises, including bog myrtle (Myrica gale), broad-leaved cottongrass (Eriophorum latifolium), grass-of-Parnassus (Parnassia palustris) and purple moor-grass (Molinia caerulea) and, around springs, yellow pimpernel (Lysimachia nemorum), bugle (Ajuga reptans) and water mint (Mentha aquatica).

Groundcover indicative of a dry acid soil includes sweet vernal grass, common bent, creeping soft-grass (Holcus mollis), wood sorrel (Oxalis acetosella), tormentil (Potentilla erecta) and bracken (Pteridium aquilinum), together with remote sedge (Carex remota), oak fern (Gymnocarpium dryopteris) and enchanter's nightshade (Circaea lutetiana).