There is a rich mixture of tree types and rare and local species are present. Bigsweir Woods are sited on Devonian Old Red Sandstone and quartz conglomerate. This produces acid soil. The wood is dominated by Sessile Oak but includes some Beech, Small-leaved Lime and Birch. Hazel, Beech, Small-leaved Lime also make up an understorey with Rowan and Holly. Ground flora includes Bramble, Bracken, Ivy, Dog's Mercury and Bilberry.
