Kennet Valley Alderwoods is a 56.8 hectare (140.35 acre) Site of Special Scientific Interest in the civil parishes of Welford and Speen in the English county of Berkshire, notified in 1997.
Located at grid reference SU400675 and at grid reference SU444669, these woodlands are the largest remaining fragments of damp, ash-alder woodland in the Kennet floodplain. The SSSI includes two woods, the Wilderness and part of Ryott's Plantation, which are important because they support a very great diversity of plants associated with this woodland type, dominated by Alder (Alnus glutinosa), though Ash (Fraxinus excelsior) is abundant in places and there is occasional Oak (Quercus robur) and Wych Elm (Ulmus glabra). In addition to the wide range of higher plants the woods support a diverse bryophyte flora including the uncommon epiphytes Radula complanata, Zygodon viridissimus and Orthotrichum affine.