Knopper Gall - Andricus quercuscalicis f. agamic

Knopper galls are a mass of ridged tissue formed on acorn cups, with a large central cavity and small inner gall.  Multiple galls can form on one acorn.  They are green and sticky at first, then reddish and becoming brown and woody.   Eventually they fall and can be found carpeting the ground under oaks.

[Note: the sexual galls are found on Turkey Oak catkins]

Andricus grossulariae has similar asexual galls on oak acorn cups; young knopper galls may be mistaken for these. If in doubt, check the numbers of chambers inside 

Photograph the gall on oak, and note the host species

English Oak Quercus robur, and the hybrid Q x rosacea (according to Redfern & Shirley 2023, they are very rare on Sessile Oak Q petraea)

very common (British Plant Galls, Redfern & Shirley, 2023). A fairly recent introduction to the British Isles, first arriving in the 1960s and now found throughout England, Wales and as far north as Scotland.

very common

See British Plant Galls (Redfern & Shirley, 2023)