I’ve been trying to tell apart the different docks and it is getting easier as the fruits develop. Four species are common around here. Broad-leaved dock is the most familiar and probably the one you reach for to neutralise nettle stings. Note the frilly ‘tepals’ around the fruit or ‘wart’.
Curled dock has distinctly curled leaf edges and broad smooth-edged tepals shielding the warts:
Wood dock and clustered dock are harder to tell apart. Both have branching stems with whorls of flowers and fruits around the stem, and more delicate leaves than broad-leaved dock. Clustered dock leaves are present further up the stems, interspersing the flowers, and the branches spread out more, sometimes angled as much as 90 degrees from the main stem. There are three tepals back to back: in wood dock, only one has a well-formed wart whereas in clustered dock all three have warts.