There are three sub-species, two of which are common:
- Rumex crispus ssp. crispus which is mostly found inland but can be on the coast with achene smaller at 1.3-2.5mm and with tubercle smaller at <2.5mm. Moreover, the tubercles are un-equal in size and often there are only one (rather than 3)
- Rumex crispus ssp. littoreus which is found only near the coast, often in the salt zone. Fruits are densely clustered with three tubercles <3.5mm long (but often not quite equal in length).
The third sub-species, Rumex crispus ssp. uliginosus grows on tidal mud in estuaries, but that is a very rare [RRR] and occurs in Southern Britain and Southern Ireland. It is often higher than 1m with the fruits loosely clustered (whereas ssp. littoreus is usually lower than 1m the fruits densely clustered).
Hybridizes with : almost every other dock, such as Wood Dock,
Broad-leaved Dock, Golden Dock,
Greek Dock, Clustered Dock,
Russian Dock and
Scottish Dock except
Shore Dock. See Rumex Hybrid Chart
The most common Dock hybrid out of all possible dock hybrids is that of
Curled Dock Rumex crispus with
Broad-leaved Dock Rumex obtusifolius which is named Rumex × pratensis. This hybrid is common throughout the UK but only where the two parents meet and is by far the most common Dock hybrid, being also the most fertile (which is not saying a lot, its fertility is low). It is recognised by its intermediate leaves and tepals (and by its low fertility).
With such a diverse and prolific number of docks with which Curled Dock can hybridize, your Author can never be sure whether all the above photos are of the pure strain. But seeing as the Sefton Coast is nowhere near Southern Britain, he can be sure that no ssp. uliginosus are mis-represented under ssp. littoreus.