Working near Rossington, Doncaster (VC63) a couple of weeks back my eye was drawn to a large Curled Dock (Rumex crispus) growing on the hedgebank. It looked unusually tall (>1.3m) and robust, with broad leaves, a huge inflorescence, large tepals and an usual deep green coloration. Then I saw another and then another. So I collected some hoping it might be the poorly known, and presumed non-native, subspecies robustus.
Further research confirmed these initial thoughts, with the tepals clearly in the size range for this subspecies (being 5-6mm wide). To be on the safe side I pressed it and sent a sample to Geoffrey Kitchener, the BSBI referee for the genus. Thanks to Geoffrey for confirming my identification.
1a. Valves ca. 5-5.5 mm wide, slightly cordate at base, ca. as wide as long, with a single inconspicuous tubercle, ca. 1.5 mm long. Lateral inflorescence branches slightly spreading, not tightly appressed to the main axis. Leaves lanceolate to broadly so, with weakly undulate margins. Plant tall, much exceeding 100 cm === subsp. robustus
1b. Valves ca. 3.5-5 mm wide (longer than wide), truncate at base, with 1-3 tubercles, the larger ca. 2 mm long. Lateral inflorescence branches often tightly appressed to the main axis. Leaves narrower and more distinctly undulate at margin. Plant smaller, rarely exceeding 100 cm (native) === subsp. crispus
The presence/absence of a well developed tubercle seems unreliable for ID purposes, with some disagreement in this in other references. Even on one plant this seems relatively variable. Geoffrey also advises that there can be a zone of overlap in tepal width, so my main advice would be to consider the plant as a whole. Does it otherwise look like typical subspecies crispus (likely to be somewhere nearby as a point of comparison)? If not, chances are its the real deal.
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