Common problems with field identification – the Polygonum aviculare aggregate BOB LEANEY
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s defined by Stace (2019), this aggregate includes four taxa: Polygonum depressum (syn. P. arenastrum) – Equalleaved Knotgrass; Polygonum aviculare s.s. – Knotgrass; Polygonum boreale – Northern Knotgrass; and Polygonum rurivagum – Cornfield Knotgrass.
The aggregate continues to cause problems on almost every meeting of the Norfolk Flora Group, both in separating P. depressum from P. aviculare s.s., and also in attempting to recognise P. rurivagum. The former problem is made worse by the fact that the main stem leaves subtending the branches, which are needed to demonstrate heterophylly or lack thereof, have usually fallen by the time the diagnostic floral and fruiting characters are apparent; but there are also frequent plants that seem intermediate and cannot be assigned to the usual species concept. Attempts to find P. rurivagum are virtually always fruitless, mainly I feel because this taxon, at least nowadays, is actually very rare!
Polygonum depressum (Equal-leaved Knotgrass): Low, decumbent form with all small, dark green, elliptic, blunt-tipped leaves. Bob Leaney
I have no experience of P. boreale, which is confined to Scotland, so will not deal with this taxon. It differs from the other three taxa within the complex mainly in having obovate leaves with quite long petioles that are well exserted from the ochreae (fused stipules). Our usual nomenclature and identification characters derive from a large morphometric study by Styles almost 60 years ago (Styles, 1962). He recognised P. aviculare s.l. as being comprised of the four species listed above, and considered species status for P. aviculare and P. depressum as being ‘beyond doubt’. However, he was less sure of the specific status for P. rurivagum, and experienced considerable difficulty in tracing populations, either in the field or in herbarium collections. Styles’ study suggests that P. rurivagum was very scarce even in his time, and confirms the supposition that the species is rare rather than under-recorded nowadays. BSBI NEWS 148 | September 2021
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