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'Hardhead' Knapweeds in Essex:

Centaurea  nigra var. or subsp. nemoralis (= debeauxii) is turning out to be the main form in Essex. With C. nigra var. or subsp. nigra less common. C. jacea has so far turned up near the coast from N. Woolwich, Shoebury and Clacton. We are going to have to revise our current records of C. nigra because of ambiguity on MapMate. The names C. debeauxii and C.nigra make the assumption that they are separate species. However, unless we are careful to record un-separated or intermediate material as C. nigra agg. we risk confusion between C. nigra as a species and C. nigra as an aggregate. The simplest way out of the mess is for everyone to adopt the two names C. nigra var. nigra and C. nigra var. nemoralis when they have separated them. Leaving the C. nigra or C. nigra agg. records to be regarded as undetermined or intermdiate. Since they hybridise and produce fertile intermediates they can't be true species anyway. The hybrid between Centaurea nigra s.l. and C. jacea has been given the name C. x gerstlaureri. SCROLL DOWN FOR ITS PHYLLARIES.