Asian Bittercress (Cardamine occulta)
- has it arrived unnoticed? - II.
On 26 April 2016 I produced a short article suggesting that C. occulta is worth looking for as a possible new invading alien species from mainland Europe where it is spreading rapidly in synanthropic environments. This attracted comment first from Dr Slenker (Czech Republic) and then from Michael Ristow at the University of Potsdam, both of whom are of the opinion that the plant I thought may be C. occulta is in fact C. flexuosa, giving reasons.
Whenever visiting local garden centres I now look out for C. occulta but have so far been unsuccessful. I also look in UK botanical literature to see if it has been recorded elsewhere but for reason unknown I actually missed reading an article by Drs Elizabeth Cooke and Steven Heathcote in BSBI News No 135, Pp 73&74. To save a lot of words I refer you to this article with its table of features differentiating C. occulta, C. flexuosa and C. hirsuta from each other, and photographs on the inner face of the back cover page. Anyone without copy please contact me.
So difficult it is to differentiate this taxon, which has some characters of either C. flexuosa or C. hirsuta, it has been informally called 'Asian C. flexuosa'. Briefly, European C. flexuosa is tetraploid (2n=32) whereas Asian C. flexuosa (C. occulta) is octoploid (2n=64), the former originating from C. hirsuta and C. amara and the latter from C. amara, C. parviflora (or unknown relative) and another yet unidentified taxon. Phenotypic plasticicity is therefore likely to cause confusion.
To answer the question, 'has it arrived?' - yes, it has been found as a container weed at a nursery in Coventry by Cooke and Heathcote in 2017. It had been bought in from a supplier in Norfolk. Like me, the authors of the BSBI article encourage close examination of Cardamine. They are happy to review specimens and pictures but warn that Cardamine taxa can show a large amount of phenotypic plasticity in response to environmental variables.
I agree with my correspondents that the specimen I used to illustrate my original article is C. flexuosa (European) based on a number of characters and is not C. occulta. I am also amazed that articles and notes on our Local Group webpage reach and attract helpful responses from such a wide audience.
Peter J Cook, 13 November 2017