Thale Cress Arabidopsis thaliana is now the commonest pavement plant in Waterloo. Each year it dominates the walls and cracks for just a couple of weeks, the rest of the year it is pretty inconspicuous, though I have seen it in flower for 10 months of the year, March to December. Native to Eurasia and North Africa it has been introduced and naturalised in the Americas, Africa, Australia and New Zealand. It was the first plant to have it’s genome sequenced in 2000, as it is has relatively few base pairs, just around  135 million. The life cycle can be completed in just 6 weeks.
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In March / April been restricted to the flora of the Waterloo area (but that is what I am studying anyway). Principally the streets and the Rimrose Valley.
There is an interesting article on the flowering times of urban Cambridge by Chris PrestonThe phenology of an urban street flora: a transect study in British and Irish Botany at https://britishandirishbotany.org/index.php/bib/article/view/44