Blue mustard

Chorispora tenella is an annual herb in the mustard family (Brassicaceae), commonly known as blue mustard or crossflower. Plants grow 10-50 cm tall from a slender taproot, with erect to ascending, branched stems that are sparsely hairy and often glandular. The basal and lower stem leaves are oblanceolate to lyrate-pinnatifid, 2-8 cm long, with sinuate to lobed margins; upper leaves are smaller, less divided, and short-petiolate to sessile. Loose terminal racemes bear cross-shaped flowers about 1 cm across, with four pinkish-purple to lavender-blue petals and four sepals; the flowers are fragrant in the evening. The distinctive fruit is a slender, curved, indehiscent silique 2-4 cm long that constricts between the seeds and breaks transversely into segments at maturity. Native to southwestern and central Asia, the species has become naturalized as an invasive weed across western North America, parts of Europe, and Australia, favoring disturbed soils, fields, and roadsides.