Sinapis arvensis

Charlock harbours an eelworm that attacks other crops and was once classed as a serious arable pest. It remains one of the most common yellow-flowered crucifers of waste and marginal ground, often found on chalky soil. This plant is rather bristly with coarsely toothed leaves that have a reddish midrib. The flowers have four, widely separated petals that reveal narrow sepals beneath. The seeds can be ground into a powder and used as a hot mustard food flavouring.