We are used to seeing fields of yellow these days, yellow with the blossoms of oil-seed rape grown for the manufacture of cooking oil and butter substitutes. We may think of this as a modern phenomenon but years ago fields were yellow with the flowers of charlock (Sinapis arvensis), also known as wild mustard. Charlock was not grown as a crop, however, as its leaves and seeds are poisonous if consumed in any quantity. Charlock is a fast spreading weed of disturbed soils (mainly on lime or chalk) in fields, waysides and waste areas. Now controlled, where it does occur it is usually present in large, somewhat untidy masses of plants.
Like other mustards, charlock is a member of the brassica or cabbage family and has the familiar four-petalled flowers of this group of plants (also known as crucifereae). The flowers emerge up the stems as previous flowers, now lower down, turn to long, cylindrical, smooth pale green seed pods. The plants grow to about a metre tall and have large lobed leaves.
Charlock is a popular food source for bees and other insects and is also a food plant for the larvae of both large and small white butterflies.