This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Widely dispersed as it is, no seeds of Charlock have yet been found in Pre- or Post-glacial beds. It is found in the Warm Temperate Zone, in Europe, N. Africa, X. and W. Asia, as far as the Himalayas, and has been introduced into America.
The Common Charlock, unfortunately for the farmer, is found in every county in Great Britain, and in Ireland and the Channel Islands. It is found at elevations of over 1000 ft.
Charlock is above all a constant denizen of cultivated ground, being evident when in flower in every cornfield, in some cases in such quantity as to give a sulphur-yellow colour to the field. But it often strays beyond arable land, and is found by the wayside with poppies on the bare ground where stone heaps have sometime stood, or along the margin of the macadam, where seeds accumulate in the gutter, amongst numerous similar stations. And then it is to be found in every stackyard and on manure heaps, or where they have once been made.
Being short, roughly hairy, and branched often into two parts about halfway up the stem, Charlock is a compact shrubby plant, with stalked lower leaves, somewhat divided, with the lobes larger upwards, rough, and the upper stalkless, entire, finely toothed, with the lower part of the stem tinged a milky purple colour. It grows profusely in a scattered manner wherever it is found. An alternative Latin name, Sinapis, was given to indicate its turnip-like aspect.
The flowers are bright yellow, and the plant is well distinguished by its smooth, jointed, many-angled pods, which are longer by three times than the single-seeded beak, which is flattened at the sides and conical. The pods are nearly cylindrical. The seeds are black and numerous.
The plant sometimes grows to a height of 18 in. It is in flower from May till August. It is annual, and increased enormously by seed.
The stigma is mature first, when the flower is in bud, opening in the early morning. At the inner side at the base of the short stamens two honey glands are situated, and two in the place of the functionless stamens that do not produce pollen.
The glands can be seen when the calyx expands, and are visible and accessible from outside. Insects can reach them without touching-any of the other parts of the flower. The insect thrusts its proboscis down between the stamens, because the flowers are so close. The stamens laterlengthen and are twisted outwards, and the opportunities for cross-pollination agree with the conditions in Cardamine pratensis. When the flowers wither and the stigma lengthens the anthers turn the pollen-covered sides upwards, bend downwards, and self-pollinate the plant. The visitors are Diptera (Syrphidae), Hymen-optera (Tenthredin-idae, Apidae), Coleop-tera (Coccinellidae), and Lepidoptera (Euclidia glyphica, Burnet Noctua).
The seeds of Charlock are dispersed by the plant itself. The pods open and allow the seeds to be scattered around the parent plant.
It is a sand plant, and requires a sand soil, which may be derived from any of the older formations, such as Coal-measures, Keuper, Lias, etc, in which there are sandstones.
This plant is infested by Ceuthorhynchus sidcicollis, Psylliodes chrysocephalus, Meligethes aeneus, Balanienus brassicae (Beetles), Athalia spinarum (Hymenoptera), Large White (Pieris brassicae), Small White (P. rapae), Green-veined White (P. napi), Turnip Moth (Agrotis segetum), Cabbage Moth (Mamestra brassicae), Bright-line Brown Eye (M. oleracea), PluteIla cruciferarum (Lepidoptera) feed on it.
Photo. B. Hanley - Charlock (Brassica arvensis, O. Kuntze)