This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Introduced. Annual and winter annual. Propagates by seeds. Time of bloom: Late May to July. Seed-time: July to August.
Range: Throughout the world, wherever grain is grown. Habitat: Grain fields, roadsides, and waste places.
"A very little Cockle is sufficient to cut the grade," says a market report from one of the wheat-growing states. The plant is particularly a weed of grain fields, and it is there because it is sown there. The seed is poisonous, and when ground with wheat the flour is rendered unwholesome and even dangerous as food. Poultry and other animals have been killed when fed with screenings composed largely of seeds of Cockle. (Fig. 94.)
Stem erect, slender, one to three feet tall, simple or with a few branches near the top, clothed with whitish, appressed hairs. Leaves opposite, a character common to the Pink Family; two to four inches long, lance-shaped to linear, the lowest slightly narrowed at the base, all softly hairy. Flowers terminal on long, hairy peduncles, often an inch and a half broad, with five spreading, reddish purple petals, which are slightly notched at the outer edge and dark-spotted near the claw; calyx ovoid, hairy, and strongly ten-ribbed, with five long, pointed lobes extending beyond the petals; styles five, opposite the petals; stamens ten. Capsule ovoid, one-celled, sometimes exceeding a half-inch in length, and containing twenty-five to forty black or very dark brown seeds, rounded triangular in shape and roughened with rows of short teeth; the size and weight of the seeds make them very difficult to remove from grain among which they are mixed. When in the soil they retain their vitality for several years.
Sow clean seed. When Cockle is first discovered among the grain, hand-pull and destroy the plants before any seed matures. If a field is too rankly infested for hand-pulling, an application of Copper sulfate or Iron sulfate spray will so injure the tissues of leaf and flower as to prevent the development of seed. Ground where Cockle seed has ripened and been distributed should not be used for grain again until after some cultivated crop has been given a place in the rotation.