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.
In a cornfield one is met at every turn by artificial agencies. Not least is the direct intervention of man causing the distribution of cornfield weeds. The cultural operations themselves are so far-reaching that there is very little scope for any other natural agency in the matter, except that of the wind, which may not only disperse the seeds in one cornfield over the whole area, but also cause dispersal from one area to another.
The operations of preparing the ground for corn are not in themselves selective so far as the dominance or otherwise of the plants associated with corn are concerned, though indirectly they are so, as has been shown. But there is a direct influence upon the weed flora when weeding is undertaken, as in clean fields it always is. In this case the farmer or his men exercise a good deal of personal discrimination as to what plants are to be specially eradicated and what may with advantage be left. Thistles are of course exterminated where possible, and so are such Grasses as Couch Grass. The Corn Buttercup and Charlock also are regarded as special enemies of the farmer, and in some areas the Corn Marigold has been laid under the ban, and a royalty paid upon its eradication. Such poisonous plants as Fool's Parsley and Darnel are harmful for wheat-growing purposes, and are also exterminated wherever they occur. Hence the character of the cornfield flora must be regarded as largely dependent upon man himself.