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.
One feature of the cornfield is its ephemeral character. This lack of permanence of conditions makes itself felt upon the plants associated with the corn itself. It can hardly be otherwise. For the time which elapses between one ploughing and the next precludes the continuity of the bulk of the plants in the cornfield beyond one season. Thus all but three of the plants whose life-history has been described in detail are annuals which must germinate in March and April, and flower and fruit from June till September.
This indeed is the only adjustment that can be made, and it is owing to their annual character that nearly all the cornfield plants bloom so late. White Campion (June to July), Alsike Clover (June to September), Corn Sow Thistle (July to August), are three cornfield perennials that grow in cornfields.
Two others, Cornflower and Blue Sherardia, both of which flower up till September, may be annual or biennial. So that there seems to be a tendency for the plants to struggle towards perennial habits.