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.
The majority of the cornfield plants do not flower so early as those in the meadows. There are, of course, as in all different types of vegetation, natural or artificial cycles of flowering. For each month is characterized by the flowering of some particular species, whilst locally some are perennially in flower, e.g. Shepherd's Purse, Groundsel, White and Purple Dead Nettle, and in the cornfield the Heart's Ease also. Some of the Speedwells are also in bloom nearly all the year round. Blue Sherardia flowers as early as March, and as late as November.
The earliest-flowering cornfield plants include Mousetail, Ivy-leaved Speedwell, Heart's Ease, Lamb's Lettuce (April). In May Corn Buttercup, Fumitory, Charlock, Venus's Looking Glass, Field Bugloss, Corn Gromwell, begin to flower. The majority appear in June, which is somewhat late compared with the season for flowering in other habitats. Such are Larkspur, Poppies, Candytuft, Charlock, White Campion, Corn Cockle, Spurrey, Flax, Alsike Clover, Shepherd's Needle, Corn Marigold, Bluebottle, Scarlet Pimpernel, Wild Oat, Darnel. Several of these continue till September, e.g. Alsike Clover, Cornflower, Scarlet Pimpernel. Still later in flowering are Fool's Parsley, Corn Sow Thistle, Small Snapdragon, Common Hemp Nettle.
The reason for this late flowering is the disturbance of the ground in February and March.