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.
Growing under variable conditions as regards the habitat, and the mode of association of the components of each florula, waste-ground plants vary a great deal in the height to which they are capable of attaining. This is largely dependent upon habit, and the relation of the latter to light. For where plants vary in respect of height, it is usually where the plants grow close together and have no elbow-room that they grow tall and lank, whilst where the ground is less occupied they are bushy and not so tall.
From 1 in. to 3 or 4 in. is an unusual height, but is the usual limit of the trailing plants, as Stork's Bill, Knotgrass, Creeping Toadflax, though they may sometimes reach a much greater height. Six inches to a foot is the normal height of the rosette plants and those with a grass habit, as Shepherd's Purse, Mouse-ear, Chickweed, Stinking Mayweed, Groundsel, Hawksbeard, Henbane, Toadflax, Purple Dead Nettle, White Dead Nettle, Good King Henry, Wall Barley; but they may also, under abnormal or especially favourable conditions, become much taller.
Greater Celandine, Goutweed, Tansy, Musk Thistle (to 3 ft.), Hound's Tongue, Viper's Bugloss, Fat Hen, Dairy Maid's Dock (to 3 ft.), range from 2 ft. upward, the habit being pyramidal. Others are 3 ft. or more, as Burdock, Spear Thistle, Chicory, Belladonna; whilst Mallow, Melilot, are 4 ft. or more, Milk Thistle 5 ft., Mullein 6 ft., Bittersweet as much as 20 ft., being a climber.
The waste place is, as a rule, a dry habitat. The soil is sandy, seldom clayey. Everything in the habitat tends to promote the early flowering of the plants. But contrary to what we should expect, the majority of the plants do not bloom till fairly late. But there are two features that are connected with waste-ground plants and their flowering that to some extent explain this.
Few, except the larger woody plants, as Mallow, are perennials. And in general it may be stated that the plants that are perennial flower early, annuals later, and biennials still later.
Another feature is the almost perennial flowering period of some plants, as Shepherd's Purse, Groundsel, which may be found in bloom almost any month, and much the same may be said of Mouse-ear Chickweed, Hawks-beard, Purple and White Dead Nettle, etc.
Relatively few flower in April, as Stork's Bill and White Dead Nettle generally, and Knotgrass. In May, Greater Celandine, Mallow, Goutweed, Purple Dead Nettle, Good King Henry, flower. In June the majority come into bloom, as Mouse-ear Chickweed, Melilot, Stinking Mayweed, Spear Thistle, Hawksbeard, Hound's Tongue, Bittersweet, Belladonna, Henbane, Toadflax, Dairy Maid's Dock, Wall Barley. A number do not even flower till July, such as Tansy, Burdock,
Musk Thistle, Milk Thistle, Chicory, Viper's Bugloss, Mullein, Creeping Toadflax, Fat Hen. These later-flowering plants bear more flowers or florets than the earlier-flowering-plants, which opens up a hitherto unrevealed principle.