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 linear nature of a roadside, and its boundary on either side by hedgerows, places a certain restriction upon wayside plants so far as the dispersal of seeds is concerned; and it is therefore, in so far as the agency of the wind is concerned, more or less definite in direction, either along the road or from one side to the other. But it must be remembered also that the field side of each hedge acts as a barrier to the dispersal of seeds from the fields, etc, from a distance, and there may thus be an aggregation of seeds, stopped by such barriers, along the highway. Moreover, the very fact that a road is devoted to traffic, as has been shown, ensures that seeds will be dispersed by external artificial agency along the way. The Clematis, Barren Strawberry, Hemlock, Hogweed, Teasel, Nipplewort, Ash, Nettle, have their seeds or fruits dispersed by the wind. A large number of fruits are edible or have hooked fruits, and are dispersed by animals, e.g. Barberry, Sloe, Bramble, Rose, Crab Apple, Hawthorn, Bryony, Hedge Parsley, Cornel, Moschatel, Cleavers, Spurge Laurel, Black Bryony, Cuckoo Pint.
In other cases, such as Hedge Garlic, Hedge Mustard, Greater Stitchwort, St. John's Wort, Herb Robert, Trailing Vetch, Meadow Vetchling, Great Bindweed, Red Bartsia, Wood Basil, Ground Ivy, the plant has a mechanism of its own for dispersing its seeds.
The soil of the roadside is liable to much alteration, not only from the length and continuity of the road, and the existence of cuttings which expose new layers, but also on account of the interlacing character of the roads. A road taken from S.W. to N.E. on the east side of Birmingham would largely pass over the same geological formation and rock soil.
A road such as the Watling Street, or Great North Road, which cuts across these in a S.E. to N.W. or S. to N. direction, however, passes across a number of different formations. In the west of England the rocks are all older, and contribute to form siliceous soils. A few plants need limestone or chalk, as Clematis or Wood Basil.
A large proportion grow on humus, as Barberry, Greater Stitchwort, Herb Robert, Bryony, Cornel; and some are equally at home on either sand or clay, as Barbarea, Hedge Garlic, Spindle Tree, Rose, Hogweed, Hedge Parsley, Lords and Ladies. Sand without humus is needed by Hedge Mustard, Trailing Vetch, Bramble, Barren Strawberry, Hawthorn, Teasel, Nipplewort, Great Bindweed, Nettle. Clay or sand is the requirement of Barberry, Crab Apple, Elder, Cleavers, Red Bartsia, Ground Ivy, Black Bryony; and pure clay is the soil for Moschatel and Bugle, as well as the Ash, which grows in a native state best on limestone. Each plant thus has a special predilection for some one type of soil.
The vegetation of the roadside is composite. There are zones of vegetation, each of which should be studied separately.
The margin of the macadam forms one zone, the greensward forms a second, and answers to the meadow type of flora. In each case a percentage of the most dominant plants should be made. A note should be made as to the soil characters here, as in the other zones, also the slope, and relation to the tree zone if it be well developed. Any unusual features of this zone, as the occurrence of scrub, of ponds, or streams that sometimes run parallel with the macadam should be noted. Where stone-heaps or gateways with open ground occur these may be treated as units in themselves.
The intersection of road drains or roadways at right angles to the macadam should be noted, and any influences these bring to bear discriminated.
The next zone, the ditch, is studied as a small stream or river, where it may show embryonic zonation or bands of vegetation of different types. When dry it may be considered as a ground flora to the semi-woodland type of hedgerow vegetation. Bridges crossing such ditches should be studied apart, and the special features recognized.
The hedgerow bank is treated separately, and the influence of the hedge upon the ground flora should be carefully studied. The hedge itself is treated in the same way as scrub, and plants in the hedge bottom as its ground flora. Where walls occur they should be studied as in the section dealing with walls, etc.