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.
As the media for traffic of all kinds it is not to be wondered at that roads afford one of the greatest means of dispersal of plants. And though this is obvious if one thinks about the matter at all, yet it does not seem, like many other facts of this nature to which attention is drawn in these notes, to have been adequately considered.
It should be noticed that the distribution is in the first instance linear, but may be later much more general, and the origin (via any particular highway) may be obscured. Another equally important fact is the extra protection afforded by the unusual closeness of the hedges, and the ample shelter they, and the ditches, afford. The greensward also is subject to interference from traffic by man or horses, etc., or mowing in summer, or the operations of the road-scraper, hedger, or ditcher.
Man himself is responsible for some dispersal of seeds. Workmen carry in their bags plants and soil, liable to be dropped in passing to and fro. People using roads who have traversed arable or even grass fields or woods are liable to leave seeds behind embedded in mud from boots or shoes, or which have been caught in the clothing. Gardening operations in allotments, etc, are responsible for a good deal also, weeds being thrown over the hedge into the road.
Birds especially are liable to carry seeds and drop them along the highway. Cattle, horses, etc, disperse them in hoofs; and in their coats, which are woolly or hairy, seeds that are furnished with hooks or spines may be caught, and so dispersed. The carting of hay, corn, stones, lime, dressings, manure, etc, is a very frequent source of dispersal on highways.
Since an integral part of the highway is the hedgerow on each side, it is best to regard the hedgerows in fields as similar in character to the roadside hedgerows, for both have the same origin. One feature of roadside hedges, however, is their continuity in a more or less parallel course, whilst hedgerows in fields are limited in extent and direction to moderately-sized rectangles; so that dispersal along the wayside is if anything more permanent.
The roadside or border of each hedge on a highway is frequently the habitat of a more numerous ground flora, as it is less disturbed in rural districts, but the hedge itself is usually kept well trimmed and layered, whereas the hedges in fields are often allowed to grow for a long period untouched.
When a series of roads has been studied and the floras of all compared, one outstanding feature will become apparent. It will be found as a rule, allowing for the possible change in soil, altitude, moisture, etc, that the flora of the roadside is fairly uniform, when the immediate effect of villages or towns upon the route is eliminated.
But a noticeable fact, which will soon become apparent, is the occurrence at variable distances from a village of certain plants, which do not travel far along the highway on either side of a town or village. Such plants are, for example, Greater Celandine, Winter Cress, Dwarf Elder, Tea Plant, Hop, Horse-radish, Chickweed, Comfrey, Borage, Alkanet, Clary, Black Horehound, White Horehound, Pelli-tory-of-the-Wall, Good King Henry, etc.
Rarely, if ever, do these plants occur in the majority of districts in any other or a possibly native station. The probable reason of the occurrence at all near villages or on the highways, is the former use of these plants for domestic or herbal or other purposes. They cannot, in fact, be regarded as truly native.