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.
Though a roadside appears to present extremely uniform conditions at first sight, in reality there is a good deal of diversity. A solitary bush by the wayside may form exactly the habitat for such a plant as Hemp Nettle, which requires such protection, but not that of a moist ditch.
The macadam at its margin or on old unfrequented roads affords a habitat for a number of characteristic plants, such as Silver-weed and Common Cinquefoil. Along the sward at the side of the road grow the usual meadow or pasture plants, varying with the soil. On clay in early spring on open ground the Lesser Celandine may be seen, on sandy loamy soil later appears the Upright Meadow Crowfoot.
The ditch affords a habitat for moisture-loving plants, such as Watercress, Willow-herbs, Figwort, and in wide ditches one may find Duckweed and Starwort, or Water Buttercup. The bank of the hedge affords a shelter for numerous plants that require shade and protection, such as the Herb Robert, Jack-by-the-Hedge, Avens, White Dead Nettle, Nettle, Docks of various kinds which grow near water, with Sedges and Rushes and many others. The three-nerved Sandwort and Chick-weed grow in the hedge bottom, as do Archangel and Moschatel.
In the hedge itself grow Hawthorn (widespread), Elder, Sloe, Buckthorn, Cornel, Blackberry, Rose, Field Maple, Guelder Roses, and such trees as Oak, Ash, Beech, Wych and Common Elm, etc.
There is frequently a little scrub at the side of the road, in some parts made up of Sloe or Furze or Bramble, amongst which many other plants, as Grassy Stitchwort, etc., will grow. Further variety is afforded by the occasional occurrence of ponds or streams by the wayside.
Soil alone does not cause the variation to be noticed in a roadside flora. Much depends upon the altitude of a road also, apart from the effect this usually has upon the upkeep of the road. Above 1000 ft. cultivation ends, and with this limit also other plants disappear. The typical vegetation above this is the moorland heather, etc., varied with Matweed or wet-soil plants, as in the bogs, which cover so large a part of the uplands. As a whole, in fact, the flora of a roadside is usually very uniform in this respect, as it is a sine qua non to provide a level road. But there are considerable variations in altitude in the same road, and the flora even at the bottom of a long steep hill will differ from that at the top, if only from the greater exposure to wind.
At low levels in flat country the roads may frequently be under water for some period of the year, or the surrounding district over-saturated with moisture, especially near rivers. In this case many plants will be dispersed, owing to the floods, by the carrying of seeds from elsewhere, and aquatic plants often spring up along such roads.
The influence of altitude upon plants in this way should be carefully noticed, and lists of plants at different heights should be made and compared.