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 tree types and hedge or scrub of a roadside may be continuous or discontinuous. In the former case, if the two sides of the road are equally allowed to attain their full development, as in an avenue, to take an extreme case, the effect upon the rest of the flora will be similar to that of a ride or glade in a wood, and the conditions as regards light, moisture, and protection will be such as shade-plants require. The latter have several types of habit, as the inversely pyramidal, grass habit, and rosette habit. Where the tree and scrub are discontinuous the conditions will be intermediate, and sun-plants will in this case be more dominant, whilst shade-plants will seek the shelter of the hedge bottom or ditch. In the opposite extreme case, where both trees and scrub are absent and the hedges layered or cut back, shade-plants will survive only in the hedge bottom and ditch.
The flora of the sward in the first case will be more akin to that of a woodland, whilst in the third case the flora will be of a dry-soil meadow type. The plants in the ditches, owing to the narrow character of the latter, will be erect and drawn up, developing spikelike flowering stems, and reduced or rosette foliage, whilst the aquatic types will be less well-developed, and in the intense struggle for existence will at the lowest level show abnormal characters. The hedgerow plants and trees are largely affected in habit by artificial trimming or layering.
The flora of the roadside is decidedly composite, so that the seasons of flowering of wayside and hedgerow plants are sufficiently representative. The meadow types that flourish on the sward are akin to those that grow in the fields, and these, except Grasses, are more or less early. Plants such as the Ragworts, Red Bartsia, and Rushes are late-flowering. The Sedges usually met with are early-flowering species, as Carex verna (or prcecox), Carex glauca, etc. The Daisy and the Dandelion are almost perennial.
The ditch vegetation, like that of truly aquatic formations, is as a whole late, e.g. Watercress, Great White Stitchwort, many Rushes, Sedges, etc, whilst Cuckoo Flower is early in flowering. The plants that lurk in the hedge bottom are representative of all months of the year. The Lesser Celandine appears almost before any other flowers, and the Spurge Laurel soon after. The Common Chickweed is nearly perennial. Moschatel is fairly early, and so, as wayside plants, are Lords and Ladies and Dog's Mercury, indicating former woodland. The Red Campion, also a woodland plant, is a little later. Ground Ivy is one of the early plants, and Germander Speedwell also.
In the hedge the Hazel is the first to bloom, then come the Sloe, Crab Apple, Hawthorn, and still later the Dog Rose, Cornel, Guelder Rose, and Buckthorn. Privet is the latest, save the Ivy. Of the trees, the Elms are very early, as are the Willows, then the Ash, the Oak, Beech, Field Maple, and Lime commence to flower by degrees.