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 continuity or otherwise of the trees and scrub in the hedgerow has a marked effect upon the rest of the roadside flora. Much depends upon the direction of the road, and the relation of the sun to the barriers that the hedges form to its path across the road.
There are roughly four types of vegetation along a roadside or a hedgerow, and the plants of each type more or less retain the same relative standard as to height, save in the case of the plants on one side which receive least sun, or are hidden by an overhanging hedge or tree belt. The height of the ditch plants is regulated by the height of the ditch. Those that grow vigorously, as Great Hairy Willow-herb, endeavour to reach above the banks on either side, and are usually abnormally long. Hence they must not be taken as typical examples. The plants at the bottom, as Watercress, are necessarily dwarfed, and consequent upon the crowded character of the ditch often lie along the bottom in a procumbent manner, and so lose height, even if they do not spread much more extensively than usual. The plants below or at the bottom of the hedge, as Three-nerved Stitchwort, usually lie on the surface, but some are elongated to reach the sun. Those that grow on the hedgebank, such as Hedge Mustard, are frequently much elongated when on the northern aspect. Others, as Herb Robert, have a straggly habit, as a result of their growing forward to the light through the hedge itself.
The composite character of the wayside flora renders it variable in respect of the way in which the plants adapt themselves to the growing season, or acquire their life duration or mode of perennation.
The Elm, Ash, Oak, Lime, Poplar, Willows, Hazel, Hornbeam, Sycamore, etc., all frequently planted by the wayside, are deciduous trees. Holly and Yew or Pine are evergreen. The scrub or shrub type is similarly deciduous e.g. Hawthorn, Cornel, Spindle, Buckthorn, Apple, Field Maple, Rose, and the Bramble, Spurge Laurel, etc., among undershrubs, are all also deciduous. Box is evergreen, but is only native on the chalk and oolite at Boxhill and one or two other places. The aquatic vegetation is largely herbaceous and perennial. The sward is made up of herbaceous perennial or annual Grasses, and some other perennials. The bulk of the annuals, as Shepherd's Purse, Wart Cress, etc., are derived from other sources, cornfields, etc.
It is thus not surprising that the bulk of the wayside plants are adapted to insect visits, which are numerous, and that most of them are cross-pollinated. But since nature has allowed for the exigencies of the weather and the occurrence of rainy periods, many of these plants are equally adapted to self-pollination, as Hedge Garlic, Greater Stitchwort, Perforate St. John's Wort, Herb Robert, Common Bramble, Crab Apple, Hedge Parsley, Cornel, Moschatel, and Elder and Cleavers are self-pollinated, as a rule. In some the anthers are mature first, as in the Teasel, Ground Ivy, and Bugle, in others the stigma, as in the Sloe, and Hawthorn, and Lords and Ladies. The Ash, as well as the other hedgerow trees, and the Nettle are largely pollinated by aid of the wind.