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 shade conditions in a woodland have a marked effect upon the periods of flowering. There are thus, apart from the general seasons of flowering in May, June, and July, when the sunlight is fuller and stronger, early-flowering plants and late-flowering plants. Those plants that flower early, seize the opportunity of doing so before the leaves of the trees appear, or at least before the foliage is fully developed. The growth season of bulbous plants is short, and they flower early in consequence.
The earlier plants to bloom in the woods are the Winter Aconite, Snowdrop, Lesser Celandine, Spurge Laurel, etc.
The trees themselves largely flower before the leaves, owing to their adaptation for wind pollination, as the pollen would be less likely to be dispersed when the leaves are fully expanded. The Willows depend partly on the wind, partly on insects, and so flower early.
The scrub is largely influenced by the same factors also. The Hazel relies on wind pollination, and is the earliest to flower. The Blackthorn also flowers before the leaves appear, since it is more conspicuous then than later. The Grasses, in spite of the fact that they are chiefly wind-pollinated, flower, as a rule, rather late in the woods.
The late-flowering groups are chiefly the Hawkweeds, rosette plants whose scapes are long. The Brambles, owing to the great output of stems and branches, also flower late. The latest plant to flower of all plants is a woodland plant, the Ivy.
A particular feature of the woodlands is the height of the dominant type, the trees. It is largely owing to their height, which is regulated to a considerable extent by the wind and soil, that they are the dominant type of plant, next to Grasses, in the world flora itself.
This character enables them to outstrip other plants in the struggle for sunlight and air. They are thereby enabled to counteract the influence of all other classes of plants, which growing below do not affect them in these respects. These facts require special emphasis.
The lower strata of plants are directly influenced as regards height (and other factors equally) by the dominance of the tree zone. This is seen in its greatest extreme in a Beech wood, where the ground flora is often nil.
The scrub, e.g. Blackthorn, Elder, etc., suffers less than the ground flora, and this is seen in clearings, where the scrub may rival the younger trees in height, etc. Like the tree zone, the scrub normally has a definite upper limit. The undershrubs, that are in turn dominated by the scrub or large shrubs and smaller trees, also approximate in height to a certain standard.
The ground flora is of course influenced most by being covered by two strata above. Consequently, as a rule, it also approximates to a certain general height, e.g. Grasses, and others with the grass habit. Orchids and bulbous plants come next. Then there are the trailers, such as the Barren Strawberry; and lastly the mosses and hepatics.