This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The exceptionally dry conditions of most heath vegetation have a noticeable effect upon the flowering seasons of the plants. A heath is exposed to intense illumination, a dry atmosphere, and hot sun, with little shade. Hence the thermal constants of the plants that grow on the sandy soil or acid humus of a typical heath are attained more quickly in the main, and the plants are not so backward as those that grow upon hills or in wet places. Here, again, however, altitude plays its part, and when Ling and Heath grow at high altitudes, the plants flower later.
A considerable number of the plants described in detail in Section IX commence to flower as early as April, as Grassy Stitchwort, Furze (which blooms more or less all the year round), Broom, Heath Bedstraw, Whortleberry, Meadow Rush, Early Sedge. In May, Milkwort, Red Rattle, Creeping Willow, and Wood Rush are first found in flower. Tor-mentil, Cat's Foot, Common Hawkweed, Harebell, Cross-leaved Heath, Crimson Heath, Cow-wheat, Green-ribbed Sedge, and Heath Hair Grass first bloom in June. Not until July do Pretty St. John's Wort, Sheep's-bit Scabious, Eyebright, Small Bent Grass, of Matweed bloom. Dodder and Pennyroyal linger till August before they open their flowers.