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 principal factors that regulate flowering are, in the case of terrestrial plants, excessive rainfall, increased altitude, exposure to wind, insolation, or interrupted sunlight. The duration of the plant also influences the flowering, and to some extent size enters into these factors. The lying of snow upon alpine heights, which are also wind-swept, poor in soil, and subject to insolation, retards the flowering of alpine plants.
The effect of size upon flowering is shown by the earliest months of flowering of some of the plants here included. Thus Vernal Whitlow Grass, an annual, blooms in March. Field Mouse-ear, Horseshoe Vetch, and Field Speedwell bloom in April. Bird's-foot, also prostrate and perennial, blooms in May. The annual Rue-leaved Saxifrage, a mural plant, also found on thin calcareous soils, flowers early in April, whilst its taller relative, Meadow Saxifrage, blooms in May, on colder clay soils. Mouse-ear Hawkweed, Mother of Thousands, and Silvery Hair Grass flower in May.
June is the month when numerous flowers first open, viz. Cheddar Pink, Sandwort Spur-rey, Navelwort, Biting Stonecrop, Houseleek, Silky Wind Grass, Flat-stalked Poa, Sand Fescue. Not a few, however, defer their flowering till July, e.g. Orpine, Golden Rod, Wall Lettuce, Rampion, Pellitory-of-the-wall, and these are largely plants that grow in crevices or walls, where the soil is poor or thin, and nutrition a slow process.