This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Branching wiry shrubs. Leaves opposite, alternate or whorled, rigid, very small, usually having their margins re-volute. Flowers in axillary or terminal racemes or umbels, tetramerous. Stamens 8; corolla persistent; anther-cells awned. Capsule 4-celled, opening through the cells, many-seeded. The name is of classical origin. This genus contains probably 500 species, the greater number occurring in South Africa. E. arborea, of Southern Europe, attains the dimensions of a small tree. Calluna vulgaris, syn. Erica vulgaris, Ling or Heath, is distinguished from this genus by the deeply 4-partite corolla, shorter than the coloured calyx, and the capsule dehiscing between the cells. It is one of our commonest species, having short thick trigonal leaves and purple or white axillary flowers. This is the only Heath that extends to North America, and even this is very rare. There are two common British species of Erica proper, namely, E. Tetralix, Cross-leaved Heath, having 4 hairy leaves in a whorl, and the rosy pink or white flowers in umbels; and E. cinerea, Scotch Heather, with 3 glabrous leaves in a whorl, and numerous reddish purple whorled flowers in long racemes. This is a very common species, rising to a height of 3 or 4 feet in some localities. There are several varieties of these, as well as of the Galluna, in cultivation, including one of the latter with double flowers. Besides the above there are three other indigenous species, but they are very local. The following are some of the hardy exotic species.
1. E. carnea, including E. herbacea. - This is one of the most desirable species on account of its early flowering season, which begins in January or February. It is a distinct very dwarf plant with linear-acute leaves and rather long urceolate slightly-lobed corollas narrowest at the mouth and projecting anthers. Flowers on distinct axillary pedurcles. There are purple, pink, and white flowered varieties. South of Europe.
2. E. Mediterranean - Another early-blooming species, though not so early by two months as the preceding. An erect shrub about 2 feet high. Leaves linear-acute, revolute. Flowers pink, axillary, on short peduncles. Corolla ovoid, the small lobes spreading. Anthers slightly projecting. E. Hibernica, a plant found in Mayo and Gralway, is referred to this species.
3. E. vagans. - This species is found in some parts of Cornwall, but nowhere else in Great Britain. An erect shrub from 1 to 3 feet high, densely clothed with linear glabrous leaves. Flowers pink, purple or white, on long peduncles, in dense axillary clusters; corolla campanulate; anthers partially ex-serted. The only other native species, E. ciliaris, has ciliate glandular leaves 3 or 4 in a whorl, flowers in a one-sided raceme. The corolla is ovoid, and the anthers included and awnless. It occurs in Dorset and Cornwall, and in Galway.
E. arborea and E. scoparia, together with some other South European forms, are erect-growing shrubby kinds 3 to 6 feet or more high. E. codonoides, syn. E. polytrichifolia, very near and perhaps a variety of E. arbbrea^ is one of the hardiest and freest of this set. It is a slender much-branched shrub with small pale green leaves and numerous many-flowered racemes of small white and pink flowers produced in early Spring.