This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
A genus of ornamental shrubs with deeply veined leaves and large trusses of flowers, which are sometimes all sterile and sometimes sterile and fertile together. H. Hortensia and H. paniculata are largely grown as pot plants for market (see Vol. II, p. 175), but are perfectly hardy in the open air in the milder parts of the kingdom, flourishing in any good garden soil. Other kinds are arborescens, quercifolia, and radiata, from North America; and petiolaris (scandens), from Japan. The last-named is a climber with large trusses of white flowers; it requires sheltered spots.
Hydrangea Paniculata Grandiflora In Bloom.
Of the thirty odd species known, only two have found favour with market growers, viz. H. Hortensia, from China, and H. paniculata, from Japan. H. Hortensia is perfectly hardy in the mildest parts of the kingdom, and makes splendid flowering bushes. For market work, however, plants are usually grown in 5-in. and 6-in. pots, and are raised annually from cuttings of the half-ripened shoots. These are obtained from old stock plants, and are inserted in spring, autumn, and winter, according to the times they are required. Cuttings inserted singly in small pots in May in a sandy soil soon root, and are afterwards potted firmly into 5-in. pots. During the summer months they remain in the open air to ensure thoroughly ripe growth by exposure to the sun and air. The best compost is turfy loam with a dash of sand and a little well-rotted manure or leaf mould, and perhaps a sprinkling of basic slag to impart vigour and substance to the flowers. From October or November onwards the plants are grown under glass in cold frames, and batches are taken into a warm greenhouse to bring them into early bloom. When coming into flower, weak manure water may be given occasionally. If a blue colour is required in the flowers it can be produced by watering with a solution of alum water. Some growers use iron filings or coal dust mixed with the soil for the same purpose. The best white-flowered varieties are "Thomas Hogg" and "E. Moulliere". Hydrangeas of the Hortensia section can be had in bloom from twelve to fifteen months from cuttings, and with heads of flowers 12 to 15 in. across.
H. paniculata is quite distinct from H. Hortensia. In Japan it forms a dense round-headed tree 25 ft. high, with a trunk 6 in. in diameter. For market work plants about 2 ft. high are usually grown in pots, the variety known as grandiflora being most popular. A rather heavy loamy soil, with a little leaf mould or well-rotted manure, is a suitable compost. When the leaves have fallen, the plants are pruned hard back to three or four eyes. From each of these a long shoot is developed in due course, and ends up in a fine pyramidal head of flowers 1 ft. or more in length. By moving the plants into a warm greenhouse, or even into a stove temperature, the flowering period is hastened. Fresh stock is raised from cuttings about 6 in. long inserted in sandy soil.