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.
The garden Petunias have originated from two South American species, namely P. nyctaginiflora and P. violacca, the first hybrid between them having flowered in 1837. Since then vast strides have been made in the evolution of the Petunia by crossing and intercrossing, and, instead of having only single flowers about 2 in. across and of one shade of colour, there are now single and double forms with flowers 4 to 6 in. across, and with shades of colour varying from the purest white through rose, pink, purple, magenta, crimson, and intermediate shades to the deepest violet - colours relieved in many forms with streaks, blotches, and bands of totally distinct shades. Instead of flowers with a simple outline there are now Petunias in which the petals of the corolla are beautifully cut, fringed, crimped, and curled in various ways (fig. 295). Petunias are really perennial plants, but it is found more convenient to treat them as half-hardy annuals. They seed freely, and many choice strains exist. The dust-like seeds are sown thinly on the surface of a nicely prepared sandy compost in February or March in a temperature of 60° to 65° F. at night, and are kept in a fairly moist condition. When large enough to handle easily the seedlings are pricked out about 1 in. apart in shallow boxes or pans of light soil. In April they will be ready for moving singly into 3-in. pots, or fine specimens may be put into 5-in. pots for special purposes. In many cases, however, the plants are simply transferred to shallow boxes, and are sold in the young stage in this state. Pot plants usually have the tip of the main shoot pricked out to induce bushiness, and by the end of May and in June the trade is in full swing for bedding out.
Fig. 295. - Petunias (single and double).
Generally speaking, the weakly looking seedlings amongst the single Petunias are those which often produce the finest shades of colour later on; and amongst the doubles the weaker seedlings usually become the most crorgeous and beautifully frilled, etc, when in blossom.
When particularly fine strains are required true to name, they are raised from cuttings. These are taken from the non-flowering side shoots in August, and when inserted in sandy soil and well watered are plunged in a brisk bottom heat of 70° to 75° F. They root readily, and are afterwards transferred to somewhat cooler places and potted up singly. During the winter months they are grown as near the glass as possible in a warm greenhouse, and in February and March the tops may be taken off and rooted as in August.