This section is from the book "The Florists' Manual", by William Scott. Also available from Amazon: The Florist's Manual.
These are very popular plants. In flower gardening they are one of the leading flowers. It appears that the garden varieties are raised from the species P. nyctaginiflora and P. viola-cea, a white and a violet species, but in the varieties now raised by selection and culture we have a great variety of color, both double and single flowers.
Howard's Star Petunias.
In large beds where there is much flower gardening to do and not a great facility for raising the plants, or where expense has to be studied, the petunia is one of our most serviceable plants, and for a flower bed the single is more effective than the double. We also find great use for them in veranda boxes and vases. We have seen the double white used as a cut flower, but that day is past.
Any fine double varieties that you wish to perpetuate must be raised from cuttings, and the plants seen in early spring in 4-inch pots are from cuttings, but for bedding it pays much better to raise them from seed. Obtain the best strain you possibly can. I have received seeds from a firm that were splendid, hardly two flowers identical in a thousand plants, and the next year from the same source they were nearly all that washed out purple that nobody wants.
Buy seed that is sold for double always. You will only get about forty per cent double flowers, and that will leave you plenty of single. There are some distinct strains that come true in form and color. A New York firm advertises a strain called Adonis, valuable for bedding, medium size flowers of a carmine color. The California strain of doubles is magnificent. The Dwarf Inimitable is also a fine single strain, of a cherry red color, with white throat. There are also many fine double varieties that are named, but the great majority of us depend on a good strain of seed, as they make a better bedding plant than those grown from cuttings, and every desirable color can be obtained.
For most places a variety of color in the same bed is preferred when filled with petunias, and they should always be given a bed to themselves, as they would give no other plants a fair show.
When choice double varieties are kept over you should select the young, fresh growths and propagate in sand before a hard frost has touched them. When rooted they should be grown on a light bench in a temperature of 50 degrees. If not allowed to get stunted these plants will give you more cuttings, which root very easily in winter when there is heat in the propagating bench. By pinching once and potting into a 4-inch you can have nice plants in flower in early May. They need a small stake to support them. Many such plants are sold in our markets.
Seedlings are the cheapest and most satisfactory. The seed of the petunia is very small. Sown in early March on a well watered, fine surface, no covering of soil is needed. Just press the surface lightly with the bottom of a clean pot. We usually cover the seed pan or flat with a piece of damp cheesecloth till the seed begins to germinate, but it should be removed directly you see the seed starting. For a few days be careful not to let the minute seedlings get parched. Neither must you let them draw up with too much shade and heat.
As soon as the small seed leaves are developed they should be near the light, and 45 to 50 degrees at night will do very well. When large enough to handle we put six or seven around the edge of a 3-inch pot and two or three in the center. I like this better than putting them in flats. About the end of April we give each plant a 2 1/2-inch pot and place on any light bench. There should be a full exposure to the sun and abundance of ventilation. They are often put into hotbeds, but I don't approve of that, as they make too rank a growth. In a cool, light house they grow fast enough and make strong, stout plants in fine condition for bedding out.
Aphis troubles petunias, so they should be fumigated with the many other plants that need it.
A good sifted loam with a third of old hotbed manure is what they like, and if you wish them to jump along quickly in May add a 6-inch pot of bone meal to every barrow-load of soil. Although the parents of our petunias are from southern Brazil and the Argentine, it must be the high elevations, for they want a high temperature at no time and grow and flower weeks after many of our bedding plants are killed.