This section is from the book "The Florists' Manual", by William Scott. Also available from Amazon: The Florist's Manual.
These are among the most important of our decorative flowering plants. Large quantities are sold for Easter church decorations, and later on large plants are in demand for outside decoration. The hardy Hydrangea paniculata grandi-flora is one of the finest of our hardy shrubs.
H. hortensis and its variety Otaksa is the common hydrangea of our greenhouses. The flowers of Otaksa are nearly always sterile, and from that fact arise their fine, showy heads of bloom. The normal color of Otaksa is a beautiful flesh pink, but it varies with certain soil, and in some parts they assume a beautiful blue color. Iron dust or filings in the soil are said to produce this. If so, it cannot be done with one season's treatment, but must be followed up from the time the plant is first rooted. When Otaksa is well colored its beautiful shade of pink can scarcely be improved by changing to a blue.
All the hydrangeas can be readily rooted from the young growths in February and March. Old plants that are given a little heat in the winter will give you fine cuttings, and they should be short, stout pieces of the very latest growth, which root quickly in the sand. For early spring use the cuttings should be propagated in February, potted on till June and have the tops pinched out, when they can go into a 5-inch pot and be plunged outside on a dry bottom, giving them plenty of room between the plants. If they grow freely give them a 6-ineh pot in August.
If strong plants in 4-inch pots, one stopping of the leading shoot is all the pinching that is necessary. If growing strong they are then shifted into 6-inch and plunged in frames in May or June in some material. Here they will grow vigorously.
I must quote here what I saw last summer, the latter part of August, at the establishment of a very large and successful Philadelphia grower of these Easter plants. A number of these plants were lifted out of the frame where their roots had been uniformly moist and the growth vigorous, and were stood in groups on the dry ground. Now this was evidently done to arrest the vigorous growth, ripen the growths and form the flower buds. It was a clever move and much can be learned from it. This is much better than subjecting the plants to early frosts, the degree of which no man can control.
When forcing these plants a shift from a 6-inch to 8-inch would be very beneficial, but it would give them an unwieldy and unsalable appearance; being crowded with roots, you should tell your customers that they must have abundance of water. The very name of the genus implies that they are gluttons for water.
Hydrangeas, especially the hortensis type, are great feeders, and should have a rather heavy but good, fresh loam with a fourth of decayed manure, and some bone flour added at the last shift will help them. Water they want in great abundance when growing and flowering, and if allowed to suffer for it they soon show it, and will show it later by yellow leaves.
There is little trouble with hydrangeas from insects. You can fumigate them should fly trouble them, and although red spider will attack the flowers it should never be allowed, as a daily syringing should be given them.
Plants that have not sold should have the flowers removed by cutting back the stem to within a few eyes of the pot. Remove some of the soil and give them a shift and plunge outside for the summer. They will make fine plants for another spring. The principal object to attain with any of these plants is a strong growth in summer, and well ripened wood in the fall. So bright sun, cool nights and a lessening of the supply of water are the requisites.
When hydrangeas get into 10-inch and 12-inch pots they take up too much room unless you are assured of a good sale. They make magnificent plants in tubs for the lawn, but those that have developed their flowers under glass are not valuable for this purpose, as they soon lose the beauty of their flowers. The best plants for this purpose that I have seen were wintered for several years in the basement of a coach-house. There was no artificial heat. It was not too dark, and with an occasional watering the plants remained dormant till it was time to return them to the lawn, when they came along naturally about the same as the hardy shrubs, and the flowers lasted the greater part of the summer.
Some such place as this should be provided for large plants, as the greenhouse, however cool, will bring them on too fast.
Some growers adopt a different plan with the young plants. Instead of growing them on in pots they plant them out in good, deep, rich soil and lift and pot in September or October. I have often done this, and for late spring sales it is a good plan, but for the Easter lot I prefer to grow them in pots all summer.
The kinds forced include Thomas Hogg, a pure white variety of hortensis. Paniculata is also forced in some places, but we think we have better plants. There is a finer variety with purplish red stems and highly colored pink flowers, rather a tall growth but very handsome. H. hortensis Otaksa is the finest variety, giving the largest head of bloom, and forcing well.
There is now a magnificent variety, or species, known, I believe, as rosea, a lovely shade of pure pink. The experts have not yet mastered the flowering of this beautiful sort.