This section is from the book "The Gardener V3", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Although the details of what is considered the best way of propagating the Azalea were given in a former paper, the more general course in private establishments of working into a stock is to purchase young plants, and, of course, where a collection has to be rapidly formed it is the quickest way. Supposing, however, that the grower is in possession of young grafted plants in, say, 5-inch pots in early spring, and the object is to grow them into as large and free-flowering plants as possible before the autumn, they should, in February, be introduced into a temperature of from 55° to 60°, with 10° more with sun by day, placing them near the glass. By careful attention to such points as keeping them steadily moist at the roots, and syringing them freely overhead morning and afternoon, they soon begin to grow freely; and presuming that their pots are well filled with roots, they should be shifted from 5-inch into 7-inch pots.
The chief points for consideration in the operation of shifting are cleanly-washed or new pots; the very best turfy peat, broken up with the hand, but not separating any of the fibre from it, and mixing about a fourth of pure pit-sand with it. If it is entirely destitute of sand this will not be too large a proportion of sand to add; but if the peat is naturally sandy, a fifth, or even a sixth portion of sand may be sufficient. The whole should be thoroughly well mixed, so that there may not be a lump of peat introduced into the pot here and a handful of sand there. The pots should have an inch of clean small crocks in them, and be blinded with the fibry part of the peat, or a very thin layer of sphagnum moss. In potting the plants they should be damp but not wet, and any inert soil on the surface of the balls and the crocks at the bottom should be removed. In fixing them in their new pots they should be three-quarters of an inch below the top of the pot, so that there may be plenty of room for efficient watering. In potting, see that the rough and finer portions of the compost are introduced regularly round the ball - that is, see that a handful of fibre is not put in one place and a handful of the fine in another.
Pot firmly, and return the plants into heat again, always keeping them near the glass. Shade them during bright sunshine, and keep them rather close for a time. If the soil was moderately moist - as it should be - they will not require water until the young roots begin to bite the new soil, but must be syringed every morning, and especially in the afternoon, when the house is shut up with sun-heat.
To grow on Azaleas speedily at this stage, they may be treated, as to heat and moisture in the air, very much like a stove-plant all the summer. As soon as they take with their shift give them a thorough soaking of water, and afterwards water them with great care, just keeping the soil moist but not wet, but being sure that they never once get mealy dry, which may kill them altogether, and is certain to check them severely. As the season advances shut them up early in the afternoon, giving them a good syringing, and keeping all parts of the house moist. Any growths that show a tendency to shoot ahead of the rest pinch back, and they will break into growth with several shoots, and so the symmetry of the plant is maintained. While recommending a warm moist stove-heat, air should be regularly given, and the plants should be allowed to become dry in the foliage for a time every day. Syringing with air on the house is one of the very worst practices in all plant or fruit culture, causing as it does the most rapid evaporation and abstraction of heat.
By midsummer the 7-inch pots will be well filled with roots. If all has gone on well they will have clustered themselves in a network at the sides of the pot, and will be seeking their way out at the bottom; and in this condition they are ready for another shift.
The size of the shift must be regulated by the habit of the variety - some grow much more robustly at both root and top than others. These may be put into well-drained 9-inch pots, using fibry rough soil and always plenty of sand. The less robust in habit will have shift enough by being transferred to an 8-inch pot, when less turfy soil must be used. They should not be put into a pot that they cannot thoroughly fill with roots by autumn. In shifting this time the same rules apply as in the first instance, and the same temperature and general treatment, until the pots are in their turn well filled with roots, and the bloom-buds can be discerned in the tips of the young growths. Then more air, more light, and less moisture in the air will be required, in order to get the wood well consolidated and ripened, so that the plants will yield a crop of bloom the following season. While they are fully exposed to all the air and light possible, be sure that at the same time they are kept moist and cool at the root.
By this stove-plant treatment it is astonishing what large plants can be produced in one season's growth. Of course, grown so rapidly, they do not produce so very dense a crop of bloom as more established plants with less robust growths do. Still, after undergoing a process of gradual but thorough exposure to sun, and consequently thorough ripening, what is wanting numerically is obtained in the size of individual blooms.
It is always found during the season of this rapid growth that much pinching and repinching of shoots is necessary to produce symmetrical and well-furnished plants.
After being perfectly matured in growth, a greenhouse temperature is high enough for wintering them in; the chief points being not to overcrowd the plants, and to keep them steadily and moderately moist at the roots.
Some varieties arrive sooner at the stas;e when it is desirable to decrease the heat and moisture, and require exposure to more light and air than others; and, if convenient, these should be drafted into another house or pit when they have formed their buds.