This section is from the book "The Gardener V1", 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.
It is unnecessary on my part to make any preliminary remarks regarding the Azalea in general; but it may not be out of place at the close of the flowering season to send you a few notes, from observations connected with their culture, and which, moreover, may prove useful to some of the readers of the 'Gardener.' If, after flowering, any of the plants present a stunted or exhausted appearance, they should be taken from the pots and the old soil shaken partially away from the roots; a sufficient quantity of turfy fibrous peat which has been previously well turned and sweetened by the atmosphere, should in the mean time be in readiness, to which add a good dash of silver sand. Some recommend a portion of decayed manure among the soil, but if the plants are in an ordinary state of vigour, I prefer to avoid it, as a firmer growth is more certain - a point, by the way, upon which free flowering the following season to a great extent depends. If they will conveniently contain the roots of the plants, no larger pots than those which they previously occupied should be used, having them thoroughly cleaned outside and inside.
Some plants here have occupied the same pots, 10 and 12 inch size, for four or more years; and as they still betoken high vigour, I have resolved to simply top-dress them, by removing 2 inches of the surface, and replacing with soil as above mentioned, as difficulty is sometimes experienced in securing well-ripened points, which might be increased by repotting vigorous plants. In the case of those which are to be potted, be careful that the pots are efficiently drained, as stagnant water is highly objectionable at the roots of any plant, and notably of the Azalea. A few pieces of charcoal may advantageously be placed over the crocks. In potting, the soil should be packed firmly about the roots, which should be disentangled from the part of the ball left; fill the pot to within one inch of the rim, which space should be reserved, in order that watering when required may be efficiently done. If when potting the soil was in proper order, water, with the exception of dewing overhead, will not be necessary for a day or two, applying it cautiously till free growth commences, when an abundant supply should be given, gradually diminishing it as the young shoots harden.
It is beneficial, in fact necessary, that the Azalea during the period of making and maturing its growth, should be placed in a close moist structure, where there is a little fire-heat. I place them in a division of a Melon-pit, from which they can be removed in time to plant a late crop if required. While there, they are regularly turned round and syringed to keep thrips at bay, that inveterate Azalea pest. I am glad to say, however, I have never observed the slightest appearance of them here. The plants are much benefited by exposure to the open air one month at least before placing them in winter quarters. If the weather at the beginning of September is stormy or very wet, the plants should be removed to the greenhouse, previously washing the pots, and attending to what tying is required. The points of management during the winter months are, to protect from sharp currents of air, and to be specially careful in watering, applying it only when actually necessary, till the flower-buds develop, when it may be applied liberally.
The following varieties have flowered equally well here this spring, viz.: Perfection, Due de Nassau, Louis Napoleon, the Bride, Standard of Flanders, Marie, Gem, Cupid, Beaute de l'Europe, Stanleyana, Criterion, Roi de Leopold.