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.
Nothing can be more opportune, or more useful to amateur cultivators of plants, than some suggestions in relation to this very important topic. In potting plants that require any cutting or pruning, the two operations should not be performed at the same time, generally; it is best to prune first, and allow the plant to make fresh growth before the potting is performed. Deciduous plants should never be repotted till they have burst into leaf; Fuchsias, for instance, which have been dried off in winter, should in no case be shaken out of their old soil till they have expanded a few leaves, and all pruning required in their case should be done before the roots are disturbed. Evergreen greenhouse plants - such as Camellias, Oranges, and Myrtles - have a particular season at which the roots elongate or increase with more rapidity than is usual at other stages of their growth; and, under ordinary circumstances, that season is immediately when they have made their growth in branches and leaves, and it is the most desirable time to shift such as require it into larger pots and more nourishing soil, just as the roots are extending, and ready to take hold of fresh soil.
Shifting a plant into a larger pot often becomes necessary when pruning is not called for. When it is desired to increase the size of a plant, it should be shifted into a larger pot as soon as the roots have circled themselves among and around the soil in their present pot to an extent that renders it safe to perform the operation without danger of the ball falling to pieces. Generally speaking, a sure criterion as to when a plant requires more pot room and nourishment, is when the roots make their appearance through the bottom of the pot. It is, however, much preferable at the near approach of winter to leave plants a little cramped at the roots, than to shift them on at such a season: under such circumstances the operation should be deferred till early spring. Azaleas and Camellias should not be shifted till after their blooming season is past and they have made fresh growth; and the exact season for potting such plants must, of course, be determined by the time at which they are forced to or retarded from making their growth.
Pots should always be scrupulously clean and quite dry when used. If they have been previously used, they should always be washed before putting another plant into them; every particle of mould or slimy matter which adheres to them and clogs up the pores should be removed, both outside and inside. When a plant is potted into a pot the inside of which is covered with particles of earth, the mischievous consequence, to say nothing of others, is that, in turning a plant out of it, the ball is sure to be broken, and, of course, the roots also injured.