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.
This genus of plants comprises a great many varieties, embracing as it does hardy annuals, hardy herbaceous plants, and greenhouse evergreen plants. The latter, to which alone the following notes refer, are all natives of the Cape of Good Hope, and all bear purple flowers. The varieties which are best known, however, and oftenest met with in cultivation, and which used to be more frequently met with than they are now, consist of the two varieties, P. Dalmaisiana and P. oppositifolia. They are both worthy of a place in the greenhouse, as they are exceedingly useful subjects to cut from, for they stand the knife well, and one can cut-and-come-again at them. They are very useful for house and conservatory work, and are good exhibition plants also, though perhaps not among the very best for this purpose. They have the further merit of taking but a moderate share of attention to grow them well. The soil which suits them best consists of two parts of good fibry peat and one part of loam, with sufficient sharp sand to keep it open. They are increased by cuttings of the side shoots, taken off when they have made about 2 inches of growth.
These should be put in among silver sand, in a properly prepared pot in the usual way, covering with a glass, and set in a warm pit until they have struck roots, when they should be potted off singly into thumb-pots, and set back into the warm pit until they get a fresh start. They should be kept well pinched back when they are young, as otherwise they are apt to grow straggly. They must be shifted on into larger pots as the others get filled with roots; and they should get plenty of light and air, so that they may be kept stocky, and that the wood may get well ripened. The winter temperature should range about 45°; and after the flowering season is past, they should be cut back pretty hard, and started away into growth again in a mild heat.
This operation should be performed annually, so that they may be kept bushy. After they have started again into growth, they should be hardened off, and placed outside in a sheltered position, but where they are fully exposed to the sun. In order that the wood may get matured, they should be put either on a deep bed of ashes, or on inverted flower-pots, so that worms may not obtain a lodgment in them. They are occasionally attacked by red-spider; but the best way to keep this in check is to maintain the plants in a healthy condition at the roots, and to syringe them now and again.
J. G., W.