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.
These plants are evergreen dwarf shrubs, of compact habit of growth, natives of Australia; and when properly treated, produce from the axils of the leaves, on the current year's shoots, their bright-coloured wax-like flowers very freely during the autumn, winter, and spring months.
Correas do not receive from gardeners generally the amount of attention that their decorative qualities entitle them to. About twenty-five or thirty years ago two or three varieties of Correa were to be seen in almost every greenhouse, but for several years past, for some reason or other, most gardeners have ceased to cultivate them. Possibly the present neglect of these and some other genera of beautiful-flowering greenhouse plants that were carefully looked after at the time mentioned, is owing in some measure to the great improvement that has been effected in the flowers of Zonal Pelargoniums. The latter plants are easily cultivated, and produce their large trusses of bright-coloured, well-formed flowers in more or less abundance all the year through, and consequently several other kinds of beautiful-flowering greenhouse plants, requiring a little more careful attention to maintain them in a healthy flowering condition than that usually bestowed upon Pelargoniums, have been in many instances crushed out of the greenhouse by the gay and easily cultivated Zonals. One difficulty, however, in the culture of Correas is in the matter of propagation, but this only applies to places where proper facilities for carrying out the process do not exist.
Correas, with the exception of C. alba, are somewhat difficult to propagate by cuttings, - at least, this has been my experience with most of them. Cuttings of half-ripened shoots of C. alba, taken off in spring, and inserted in a mixture of equal parts of silver-sand and leaf-mould, firmly pressed in well-drained small pots or shallow pans, supplied with a bottom-heat of 70°, and covered with bell-glasses, will, if duly looked after in the way of shading from sunshine and supplying them with water, emit roots in from four to six weeks, and be ready for potting off into small pots in eight weeks from the time of being put in as cuttings. C. alba is the strongest grower of any of the species in cultivation, and being easier to propagate by cuttings than the others, it is employed as a stock on which to graft the weaker-growing kinds. In twelve months from the time the cuttings were put in, they will be strong enough, if rightly treated during that time, to receive the grafts of the sorts that it is desirable to increase.
The methods of grafting and subsequent treatment up to this time, when the union of stock and scion is complete, are the same as those recommended in the case of Azaleas in last month's issue of the ' Gardener.'
Established plants should be repotted about the beginning of April, and when doing so, drain the pots efficiently and carefully, making the fresh compost moderately firm about the roots of the plants. Any cutting back or pruning of the shoots that may be thought necessary to keep the plants in shape, should be done at the same time. After being repotted, place the plants in a position where they can have a slight shade from sunshine, and where a temperature of from 60° to 70°, with plenty of atmospheric moisture, can be kept up. Let the plants remain in this position till the end of May, and then remove them to a cold pit or frame, plunging the pots in a bed of ashes, as near to the glass as the size of the plants will permit. A slight shading should be applied to the glass, and the pit or frame should be kept rather close for a week or so after the plants are placed therein. Here the plants may remain till the first week of September, when they will be commencing to flower, and should be taken into the greenhouse and have a position where they will receive as much light as possible. Correas require to be carefully attended to with water at all seasons. They are easily injured by over-watering, and stagnant water about their roots kills them in a very short time.
While the plants are in the cold frame or pit, they will be benefited by an occasional application of liquid manure; the dose, however, must not be strong. As a rule, the roots of hard-wooded plants are much easier injured by an over-strong application of liquid manure than those of soft-wooded and quicker-growing plants.
Correas must also have due attention in keeping them free from insects. Brown scale will attack them, and must not be allowed to make headway on the plants. If permitted to do so, it is difficult to get the plants clear of the pest again. Correas when in flower may be employed as table-plants in the dwelling-house; and for this purpose, plants of the right size of C. brilliant and C. Harrisii are very appropriate. C. cardinalis and C. ventricosa are also beautiful kinds for the decoration of the greenhouse or conservatory. They are, however, of a more slender habit of growth than the two former species, and require to be closely cut back at pruning time for the purpose of imparting to the plants a more compact and bushy shape than they naturally assume when not treated so. In conclusion, I would recommend those who have not yet cultivated Correas to get half-a-dozen each of the above four species from their nurseryman, and carefully attend to them during the coming summer, and I have no fear but the result next winter will be satisfactory to the cultivator.