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.
This lovely genera of plants ranks amongst the most beautiful of stove-climbers, and deserves to be grown largely on the roof of a plant-stove, where plenty of heat and moisture can be afforded them. Dip-ladenia Brearleyana makes one of the finest exhibition plants. However beautiful the older varieties of a few years ago were, they are certainly superseded by the newer varieties. D. amoena and amabilis are beautiful; while D. insignis is a great acquisition, with its rich rosy-carmine flowers, which are of a good size, and having a white tube or throat, renders it an attractive plant for exhibition purposes. D. regina will undoubtedly, with its light-coloured flowers, prove to be, from its free-blooming habit, a great acquisition.
We intend to confine our remarks to that beautiful evergreen plant D. Brearleyana, which we consider far exceeds any other variety, either as a stove-climber or for exhibition purposes, both in freeness of growth and for its floriferous character. Cleanliness is a point of great importance in the culture of this plant, as it is very subject to all kinds of insects which infest plants, especially mealy-bug; and unless the plants are free of these, little success can be anticipated. When once attacked with bug, it in a very short time arrests the growth of the plant, and the foliage soon turns yellow and falls off.
Watering is another particular point, and this plant is very impatient of carelessness in this respect; it cannot bear being kept so wet at the root as the majority of stove plants. If the waterpot is judiciously used, and the plant kept clean, combined with light, heat, and moisture, the cultivator will not fail to be successful with D. Brearleyana, and have a quantity of large, fine-coloured flowers of great substance. This magnificent hybrid is a very attractive and striking object when in flower, from its peculiarity of having flowers of two colours on the plant at the same time, - opening as it does pink, and then with age the colour intensifies until it is of the richest crimson. This variety flowers very freely. It, like several other stove-climbers, requires a large number of young shoots to be trained under the glass before a great profusion of flowers can be produced at one time. This is particularly necessary if required for exhibition purposes. Like the Stephanotis, it is best trained on string for that purpose, until the flowers are well advanced. They should then be carefully trained round the trellis they are intended to be exhibited on, placing the flowers as near equal distances from each other as possible. Dipladenias cling to the support upon which they are trained.
When trained on string, it can be cut in short lengths, and easily drawn out, making it more convenient to take the plant down when it has to undergo the ordeal of cleaning. The young shoots are sure to be injured more or less if allowed to twist round wires.
Dipladenias do best when allowed to ramble and twine to string at will. Care should be taken that each shoot, if only a few inches apart, is supplied with a separate support.
Propagation is best effected by means of cuttings in the early spring. The young wood, when a few inches in length, will strike freely if numbers are inserted in a 6-inch pot, or singly in small pots, and plunged in brisk bottom-heat, and a bell-glass placed over them. When rooted, the young plants should be potted in small pots; if rooted singly in small pots, so much the better. If, on the other hand, a number are put in one pot, they must be potted as soon as the formation of roots has commenced. Care must be taken of the young plants after potting; if they receive a check, they do not start into growth for a very long time, and in some cases do no good afterwards. They must be grown on in bottom-heat, and liberal attention bestowed on them. They will grow rapidly when once fairly started; and by the end of the season will make nice young plants, provided they are rooted early, which is important. The young plants are best in a temperature during winter of 58° to 60°, according to the external atmosphere; and then with increased light and moisture, after the commencement of the year, the young plants will soon start freely into growth. If the plants grow well the first seaons, they will before winter be well established in 6-inch pots.
After they begin to root freely, they should be transferred into 8- or 9-inch pots, using a compost of two-thirds fibry peat, one-third of fibry loam, with lumps of charcoal mixed, a dash of small bones, and plenty of coarse white sand to keep the whole porous.
The wood made the first season should be laid as much horizontally as possible, so that as many young shoots can be encouraged to lay a proper foundation for a good quantity of wood the following season. As the young shoots extend themselves, they will show flower-spikes from the axils of the leaf, and as they continue growing, will keep flowering over a lengthened period of time. Should it be deemed necessary to repot the plant a second time in the season, if the 8- or 9-inch pots are well filled with roots, and the plant is making rapid progress, the operation can be performed, but not later than the middle of July, so that the plant will be thoroughly established before the winter sets in.
Another particular point that must not be lost sight of in the successful management of Dipladenias is, they do not like being disturbed at the roots. In repotting, the crocks only must be carefully removed, and any loose soil that may exist on the surface of the ball. If the soil recommended is of a good fibry nature, and sufficient materials used to keep it open, the watering carefully attended to, which is absolutely necessary, the compost will not be in a sour condition for a very long time.
Although the Dipladenia cannot be used for all purposes, it is not only a worthy subject of the exhibition-tent and stove decoration, associated with Allamandas, Clerodendrons, etc, but it is useful in a cut state for shallow vases or dishes: mixed with these and Gloxinias, with a little Fern intermixed, Dipladenias are useful and striking.