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.
All the species of this genus in cultivation are evergreen shrubs of dwarf growth. Their time of flowering in this country extends from the beginning of March until the end of June. The majority of them are of a slender habit of growth, and to form them into neat compact specimens a certain amount of staking and training is necessary. When properly managed, Boronias are beautiful and interesting plants at all seasons of the year, and when in bloom they are effective for the decoration of the greenhouse or conservatory. B. pinnata, B. Drummondii, and B. serrulata, are splendid subjects when large enough for exhibition purposes, as they will stand a considerable amount of rough usage, without injury to the flowers at the time, or to the health of the plants afterwards.
All the species commence flowering when the plants are comparatively small, hence, of whatever size they are, they reward the cultivator with a crop of flowers proportioned thereto. Small plants, when in flower, are suitable for taking part in floral decorations in the dwelling-house, and they will suffer as little injury from being exposed to the dry atmosphere that necessarily prevails in rooms of the dwelling-house, as any hard-wooded plants with which I am acquainted; and this alone should secure for them the attention of those whose duty it is to provide a supply of varied and choice flowering plants for the purpose named. Boronias are increased by cuttings, but, like a few genera of which I have treated in former papers, it requires a few years before they get to a size to be of much service as decorative subjects. It is therefore the better way to procure a stock of small plants from a nurseryman; remembering to stipulate when giving the order, that the plants are to be free from scale and mealy-bug - as it is not an unfrequent occurrence for some nurserymen to supply gratis, along with certain species of the vegetable kingdom, a host of representatives of some families belonging to the animal kingdom, - the latter, as a rule, being in a highly satisfactory condition as regards their health and power of reproducing their kind, whatever may be the condition of the former.
Boronias should have an airy position as near to the glass as possible at all times of the year. When bright sunshine occurs during the summer months they should be slightly shaded, but continued shade at any season is not beneficial to them.
As soon as the flowering season is over, the necessary pruning or cutting-back of the plants should take place. Those species of a habit of growth similar to that of B. Drummondii should be cut well back annually. By so doing the plants are kept well furnished with shoots, thereby giving to them a bushy and well-formed appearance. In the case of B. serrulata, it is sufficient to pinch out the points of the shoots annually, after the flowering season is over. This species does not make as long shoots annually as most of the others; hence it is only necessary to nip out the points of its shoots until such time as the plants are of the desired size - then of course it should be treated similarly to the others in the matter of pruning or cutting back of its annual growths. After the plants have been pruned they should be placed on their side, and well washed by means of a syringe. If at the time the cultivator entertains any suspicion that there is scale or mealy-bug lurking about them, he should use at first paraffin and water, in the proportion of two wine-glassfulls of the former to one gallon of the latter, heated to a temperature of 95°, and kept thoroughly mixed together during the time of application.
Let the mixture remain for a few minutes on the plants, and then give them a thorough syringing with pure water. This will clear them of the paraffin and the greater number of insects, supposing any of the latter were present on them previous to the application. After undergoing cleaning in the way just referred to, the plants should be placed near to the glass in a cold pit or frame, and duly attended to in the matter of admitting air and supplying them with water at the roots. In this position they will soon commence to push forth fresh growth; and whenever this is observed they should, if in the cultivator's opinion they require it, be re-potted. When doing this much care ought to be taken, as, like all hard-wooded plants, Boronias will not thrive satisfactorily unless the soil about their roots is maintained in a thoroughly sweet condition; hence it is highly important to supply an adequate amount of efficient drainage at first. After the plants are repotted, they should be again placed in a cold frame or pit, and the latter kept rather close and shaded for a week or two, until the roots have laid hold of the fresh soil. They should remain in this position up to the end of September, and during the time air should be admitted freely to them both day and night.
By the first week of October they should be placed in their winter quarters, and, as before indicated, in a position as near to the glass as circumstances will permit. Large plants of Boronias will remain healthy for several years without repotting, provided the drainage keep in good working order, and they are properly attended to in other ways. Good peat, such as a Heath-grower would select for his favourites, and coarse river-sand, in the proportion of one-third in bulk of the latter to two-thirds of the former, make a good mixture for applying to the roots of Boronias; and on all occasions when they are repotted, the compost should be made as firm as possible about the roots. In the matter of applying water to the roots, the cultivator should exercise much care at all seasons, as an excess is very prejudicial to their health; and on the other hand, the soil must not be allowed at any time to become what might be termed dry. Another matter to be attended to in their culture is to keep them clear of mildew. Some species, particularly B. serrulata, are subject to attacks of mildew, and a strict watch should be kept for its appearance.
If at any time it is discovered on the plants, they should be laid on their sides and dusted with flower of sulphur, which will have the effect of arresting the further progress of the enemy. In addition to the three species already named, B. crenulata and B. megastigma are deserving of being cultivated by all lovers of choice greenhouse plants. The latter, when in good health, is an elegant and graceful plant when not in flower; and when in bloom it is surpassed by few in the quiet beauty of its flowers. J. Hammond.