This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Under this title there are four distinct kinds of plants grown for market, viz.: (1) Zonal Pelargoniums (including green-leaved, silver-leaved, bronze-leaved, and tricolor-leaved varieties); (2) Ivy-leaved Pelargoniums; (3) Show and Fancy Pelargoniums; and (4) Oak-leaved or Sweet-scented Pelargoniums. These are all popularly known under the name of Geranium, a genus which has already been dealt with at p. 47.
These constitute the most important group, and millions of plants are raised and sold annually by market growers, to say nothing of the hundreds of thousands that are also raised in private-garden establishments. The different varieties have originated from Pelargonium zonale and P. inquinans, both natives of South Africa. The first-named has roundish smooth or softly downy leaves, with a distinct dark zone or band, which gave rise to the popular name of Zonal. The flowers vary from red, scarlet, and crimson to pure white. P. inquinans has more kidney-shaped leaves not so deeply cut or crenated on the margins as P. zonale, and the flowers vary from intense scarlet to rose and white. During the past two hundred years and more gardeners have been improving the progeny of these two species until one would scarcely imagine that the large circular broad-petalled varieties had anything to do with the species that had narrow-petalled irregular flowers such as are produced by the "Oak-leaf" section.
Zonal Pelargoniums of all sections are easily propagated from cuttings taken almost at any time of the year so long as they are not too young and sappy. Spring and autumn, however, are the two chief periods for taking cuttings. These vary from 1-3 in. in length, and are cut beneath a joint, and trimmed of unnecessary leaves and stipules. They are then inserted round the edges of 5-in. or 6-in. pots in gritty soil, or three cuttings may be put in a 3-in. pot filled with the compost, the cuttings in all cases being dibbled in. Some growers use a little silver sand on the top of the soil, but the vast majority do not trouble about this detail. Besides pots, shallow wooden boxes or trays known as cutting boxes, and measuring 15 in. long by 9 in. wide and 2 in. deep, and costing from 8s. to 10s. per 100 or per gross, are extensively used. Each box is filled up with gritty soil, pressed down firmly with the fingers, and will hold two dozen cuttings comfortably. These boxes are very useful for placing on shelves over the pipes in many small greenhouses where space is a consideration, and where pots would take up much more room. From January to March and April hundreds of thousands of Zonal Pelargoniums are propagated in this way, and in a temperature of 65° to 70° F. they root readily. When well established they are moved into 3-in. pots and perhaps later on into 5-in. pots, according to circumstances. Cuttings are also struck readily in hotbeds in spring, the young plants being potted on as quickly as possible. During August cuttings root quickly in the open air in any good garden soil, and the old plants are frequently planted out in May or June to produce a supply for this purpose. Autumn-struck cuttings produce bigger plants in the spring, and are usually sold in 5-in. pots. Cuttings are often taken from them up to February and March. This makes the older plants - which during the winter are stood pot to pot - more dwarf and bushy, with the natural result that more space must be given them about the end of March or early April. Unless grown specially for cut flower, no flower trusses are allowed to develop until about six weeks before the sales usually begin. They are pinched out, thus throwing all the vigour of the plants into growth. To hasten the development of fine specimens in a short time, a little weak liquid manure is given from time to time, and perhaps a little special manure is worked in about the middle of March or early April. If a little basic slag is mixed with the soil at the last potting it will help the plants considerably also. From this time onwards as little fire heat as possible is given - perhaps only at night to prevent damping - and on all genial days as much air as possible is given. This hardens the plants off by the middle or end of May, when the bedding out season may be said to commence in earnest. The plants are sold either in 3-in. (60's) or 5-in. (48's) pots, fetching from 2s. to 2s 6d. and 4s. to 5s. per dozen respectively; and in cutting boxes, containing two dozen "rooted" cuttings, fetching 1s. 6d. to 2s. 6d. per box.
Apart from the sale of plants, Zonal Pelargoniums are also grown for the production of cut flowers in winter, spring, and summer. This trade developed many years ago, when there were fewer flowers to be had and prices were better. The single varieties even in those days sold, but the flowers soon dropped, and it was not until the idea of "gumming" blossoms arose that an extensive trade was done. It is said that the gumming of single "geranium" blooms was first practised by a costermonger, who found that when he stuck the petals on, as it were, he did a more flourishing trade. The double-flowered varieties need no gumming. To secure plenty of bloom in winter, cuttings should be struck in spring, and the plants potted up and grown on out-of-doors during the summer months. The flowyer trusses are pinched out when they appear, and not till September or October according to circumstances are they allowed to develop. Not only must plenty of light be given to winter-blooming Zonals, but the atmosphere must be kept fairly dry, and the temperature should not fall below 65° F. at night, wrhile 70° to 75° will not be too much during the day.
The following are some of the best market Zonal Pelargoniums at present: Scarlet and crimson flowered: Paul Crampel, Vesuvius, West Brighton Gem, Corsair, George Potter, Admiral Togo, Henry Jacoby (rich crimson), John Gibbons, Paul Farrer (brilliant orange scarlet), King Edward VII, Jacqueri, etc.
The first-named - "Paul Crampel" - seems to have taken the world by storm. It has practically displaced the old favourite "Vesuvius", and has reduced the immense popularity held for so many years by the finest of all crimsons - "Henry Jacoby". "West Brighton Gem", with its whitish and green-striped stems, sturdy bushy habit, and fine scarlet flowers, still holds its own pretty well.