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.
Few among the more popular stove-plants are more deserving of cultivation, or will more amply repay any extra care bestowed upon them, than Gloxinias. They can be had in bloom for at least six months of the year; and the most of the time they are not in active service they can be stowed away in very small compass, and require no further trouble than an occasional look to see that the bulbs are keeping securely. Some of the varieties now in cultivation, and especially the upright-flowering ones, are exquisitely beautiful, and comprise among them many most delicate shades of colour in beautifully-blended combinations. They have the additional recommendation of not being difficult to grow successfully, and may take the same rank among warm-temperature plants that the Cyclamen takes amongst those that adorn the greenhouse in spring.
Any one who wishes as quickly as possible to get up a stock of Gloxinias, cannot do better than purchase a packet of seed which has been saved from a good named collection, and sow it about the middle of September. Take equal proportions of loam, peat, and leaf-mould, with a fifth of the whole of sand, and fill a well-drained seed-pan to the brim, finishing with a quarter of an inch of finely-sifted soil, to give an equal surface for the fine powdery-looking seeds. On this surface sow rather thinly than otherwise; and after merely covering the seed with fine soil, water well through a fine rose, cover the pan with a pane of glass, and place it in a house or pit where the temperature ranges from 65° to 70°. Shade from sun till the young seedlings are well through the soil, then raise the pane of glass, and after a few days remove it entirely and place the pan near the glass, shading from the sun when at its hottest, till the young plants are hardy and able to bear the full light of the season.
Whenever the plants are ready to handle easily, prick them off into pans filled with the soil already recommended, with perhaps a little finely-sifted old cow-manure added to it. In pricking them off give the plants sufficient room to keep them from getting crowded before February, say about 1½ inch each way. The object now is to keep them growing all winter, and at the same time to keep them stocky; and room and light are the two principal conditions requisite to this end. Consequently place them in as light a place as possible through the winter, and in a temperature ranging from 60° to 65°. Do not over-water them, but at the same time keep them regularly in a moist condition. By the middle of February they will be ready to pot singly into 2-½-inch pots. Choose equal proportions of free fibry loam and peat, and pass it through a ½-inch sieve, fibre and all; add a little rotten cow-manure and sufficient sand to make it sparkle - say a sixth of the whole. Drain the pots well, and when the plants are potted they will do all the better if they can be plunged in bottom-heat near the glass, but this is not indispensable.
They dislike a dry parching atmosphere, and grow most vigorously in moist stove temperature in a subdued light; but the latter condition must not be in excess, or they will grow weakly and not flower so well. They will now grow rapidly and make fine strong leaves. As soon as the roots have got well to the bottom and sides of the ball, let them become moderately dry, and shift them into 5-inch pots for blooming in, using the same soil and subjecting the plants to the same growing conditions recommended for them when first potted, only the heat may increase as light increases. Shade just sufficient to keep the sun from scalding and blotching the leaves, and attend carefully to their wants in the way of water.
In this stage they will grow rapidly and make fine foliage, spreading over the surface of the pots and curling over their sides. They will soon show signs of an abundant crop of bloom, and as the flower-stems are thrown up and begin to open, care must be exercised that they are not subjected to a parching arid atmosphere or sudden bursts of brilliant sun. They will flower most satisfactorily in a moderate stove temperature where 65° is the maximum night-heat, but may be bloomed in a house considerably cooler; but the flowers will not be so fine, and are more subject to damp off prematurely. Managed thus they will bloom splendidly for at least two months, and a packet of seed sown in January will keep up the succession till autumn.
When they are done blooming, do not withdraw water and send them prematurely to rest, but keep the foliage healthy as long as possible, so as to have a large well-matured bulb for next year. It is presumed that only the best varieties are selected for keeping over the winter. Withdraw water by degrees, and when the leaves have all decayed in their natural order, dry them off entirely, then shake them entirely out of the soil, and store them either in a large flower-pot or pan mixed and covered up with dry sand, and place them for the resting season in any dry place where the temperature ranges from 55° to 60°. This is less trouble than storing them in the pots in which they bloomed, and a far more certain way of keeping them without any decay or loss among them.
It may be observed that the multiplication of any fine variety is easily effected by taking leaves with part of the stem attached and putting it in a pot of sand, like a cutting, in bottom-heat - each leafstalk produces one strong bulb. But if numbers be required, nick the midribs of each leaf and peg it down closely on the surface of the sand, and at each wound or incision a young bulb is formed.
The time to start bulbs into growth must be determined by the time at which they are required to bloom. Those started early in February will be in full bloom in May, and by starting a number of bulbs at three or four different times, a fine bloom of Gloxinias can be kept up for a great part of the year.
They thrive best in a rich sandy soil, such as has been already recommended, and the pots should always be well drained, as they will not thrive if the soil becomes the least water-logged and soured. Avoid over-potting even large bulbs. A 6 or 7 inch pot will grow an immense size of a plant. And it is best to start them in smaller pots and shift into fresh soil as the pots become moderately filled with roots. Gloxinia.