This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
A garden is scarcely considered furnished during the summer months without some Pelargoniums, Calceolarias, Verbenas, Heliotropes, Lobelias, Ageratums, Dahlias, and several other things. To raise and winter these plants a small greenhouse or warm pit is indispensable. Calceolarias are nearly hardy, and cuttings taken early in the autumn and bedded in thickly together will throw roots and merely require the protection of a frame during winter. Next in point of hardiness are the Pelargoniums: these likewise are propagated from cuttings in the autumn, either out of doors or several together in pans. They may be left in the beds or pans with ample protection from frost until the end of February or beginning of March, when they should be potted singly to enable them to form strong plants. The principal point to guard against during the winter, especially if they are stored where the temperature is low, is superabundant moisture. The beds or pans should be well drained, and water almost entirely withheld in severe weather. All dead leaves and decaying matter should be removed as soon as observed, or the young plants will be liable to damp off. Lobelias, as we have already mentioned, are raised by preference from seed, which should be sown early in the year. Verbenas, Heliotropes, etc., being rapid-growing plants, and rather tender, the simplest plan is to store a few old plants to obtain cuttings from in the spring. A little more heat should be applied about the beginning of March to stimulate the old plants into making new growth, and as soon as the shoots are two or three joints long, they may be taken off and put into the cutting pots, a hot-bed having been previously prepared for their reception. If healthy, and the hot-bed quite sweet, they will soon strike, when they should be potted off before the roots become matted together. A great deal depends upon their being kept free from parasitical vermin and mildew. Dahlia tubers should be stowed away in a moderately dry place where no frost can reach them. The beginning of March is the best time to start them into growth, the more gently the better. They are propagated by division of the tubers and from cuttings. Our concluding remark is, Do not turn out bedding plants too early, or without being properly hardened off.