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.
Plants of annual or biennial duration possess so many valuable qualities that they are quite indispensable in the flower garden. Some are cherished for their fragrance, as the Sweet Pea, Mignonette, and Stocks; others for the showiness and the variety of their brilliantly coloured flowers, as Poppies, Zinnias, and Asters; others for their diminutive, compact habit, and profusion of flowers, as Leptosiphon and Ionopsidium; others for the duration of their scarious flower-heads, as Helichrysum and Waitzia; and others again for the elegance and grace of their inflorescence, as Humea elegans and Agrostis nebulosa. A large number of perennials, many of them tender, are commonly treated as annuals, and flower the first season. The fact of their being amenable to this mode of treatment is usually mentioned under the description. A notable case in point is the Lobelia Erinus, a difficult plant to preserve through the winter, but easily raised from seed, which it produces in great abundance. Annuals are of the greatest service for filling up vacant spaces, or, when judiciously selected, for growing by themselves in beds or borders. Such plants as the China Aster, Zinnia, and Phlox Drummondii, make very effective beds, either with their varieties mixed, or in separate colours. Another recommendation to favour is the short period and little trouble required to raise many of them for succession, filling up or replacing failures. Annuals may be divided into three groups, namely, hardy, half-hardy, and tender. Although many of the tender species are either described or noticed in this work, they need not occupy our attention here; for all coming under this designation cannot be raised early enough to flower in the open air without artificial heat, and many of them are so delicate as to succumb to the least unfavourable changes of the weather, and at best their beauty is of short duration; still, with time and convenience for hot-beds, and warm, sheltered borders, with a light, permeable soil, they may be cultivated, if only for the sake of novelty. The strictly hardy annuals, or species treated as such, are of the first importance to the amateur of limited resources; and if they are not quite so numerous and brilliant as the half-hardy species, there is yet sufficient choice to admit of an effective display when associated with a small collection of perennials. If we include those species that merely require a little protection during cold nights, such as a hand-light, bell-glass, or inverted flower-pot, our list would contain nearly all those in general cultivation. Naturally these half-hardy species are better raised in a frame, either with or without a little artificial heat, because they may by these means be had in flower much earlier. Hardy annuals are those which may be sown in the open ground without any covering or protection whatever; amongst the most familiar we may enumerate - Candytuft, Sweet Pea, Lupins, Common Marigold, Larkspur, Nemophila, Clarkia, Saponaria Calabrica, Convolvulus tricolor, Mignonette, Love-lies-bleeding, Collinsia, Eschscholtzia Californica, and Collomia coccinea. These and numerous others may be sown in suitable weather at different times, from the end of February onwards, according to the requirements of the establishment. Where sown in patches in the mixed borders, the spaces should be thoroughly forked, and, if poor, a little leaf-mould and thoroughly rotten stable-dung from an old hot-bed, if attainable, should be incorporated with the native soil; the surface should be even and fine, and if dry and light, a little pressure will be beneficial after the seeds are sown. The latter should have a layer of mould over them about equal to their own volume. The seed of most annuals being very cheap is frequently the cause of their not attaining their normal development, for it is sown too thickly by ten times, and the surplus plants never rooted up. As a rule, there are from half-a-dozen to a dozen plants where there is only space for one, and the consequence is mutual starvation. Watering should be carefully done with a fine rose when really necessary, but it is better not to water, especially on a stiff soil liable to cake, except during a prolonged drought. In the summer, when the plants are grown up, frequent waterings in dry weather will, however, prove beneficial. The removal of the seed-vessels will prolong the flowering season of many species, not only of this, but of all other classes.
Half-hardy annuals require raising in artificial heat, or where there is at least sufficient protection to exclude frost. They should be sown in March or April, and planted out at the same time as the bedding plants, about the middle of May. The same treatment may be adopted for these, as recommended under General Remarks for perennials, except that there is less necessity for a second frame or pit to remove them to according as they come up. Care should be taken not to remove them suddenly from a hot-bed to a cold pit. A very gentle heat is all that is required, and gradual hardening off is imperative before transferring them to their quarters in the open air. To obtain good strong plants the seedlings should be potted off when they are quite small, placing about three or four in a six-inch pot, and it should be borne in mind that a few vigorous plants will make a finer display than a great many weakly ones. The beds or borders should be renovated during the winter, and it is always better not to grow the same description of plants in the same places year after year. Asters, Zinnias, French and African Marigolds, Helichrysum bracteatum, Phlox Drum-mondii, ornamental Gourds, and most of the herbaceous climbers come under this head.
Biennials offer less variety, and only the hardy species are generally cultivated. Some, it is true, are treated as annuals, but the majority must be sown towards the end of summer, in order to flower the following spring. Brompton and Queen Stocks, Honesty, Hollyhock, and the Common Wallflower are familiar examples of the hardy members of this class. The Wallflowers are really perennial, but young plants flower more profusely than old ones. The double-flowered varieties of the Wallflower are propagated from cuttings, and the Hollyhock from offsets. Humea elegans is one of the most desirable of tender biennials. It may be treated as an annual if sown early in the year, but it neither grows so strong, nor flowers so freely as when raised during the preceding season.