I will now say a few words on the greenhouse plants that you will want for planting in the open ground in your flower-garden. Petunias may be all raised from seeds with the other half-hardy annuals; as seedling plants both grow and flower much more vigorously when planted out into the open ground, than plants that have been raised from layers or cuttings. Célsia or Alonsòa urticifòlia may also be raised from seeds; as may Thunbérgia alàta, and its white variety. Phlóx Drummóndi is almost always raised in this manner; as are the beautiful climbing plants, Lophospérmum scándens and its varieties, Maurándya Barclayàna, Cobœa scándens, Eccremocárpus or Calámpelis scàbra, Rhodochìton volùbile, the beautiful canary-bird flower (Tropœ'olum peregrìnum), the most splendid of the ipomœas, and several other well-known plants.
Geraniums, or pelargoniums, as they are now more properly called, being half-shrubby plants, require to be raised by cuttings. These are generally taken off the points of the shoots in autumn; and, a good many being put into one pot, they are plunged into the hotbed till they have struck root, and then gradually hardened and placed on the back shelf of a greenhouse, or in a cold frame, till the spring, when they are removed to separate pots till they are wanted for planting out. Some gardeners do not put themselves to the trouble of potting them, but keep them in the same pots in which the cuttings were struck till they are wanted for planting out; but this is a slovenly mode of culture, as, when the plants are kept so long in one pot, they become drawn up, and never have the compact bushy appearance that they have when they are properly transplanted early in spring. Verbenas may be either preserved by cuttings or layers, or raised afresh from seed. The usual way of propagating them, however, is by layers, as they strike root readily at the joints, if the joints are covered with a little earth. All the other greenhouse plants which you may want to grow for planting out may be treated in the same manner as those I have mentioned.
A cold frame is a bottomless box of the kind described for a hotbed, but formed of brick or stone instead of wood. These frames have a glass sash at the top, but contain no manure; and they are generally sunk in the soil, that the warmth of the earth around may aid in protecting the plants they contain from the frost. These frames, if they have only one light, are generally five feet in width; that is, from the back to the front; but, if they have two or three lights, the width is generally seven feet, as these are the dimensions of the frames used for hotbeds in kitchen-gardens. The greenhouse plants that are to be preserved in the cold frame are merely set in their pots close together, and, the glass sashes being then closed, mats and other coverings are laid on to keep out the frost.
Sometimes greenhouse plants which are left in the open ground are preserved from the frost by coverings of wicker-work like beehives being put over them, or tin hoops over which mats have been stretched; or, where the plants are small, a flower-pot may be turned over them, or a hand-glass used for the same purpose. It is seldom, however, worth while to take much pains to preserve greenhouse plants that have flowered in the open air. The ordinary way is to make abundance of cuttings in autumn; to strike them in a hotbed, and then, after hardening them by degrees, to preserve them in a small greenhouse, or in a cold pit, till the time for planting out next year.
[LETTER VIII.]
USE OF PLANT-HOUSES.—NATURE OF CLIMATES—DIFFERENT KINDS OF HOTHOUSES.—THE DRY STOVE, THE BARK STOVE, AND THE ORCHIDEOUS HOUSE.—CULTURE OF PLANTS IN THE BARK STOVE.—AQUARIUM AND WATER PLANTS.—RED SPIDER.—CULTURE OF SUCCULENT STOVE PLANTS.—CULTURE OF ORCHIDEOUS PLANTS.—THE GREENHOUSE.—THE AUSTRALIAN HOUSE, AND CULTURE OF ITS PLANTS.—THE COMMON GREENHOUSE, THE HEATH HOUSE, THE CONSERVATORY, THE ORANGERY, AND THE CAMELLIA HOUSE.—THE CULTURE OF PLANTS IN THE COMMON GREEN HOUSE.—POTTING PLANTS.—HEATHS.—CULTURE OF PLANTS IN THE CONSERVATORY.—CULTURE OF ORANGE TREES.—APHIDES.
Before I say any thing of the management of the plants in your greenhouse, I must remind you that, in order to grow plants well, it is not enough merely to preserve them from the frost, but we must imitate as well as we can their native climate: that is, the degree of heat, light, and moisture they have been accustomed to in their native country, together with the air and the soil. The latter is the easiest condition to fulfil; as, by combining different kinds of earth, we can, without much difficulty, produce a very tolerable imitation of any soil we please: but it is not so easy to give plants abundance of light and air in combination with heat; and, though we can readily give plants plenty of water, it requires some management to surround them with a moist warm atmosphere, like that they have been used to in their native woods. To meet these difficulties, buildings have been constructed, suited for the reception of plants, with various contrivances for producing heat and distributing air and moisture, and with a glass roof, front, and sides, to admit abundance of light.