Many persons injure greenhouse plants by keeping them too warm, and giving them too little air during winter, and then are surprised that their plants become sickly and remain without flowering, notwithstanding all the care and expense that have been bestowed upon them. No greenhouse ought to be kept at a greater heat at night than from 35° to 40°; and in the daytime it should not be allowed to rise above 50°, or at most 52°. When there happens to be sunshine, the fire ought to be lessened, and whenever the air is not frosty the windows ought to be opened from twelve till two every day. If a greenhouse is kept too warm, it will induce premature vegetation, and the plants will waste their strength in an attempt to produce flowers and fruit, at a season when nature requires them to be kept in a state of complete repose. Greenhouse plants should be watered generally every morning; but in frosty weather water need not be given every day, and some plants will not require watering oftener than once a week. This, however, must depend in a great measure on circumstances, and, as a general rule, it may be observed, that water may always be given in small quantities when the surface of the earth contained in the pot looks dry. The pots should never be allowed to stand in saucers, as stagnant water is peculiarly injurious in winter. Whenever the earth looks black and sodden, the plant should be turned out of the pot, and, after the black earth has been carefully shaken from the root, it should be repotted in fresh soil, an inch or more in the bottom of the pot being filled in with small pieces of broken crocks.
In February or March greenhouse plants should be looked over, and repotted where necessary; those that are too tall should be cut in, and cuttings made of their shoots. The young plants raised from cuttings made in autumn should be repotted in larger pots for flowering; and where the plants do not require fresh potting, but have the surface of their mould become green and mossy, the moss should be taken off, and the ground slightly stirred with a flat stick, taking care, however, not to go so deep as to injure the roots. When trouble is not an object, all greenhouse plants are the better for repotting once a year, either in spring or autumn; and when the ball is taken out of the pot for this purpose, it should be carefully examined, and all the decayed parts of the roots should be cut off. Sometimes, when the ball of earth is turned out, nearly half of it will fall off almost without touching it; and when this is the case, it will generally be found that there is a worm in the pot. Worms do a great deal of mischief to greenhouse plants in cutting through the roots, as their instinct teaches them to make their way through the earth straight across the pot and back again, and they cannot do this without tearing the roots asunder every time they pass.
Another point to be attended to in the management of a greenhouse is, keeping the plants as near as possible to the glass, as, unless this be done, the plants will become what gardeners call "drawn up," that is, they will be unnaturally tall and slender, from the efforts they make to reach the light.
As I have frequently mentioned repotting, I may as well tell you here the best mode of performing the operation. The pot to which the plant is to be removed should always have been previously washed quite clean, and be perfectly dry. Some bits of broken pots, called potsherds, should then be put at the bottom of the pot, the quantity varying from three or four pieces (so as just to cover the hole) to a mass an inch in depth, depending upon the nature of the plant. If the plant has not been in a pot before, the roots are then placed just above the potsherds, and the earth is filled in, the plant being occasionally shaken so as to allow the earth to get amongst its roots. The soil is next consolidated by shaking the pot, and then lifting it up and setting it down again with a jerk; and is rendered firm and neat round the rim by means of a broad smooth piece of stick shaped somewhat like a table-knife, and called a potting-stick. When a plant has been in a pot before, and is repotted or shifted, as it is called, into a pot a size larger, the plant is turned out of its old pot by putting the hand upon the earth and turning the pot upside down; or, if the ball of earth does not come out readily, striking the rim of the pot against the edge of the potting-table or shelf. The ball containing the plant will thus drop out into the left hand; and the potsherds that adhere to the bottom of the ball having been picked off, and any part of the root that appears decayed having been removed, a little mould is put on the drainage in the new pot; and the ball of earth, containing the plant having been placed in the centre, the space between it and the pot is filled in with light rich mould, and made firm with the potting-stick. The operation is concluded by shaking the pot, and then taking hold of the rim with both hands, and striking the bottom of the pot two or three times with a jerk against the potting-bench. The plant is then watered, and set in the shade for the remainder of the day.
Heaths are very difficult plants to manage; but a great improvement has taken place in their culture within the last few years. They are grown in what is called heath mould, that is, a mixture of peat and sand; and when this earth is put into the pot, it is mixed with good-sized entire pebbles, some of which are suffered to protrude through the surface of the soil. The roots of heaths are extremely fine and hair-like, and the shelter afforded by the pebbles is so congenial to them, that, if one of the stones be taken out, a cluster of fine, white, vigorous roots will be found below it. The plants are always potted high, so as to let the base of the stem be above the level of the rim of the pot, as the plants are very apt to damp off if the collar of the plant be buried in the ground. Heaths should never be suffered to become too dry, and never kept too wet. They require very little heat; and many experienced cultivators never apply fire-heat to their heatheries at all, but merely keep out the frost by having wooden shutters to the sashes, and covering them with mats. Heaths, when growing rapidly, should be repotted whenever the roots have filled the pot; but they should not be shifted too often; and, when they have attained their full growth, they may be suffered to remain in the same pots three or four years without injury.
As plants in the conservatory are grown in the free soil, they are in a much more natural state than any plants can be in pots, and consequently they require much less care in their culture. There is usually a walk all round the conservatory, next the glass, and one down the middle, on each side of which are the beds containing the plants, and under which are placed the hot-water pipes that warm the house. The consequence of this arrangement is, that the beds on each side the middle walk are so planted as to have their highest shrubs in the centre, shelving down to those of lower growth on each side; and hence the centre is generally planted with tall camellias, acacias, metrosideros, eucalyptus, &c.; while near the walk are placed oleanders, myrtles, fuchsias of different kinds, together with chorozemas, and many of the other most ornamental New Holland plants; and up the pillars that support the roof are trained kennedyas, bignonias, ipomœas, and passion-flowers in great variety. Cliánthus puníceus and Polýgala oppositifòlia ought to find a place in every conservatory; and a plant of Wistària sinénsis may be trained under the rafters so as to afford shade to the camellias; as, under shelter, the wistaria will flower twice in the year, and its flowers will yield a delightful, though very delicate, fragrance.
As it is of the greatest importance to the health of the plants to have the soil in a conservatory well drained, many persons form the beds by excavating pits of the proper size, about two feet and a half deep, and put at the bottom a layer of brick-bats, stones, and other materials for drainage, about six inches thick. On this is deposited a thin layer of coarse rough gravel; and on the gravel a layer of rich mould, which should be about two feet thick in the centre of the bed, where the largest shrubs are to be planted, and shelving off to about sixteen or eighteen inches at the sides next the walks. All the sashes should be made to open, and there ought to be large glass doors in front, which should generally stand open during the day in summer, in order to admit as much air as possible.
In some places a movable frame is contrived for a conservatory, into which sashes fit in winter, and which, in severe weather, is covered with tarpauling, made to pull down, like a blind, from a roller along the ridge of the roof; the whole frame being so contrived as to be entirely removed in summer. The upright posts of this frame are let into holes in the ground, like the posts in a drying-ground, so that when the posts are taken out, the holes may be stopped up with wooden plugs with rings attached; and the roof and horizontal pieces fit into each other, and into the uprights, the whole being kept firm by bolts. When a conservatory of this kind is to be removed for the summer, the side sashes and doors are taken away first, generally about the middle or end of April. A week or ten days after, the sashes of the roof are taken off, but the frame and tarpauling are left in case of spring frosts; and, when all danger from these is over, the whole of the framework is removed, and the orange trees, camellias, and other exotic trees that have been planted in the conservatory, appear to be growing in the open air.
The orangery is often contrived so as to be used as a kind of living-room during summer, as it is only intended for the reception of the orange trees, and other plants belonging to the genus Citrus, during winter. The trees are generally grown in large tubs and boxes, in a rich loamy soil, and are set out in the open air during summer, when they require but little care, provided they are frequently watered over the leaves, though they do not like much water to their roots. When young plants are raised from seed, they seldom flower till they have been budded or grafted from an old tree. Orange trees are generally put in the open air in May, and kept there till September or October; and they are very seldom shifted. They require scarcely any light or water during winter, and no heat beyond what is necessary to protect them from the frost.