Ventilation
Ventilation, or air-giving, is one of the most important operations in garden management, and the cultivation of all kinds of trees and plants under glass. Ventilators are requisite in all horticultural structures, for the purpose of regulating temperatures, and affording an interchange of air in their interiors. The amount of space made available for opening to admit air, depends on the kind of plants cultivated in any particular structure, as, for instance, conservatories and greenhouses. Peach houses and vineries need provision for the admission of an abundant circulation of air whenever it is required, or when circumstances and weather permit; while in plant stoves and houses in which the occupants need a more or less tropical atmosphere at all times, a smaller number of ventilators will suffice. It is always advisable to have some plan of giving Ventilation at, or near, the top: this is provided in most houses of modern construction, so that the least supply, or nearly the full amount, of air may be admitted without rain getting in, except from an occasional splashing. Ventilating gearing has been greatly improved during recent years, so that either top or bottom sashes along a house, say 30ft. long, may be easily opened or closed with one hand, working a lever, and the sashes may be fixed with the other hand to wherever they are required to remain. Where there are no side or front sashes, as in many lean-to houses, wooden door ventilators are best, fixed in the front wall; and if the air from these can be made to pass over the hot-water pipes, it will become warmed before reaching the plants. Wooden ventilators may be similarly placed in a back wall; but wherever convenient, lifting sashes are preferable. There are numerous details attending the admission of air which depend on widely varied circumstances, and can only be learnt by practical experience; the state of the weather, which is often very changeable, and the difference between outside and inside temperatures, being, perhaps, the most important points to consider. At different seasons, too, the same plants will need very dissimilar treatment according to their stages of growth. During spring, the greatest care is necessary regarding the admission of air, as sudden changes of temperature, caused by improper Ventilation, quickly show their evil effects on young and tender foliage. In old-fashioned houses, with small panes of glass, the sun never has the same effect in raising the temperature inside, as in most of those of modern construction with large panes; hence the necessity of early attention to Ventilation becomes much greater with the last-named. Happily, many of the improved systems of applying air, as already referred to, afford the means of opening or closing the ventilators in a tenth part of the time occupied in handling each sash separately in a large house. In daily management, when it is known that air will have to be admitted to a house, the ventilators should be opened very gradually so soon as the temperature begins to rise in the morning or early part of the day. It is never good practice to allow the temperature to get high and then put on what air is required for the day at once, or even at twice. Under such management tender foliage often droops quickly because of a sudden change and excessive evaporation taking place. Few plants or trees are able to withstand this: their leaves often become scorched during the day, and the attacks of insects are much encouraged. In spring, the ventilating of forcing-houses and other structures may require somewhat different management nearly every day in order to keep near the requisite degree of heat. The quantity of air and the mode of applying it are, therefore, matters which must be determined in dealing with the culture of plants under such varied circumstances. What may be called general advice is to begin ventilating early, as already noted, when it is pretty certain that airing will be necessary, and apply a little more at frequent intervals until sufficient for the day is put on. Secondly, draughts should always be avoided: if the air is cold or the wind rough, never open ventilators at the front and back parts of a house at the same time, unless the occupants are such as take no harm--and this is seldom the case. In summer, when the inside and outside temperatures are much more uniform than at other seasons, air may generally be most freely admitted without causing much injury.