This section is from the book "The Florists' Manual", by William Scott. Also available from Amazon: The Florist's Manual.
Ventilation is primarily afforded to keep down temperature when the sun's rays have heated up the houses, but to give and afford a free circulation of fresh air to the plants is quite as much a reason for ventilation. Our large glass of the present day quickly runs up the temperature with the sun shining, and if ventilation is not faithfully attended to at the proper time great harm is done, and in the case of roses months of hard and faithful labor can be ruined.
The necessity and benefit of ventilation is too well known to need any lengthy remarks. It is the mental side of the business. It wants watching like the water gauge of a steam engine, but more scientific. It is the same sort of science that is brought into use by Mary Murphy when she knows the potatoes are just done, and cooking is a science. You can lay down rules that this or that house should be ventilated when the thermometer registers a certain degree, but to that should be added some knowledge and judgment. In rose growing it is perhaps the most particular of all work connected with them, and if a man has charge of three or four houses it will almost keep him busy in the spring and fall months running from one to the other regulating the ventilation.
A man should be able to tell without even looking at the thermometer whether the sashes are too much or not enough open. The thermometer is of course an infallible guide and authority, but there are times when even a few degrees higher is of less injury to the plants than a keen, cutting draught of air. Often the sashes are opened six inches when two inches would be plenty. There is one good rule and that is to begin ventilating early and take it off early. Too many are guilty of waiting till perhaps 10 o'clock on a bright morning and then opening up wide; first subjecting the plants to an enervating heat and then giving them a sudden chill. By shutting up early in the afternon you have utilized the sun heat and saved coal, and sun heat is always better for the plants than fire heat. There are thousands of houses throughout the country that are sadly inadequate in ventilation, and in such houses roses, carnations and all our flowering plants will draw up weak.
What we are most concerned about is not the mistakes that have been made, but to prevent any more. In houses that are attached it is obvious that side ventilation can only be given on the two exterior walls, and in any rose house we would not have any ventilation in the side wall or glass, even if it cost nothing, because we could not use it. In plant houses or in carnation houses side ventilation is perhaps desirable, but I think not at all necessary if ample ventilation is given at the ridge.
In equal-span carnation houses we have in addition to the ordinary ventilating sashes on the south side large sashes about 3x5 hinged on the north side, eight feet between each. We have no ventilating gear attached, but after settled warm weather, or when there is no longer danger of weather that would hurt carnations, we raise these sashes eight or nine inches on blocks of wood, and then tie them securely down, leaving them so till October 1. Believing that if ample ventilation is provided on one side of the ridge it is enough, and we do believe it from experience and observation of other people's houses and crops, then it is useless to discuss the matter further.
Ventilation should be provided the whole length of a house. If it is wanted at all in one place in the roof then it is wanted the entire length, and it must be better to give three inches all along than six inches only in spots. This will apply not only to a rose house but any greenhouse for whatsoever use intended. Though you may need but one inch of ventilation throughout the whole month of January, in June our climate demands the utmost you can give. It takes no more glass to have continuous ventilation, no more in cost of apparatus except a few arms, and only a few dollars more in extra ventilators.
In the long-span-to-the-south house the ventilation is always on the south side of the ridge, and the same in equal-span houses whose ridges run east and west. In the short-span-to-the-south the ventilation is on the north side of the ridge. In houses where the ridge runs north and south, always equal-span, the ventilation should be on the east side. You can open the sashes earlier and our prevailing winds are from the west. We are often able to give an inch of air at the ridge when cold outside without feeling any draught, whereas if the ventilators were hinged at the ridge and opened two feet down the sash, we should feel a draught. And if it is good for one house or one kind of plant it certainly is for all. So that is the way to hang your ventilators; let them all open at the ridge.
Since writing the above we built a number of equal-span rose houses running east and west. After considerable study of the question we decided to place the Ventilators on the north side of the ridge, hinging them on the headers and opening at the ridge, and it has turned out satisfactorily. If the latter plan is followed one lifting apparatus will do the work that would require two if the ventilators were hinged at the ridge. It is a difference between lifting a fifty-sixpound weight over your head with your elbow bent and raising the same weight with your arm perfectly straight. Aspect and prevailing winds must be your guide in ventilating.
While you are having sashes made have them large enough. If the house is from nineteen to twenty-three feet wide the ventilators should be from thirty to thirty-six inches deep and continuous. The length of each section should be not over five feet or the sashes will be too heavy to lift easily, but there is not nearly so much weight to lift when they open at the ridge as when hinged at the ridge. The ventilator man will tell you how many machines you need.
No one would think in this day of ventilating without the use of one of the machines which do their work so admirably. They will pay for themselves easily in labor saving in one year, and without them I can't see how you could manage. Yet some struggle on without them. It is not the labor saving alone, it is the plants that suffer when the sashes are moved by ropes or rods or sticks. To raise or lower a lot of sashes by those crude methods is quite a chore and too often if you are busy and you think actual necessity does not compel, you are too apt to say, "It's pretty warm, but I guess it won't hurt." You are shirking the job, but how easy to say, "Jim, put on a crack of air," and Jim turns the handle and up go a hundred feet of sashes in a moment, and only fun to do it. There are several good appliances. I have five different makes, and like best the "Challenge" ventilator.