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Greenhouse - Moisture And Water Requirements

( Originally Published 1920 )

MOISTURE here means the humidity present in the hothouse atmosphere. The importance of this subject is as yet little appreciated by the practical man. The investigations of Blake* on the moisture requirements of roses point to the urgent need of similar experiments on other commercial hothouse crops.

In the greenhouse, temperature and humidity rank with food in their importance to the plants.

Effect of Humidity on Rose Foliage. Frequently when a greenhouse crop fails, the soil, the fertilizer or the water receives the burden of the blame. Little does it occur to us that the cause of the failure may be due to improper adjustment of humidity to temperature and watering. That each crop requires different humidity conditions is well conceded. The maidenhair fern, for instance, will not thrive in a house with a dry atmosphere, a condition which is ideal for other plants such as ornamental cacti. With roses a low humidity tends to reduce the size of the leaves, and the latter become "hard" and lose their flexibility. Again, vigorous, dark green foliage indicates a proper degree of humidity.


The investigations of Blake show (Table 12) that the humidity decreases in direct proportion to an increase in temperature. The opposite occurs when the temperature is lowered.

From Table 12 it is evident that heat lowers the humidity. It is also to be noted that the decrease in humidity is rapid on bright days even when most cold. It is to be further noted that not only the heat, but the ventilation of the house during the day when the heat is turned on in the pipes, tends to reduce the humidity still further. Rose growers, for instance, are aware that at the approach of spring the plants suddenly improve greatly. This is generally attributed to longer and brighter days. However, according to Blake, this change is due to an increase in humidity in the hothouse due to the shutting off of the heat in the pipes.


For further information on this subject, we have to refer again to the researches of Blake. He finds that the American Beauty rose, for instance, as a result of insufficient humidity fails to produce new shoots from the base of the plant. Moreover, the foliage, except at the tips of the growing shoots, becomes hardened and toughened. The older leaves turn yellowish and fall off prematurely, leaving bare stalks. As a result of this the younger and smaller rootlets die out. Blake also found that a high humidity tends to increase the size of the foliage, and the flowers seem to be likewise favorably affected. With low humidity the leaves of small rose plants wilt on bright days, even though the soil is kept moist. Frequently the leaves turn black, which is really a form of sunburn. This is undoubtedly caused by the dryness of the atmosphere in which the moisture in the leaves is given off faster than the roots can supply it.


There seems to be a tendency among modern greenhouse men to build benches and sidewalks of cement. While this may be very desirable from a hygienic viewpoint, it is objectionable for the maintenance of the proper degree of humidity. It is extremely difficult to raise the humidity of a house with cement walks. This is true even when water is applied to the walks and sprinkled on the plants. In houses devoted to roses and similar plants, where it is desirable to maintain a humidity of at least 75 per cent., cement walks become quite objectionable; cinder walks are to be recommended instead. Cement walks may be readily transformed by covering them with a layer of six inches of cinders. This covering will make it possible to maintain a higher and more uniform humidity.

Humidity is also an important factor in the heating of glass houses. The greater the humidity the greater is the evaporation of moisture, and the greater is the amount of heat required to maintain a uniform temperature. 'Without proper attention to these conditions, the hothouse crop may be doomed to failure.


Perhaps the quickest and safest way of determining the relative humidity of the air in the hothouse is by means of a sling psychrometer (fig. II.). This instrument is very simple in design. It consists of a wet and dry bulb thermometer attached to a wooden or metal support. The handle arrange-ment permits the instrument to be whirled in the air while taking the reading. The wet bulb thermometer is covered with muslin and is thoroughly moistened by being plunged into a cup of water which should be of the same temperature as that of the air of the hothouse.


After wetting the muslin of the wet bulb thermometer, the instrument is whirled steadily for a few seconds and the reading of the wet bulb thermometer noted. This whirling is repeated several times, until the reading of the wet bulb thermometer is constant. At this stage, the difference of temperature between the wet and dry thermometers is re-corded. After this difference has been obtained, we turn to Table 13 to get the exact reading of the relative humidity of the hothouse air. To make this clear to the reader, let us take a specific example. Suppose that the reading of the wet bulb thermometer was 64 degrees, and that of the dry bulb thermometer 62 degrees. This, then, will give us a difference of two degrees between the wet and the dry bulb thermometer. Let us now refer to the table under the column Dry bulb thermometer degrees, where it is marked 62 degrees. Read across Table 13 under the column depression of wet bulb thermometer in degrees until the column indicates a difference of two degrees. In this case it is the fourth column. The resultant figure, 89, will be the relative humidity of the hothouse air. In other words, a temperature of 62 degrees of the dry bulb thermometer with a difference of two degrees of the wet bulb thermometer will give a reading of 89 relative humidity of the air. In like manner, and by referring to Table 13, which should be hung up at a convenient place in the hothouse, the relative humidity of the house may be obtained. There are other simpler instruments by means of which the reading of the relative humidity may be obtained directly without the use of tables. The Mithoff hygrometer, for instance, is a type of such an instrument. However, they may readily get out of commission, and thus become unreliable.


The importance of water for greenhouse plants cannot be too emphatically stated. It has been truly said that "he who does not know how to water plants does not know how to grow them." Water is essential to plant life. It has been intimated that some crops evaporate from the leaves an amount of water equal to about three hundred times the weight of the dry matter which they contain. The amount of water in the soil is also an important consideration. The more water a soil contains the less air it will have. The presence of too much water in the soil often brings about serious complications in the health of plants such as suffocation of the roots, weak growth and a loss in power of resistance.

The improper use of water may affect the physical structure of the soil and injure the plants. The careless dashing of water on the surface of hothouse benches will compact and puddle the soil, and tend to. wash down the smaller grains to the bottom, changing thereby the capacity of that soil to retain air or heat, and thus indirectly affect the health of the plants.

Greenhouse plants depend on irrigation for their water entirely. Surface watering is still in use by the majority of hothouse men. But at best, this method often does no more than pack the soil in-stead of saturating it. Moreover, while safe enough for the experienced grower, it becomes extremely unsatisfactory when entrusted to careless or inexperienced labor.


It has been hinted previously that subirrigation has not found general favor with greenhouse men. Yet this has proved both experimentally and in practice to be far superior to any other form of greenhouse irrigation. In subirrigation water is applied through tiles underground.

Effect of Subirrigation on Vegetable Crops. Investigations by Rane have clearly shown that parsley, tomatoes, long rooted radishes and spinach are greatly benefited by subirrigation. Lettuce, especially, seems to be most favorably influenced by this method of watering (fig. 12, a-c.). Little is known of the effect of subirrigation on flowering plants. Investigations along those lines are especially desirable.

From the health viewpoint, subirrigation should appeal to greenhouse men. Where lettuce drop is prevalent subirrigation seems to check it materially. The same is also true of damping off.

In the greenhouse, subirrigation may be adapted to any form of bed used, whether raised or solid. In either case the bed should be practically water-tight. To prevent the rotting of wooden beds Taft t recommends coating the inside of the beds with a cement made of one part of water lime and three of sharp sand. This is made into a thick paste and spread over the surface about one-fourth of an inch thick. For a bed with tile or slate bottoms a similar covering will render them sufficiently tight. With wooden benches it is desirable that the supports be close enough to prevent sagging of the beds. In case of solid beds, a tight bottom about eight inches below the intended level of the bed is necessary. If the subsoil is a stiff clay it may be desirable to spread an inch of gravel. After thoroughly ramming it is covered with a thin layer of the cement as described above.


This method is in greater use than any other practice of greenhouse watering. The advantages claimed for it are the cheapness of installation and the more uniform way in which the water is applied. By this method, too, the dry atmosphere of the house can be quickly changed. This is especially desirable during the hot summer days. The disadvantages of this system are the packing of the surface soil and the encouragement of disease through the excessive moisture applied to the plant.


Next to watering, ventilation is of utmost importance from the health viewpoint. Many of the plant diseases which are confined to the greenhouse are encouraged by improper ventilation. The lack of it is as harmful as an excess. The practical grower will give this careful thought and consideration. As a rule, plenty of ventilation should be given whenever weather conditions permit it, avoiding, however, draft and strong air currents.

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