This article throws light upon the top two types of structures for green houses. The types are: 1. Temporary Structures 2. Permanent Structures.
Type # 1. Temporary Structures:
Cold or Storage Pits:
In early times, pits used to dig about 90 cm deep to store the pot plants or germinating seedlings, the top was covered with transparent material or glass in frame in slanting position keeping higher side in north side and slope on south side. Thus the plants were remained too much below of frost line so they were not damaged due to prevalence of low temperature.
Hot Beds:
In such beds, farmyard manure is spread on the ground ranging from 30-45 cm thick for mild climate and 60-70 cm thick for severe climatic conditions. On fermentation, manure provides heat for three to six weeks.
Sash Beds:
Under severe adverse climatic conditions to save the crop from top, crop is covered with transparent glass fixed in frames. The frames are made up of wood. The length of frame is about 180 cm and 150 cm wide. These frames are made in such a way that one or more layers of glass can be accommodated. The usual height is 30 cm on one side and 15 cm on other side.
Higher side is placed on north side so that sloppy side faces south side to get the maximum benefit of sun light. The thickness of glass is 4 or 6 mm. The thicker will be heavier to handle but will be more durable. Under severe winter conditions double glass layer is used. Generally these frames are heated with farmyard manure, flues or electric cables.
Cold Frames:
When sash beds are not heated, these are called as cold frames. These are successful in mild climatic conditions.
Forcing Boxes:
These are small cold frame with a single glass or glass substitute like polythene, polyvinylchloride or Rein forced plastic fiber which is over a single plant in the field during early spring. They provide sufficient heat to help in establishing them and maintain them in healthy conditions.
Type # 2. Permanent Structures:
(a) Lean to Type:
It is simple to construct and maintain and is commonly called as shed roof house. It is constructed on the south side of the building so that it get maximum sunlight during winter when it is required most. Since it is constructed against the southern wall of the house, its three sides are made with glass structure.
Slope is made towards south side. Lean to types are mostly used to protect the potted plants, growing plants which require less light like ferns, foliage plants or for starting early plants or raising nursery seedlings.
Being simple to construct, it has several advantages as follows:
Advantages:
(i) Less expensive,
(ii) Less cost of maintenance,
(iii) Glass area is less so less breakage,
(iv) Radiates less heat, and
(v) Highly suitable for amateur use as they are attached to the house.
Disadvantages:
(i) Less productive and
(ii) Receive sunlight mainly from one side.
(b) Even Span Type:
As the name indicates such type of green houses have equal length on both the sides from the centre. These types of houses are very popular and more than 80% of this type is being constructed. The width of such green-houses varies from 4-20 m and length is 50 m. More length then this result in too much walking off workers. Larger green-houses should have a larger garage type door in the centre so tilling equipment and small trucks can move in and out readily.
Advantages:
(i) Heating or cooling is achieved uniformly and efficiently.
(ii) Full advantage of sunlight is achieved.
Uneven Span House:
In these types of houses one side of roof is longer than the other. It makes a small angle with the horizontal and thus has a lower pitch. Such type of green-houses is very popular on sloppy terrain and to utilize the maximum sun light, smaller arm kept on north side whereas longer arm is facing south side. The shorter arm is normally one third of longer arm.
Curvilinear and Curved Eave Type:
These types are highly suitable for conservatories and are features of mainly city parks, conservatories, botanical gardens and of rich individuals. Such type became part of landscape plan and improves the aesthetic of surroundings. Many of these conservatories are used for seasonal displays and exhibition of floral crops. These types are rarely used for commercial flower production but add much landscape value.
Ground to Ground Types:
These types are invented during World War II. The frame work of such structure is galvanized pipes of various diameters varying from 1.25 cm to 4 cm and central height is kept 2.7 to 3.0 m. The length is kept 16 m to 25 m and width is 6.5 m. These pipes are fixed in the ground or fixed grouted pipe of larger diameters at the gap of 2 m from pipe to pipe.
In the centre pipes are connected with main pipe so structure become rigid and stable. This structure is covered with polyethylene or rigid fibre glass. On one side gate is kept at entrance. In these type, some land of sides used to remain unutilized so after improvement new type have been made which is called as Quonset type.
Quonset Type:
In these type pipes are straight from the ground about 2 m so that up to the last end of ground is utilized. Now a-days these are most popular and being fabricated for growing cut flowers. Length, width and height are the same as above. Above types can be made separately (single) or detached type or many are attached with each other. There are certain advantages of each system.
Advantages of detached green house or single units:
(i) It is easier to programme and maintain the temperature to meet specific requirement of crop.
(ii) It is easier to ventilate without exposing the plants to blast of cold air.
(iii) Fewer ventilators and less ventilating machinery is required.
(iv) Light distribution is uniforms.
(v) Care of the house is easier.
(vi) Appropriately spaced detached green houses are easily cleared of snow without damaging structures.
Disadvantages of single houses:
(i) The greater height of a single detached house requires more energy of heat the large volume of contained air.
(ii) More height is also exposed to winter winds and hence, greater loss of heat than low houses.
(iii) They have higher initial cost.
Now-a-days for commercial production larger area is covered under green-house both detached or many are attached at single location. Such units should be economically viable. By setting up such units, over-head expenses are reduced drastically. There are certain considerations which should be kept in mind before starting up such commercial activities.