Agriculture for Engineers 4 (3+1)

Lesson 40. Plant Growing Structures

40.1 PLANT GROWING STRUCTURES

Following are most commonly used plant growing structures for horticultural plants:

40.1.1 Shade Houses: Shade houses in nurseries in tropical and sub-tropical regions offer many advantages like raising of seedling in bags directly, protecting the grafts from hot summer months, effective irrigation through upside down overhead micro sprinklers. The shade house made with shade nets ( 50% or 75%) for regulation of shade are particularly very useful in arid regions where the humidity is very low during summer months.

40.1.2 Green Houses /Poly Houses: Grafting or budding of several fruit species under poly house or low cost green houses with natural ventilation will enhance the percentage graft / bud take besides faster growth of grafts due to favourable micro climatic conditions of poly house. In green house construction a wood or metal frame work is built to which wood or metal sack bars are fixed to support panes of glass work. in all poly house/greenhouse means of providing air movement and air exchange is necessary to aid in controlling temperature and humidity. It is the best, if possible to have in the green house heating and self –opening ventilators and evaporative cooling systems. Plastic covered green houses tend to be much lighter than glass covered once with a build up of excessive high humidity.

i. Polythene film: This is the most inexpensive covering material but it is the short lasting one. However, UV ray resisting polyethylene film of various thicknesses is usually recommended which lasts longer.

ii. PVC film : This material is pliable and comes in various thickness and widths unto 6 Ft. it is longer lasting than polythene and is more expensive PVC surface of films tends to collect dust and lower the light intensity in due course of time.

iii. Polyester film:  This is strong material with excellent extremes of heat or cold through it is costly than polythene film /PVC film.

iv. Fiber glass: Rigid panels, corrugated or flat fiberglass sheets embedded in plastic are widely used for green house construction, fiberglass is strong, long lasting, lightweight and easily applied which is coming in a variety of widths, lengths an thickness. It is costlier than poly thin film/PVC film.

40.1.3 Hotbeds: The hotbeds is often used for same purpose as a green house but in a smaller scale. Amateur operations and seedling can be started and leafy cuttings root easily in the cold seasons in such structures. Heat is provided artificially below the propagating medium by electric heating cables, pot water, steam pipes or hot air blows. As in the green house, in the hot beds attention must be paid for shading and ventilation as well as temperature and humidity control.

40.1.4 Lath Houses:- These structures are very useful in providing protection from the sun for container grown nursery stocks in areas of high summer temperatures and high light intensity. Well-established plants also require lath house protecting including shade-loving plants. Lath house construction varies widely depending on the material used. Aluminium  pre-fabricated lath house are available but may be more costly than wood structures. Shade is provided by appropriate structures and use of shade nets of different densities allows various intensities of light in the lath house.

40.1.5 Miscellaneous Propagating Structures:-

1. Mist Beds: These are valuable propagating units both in the green house and out doors and are useful mainly in rooting of leafy cuttings.

2. Mist Chambers: This is structure used to propagate soft wood cutting, difficult to root plants and shrubs. Here the principle is to spray the cutting with a minimum quantity of water. This is achieved by providing the cutting a series of intermittent spraying rather than a continuous spray. The intermitted spraying can be done easily by means of a high-pressure pump and a time switch. The pumps leads to a pipeline systems inside the propagating structure. The mist nozzles are fitted to these pipelines and suitability spaced over the propagating material.

3. Nursery Bed: These are raised beds or boxes made of bricks and morter, provided with drainage holes at a bottom. The dimension of the boxes are 60 cm high, 120 cm broad and length as required preferably not exceeding 10 m roof structures for planting on both sides and forming ridges at the center are constructed on the top of the nursery beds. These structures may be made permanent with angle iron or maybe made of wood. bamboos mats, palm leaf mats are placed over these structures to protected the seedling from hot sun and heavy rains.

4. Fluorescent Light Boxes: Young plants of many species grow satisfactory under artificial light from fluorescent lamp units. Although adequate growth of many plant species may be obtained under fluorescent lamps but not up to the mark compared to good green house conditions.

5. Propagating Cases: Even in green house, humidity condition are often not sufficiently high for rooting. The use of enclosed frames or cases covered with glass or plastic materials may be necessary for successful rooting. In using such structures, care is necessary to avoid the build up of disease organism due to high humidity.

6. Plastic Mulch: Mulch is a material used for covering the soil  in order to prevent weed growth avoids direct evaporation of the water from the soil and to increase soil temperature, mulching seedbeds with opaque black plastic film prevents weed growth completely and because of higher soil temperature and better conservation of soil moisture, brings about early germination and faster seedling growth.

7. Mini-Green Houses: The plants need carbon dioxide for making food through photosynthesis. The green house helps in providing additional carbon dioxide to the plants to enhance their rate of photosynthesis. This can be achieved by enclosing the plants in a box like structure made out of bamboo and colourless transparent plastic, with a lid at the top. The lid is closed after sun set, so that carbon dioxide produce due to respiration accumulate in the box. After sun rise the plant start photosynthesis and since there is higher carbon dioxide content around the plant, the rate of carbon assimilation is higher. After a few hours the lid is opened to prevent over-heating alternatively. A plot or beds of plant can be provided with plastic films skirting supports by bamboo stakes, about 12 cm. high. Since CO₂ is heavier than air it tends to settle around the plants.

8. Light Chamber: Several plants to go into winter dormancy when the day length shorts. Additional light from tube lights, given after sun-set, creates long day conditions that prevent the plants from going into winter dormancy. Light given at the end of the day. Also encourage growth of green leaves; they grow tall without developing lateral branches. On the other hand, if they are exposed to fluorescent light from tube-lights laid on the grounds, they developed side branches and show a bushy habits.

9. High-Humidity Chambers: This technique resolves the common problems of graft or cutting dying due to desiccation when planted in the soil for rooting, by ensuring a humid atmosphere around the cuttings, thus wood preventing excessive evaporation. The cutting/grafts are planted on a sand beds, enclosed on all the sides by a dome made of GI wire and covered with a transparent, colourless plastic film. The sand is watered to field capacity, and plastic film traps evaporation inside the chamber creating a highly humid atmosphere. The dome must be shaded, since direct sunlight will heat up the internal atmosphere of the dome, killing the plants.     

40.2 GREEN HOUSE                                                          

Plants under natural conditions grow and bear flowers, fruits and seeds in their own specific manner but there are some hurdles like unfavorable weather conditions, attack of insects, pests and diseases etc., which restrict their development and growth. There is a need, therefore, to protect the plants from such adverse conditions so as to allow them to grow vigorously. This led to the invention of a shelter where the plants can grow or pass over their critical period of growth under most favorable condition. The shelter so prepared is known as Green house which is generally a structure covered with a transparent material to utile the solar radiant energy and to provide suitable environmental conditions for the optimum growth of plants.

40.2.1 Types of Green Houses

 A greenhouse is a place where plant can grow under controlled conditions so as to grow, develop and produce at the will of the grower. In the beginning, these houses or structure were fully covered with attached roof or a covering, which could be lifted with ease. But later on, they were being covered with stales and further, with glass and as such they came to be known as glasshouses. Now, many types of materials are available to cover them. Hence different other names depending upon the type of covering used are common.

Glasshouse is with a roof and sides off glass of different sizes. Instead of glass, fiberglass, which is stronger, durable and does not break into pieces, is used and so the name of such a house is fibre glasshouse. This is hardier than the glasshouse but is costlier than that, though the chances of its breakage are much less. Plastic house or plastic  green house has a cover of a transparent alkathene or polythene which is strong enough to withstand the varying temperature of summer and winter, hail, strong winds etc, these are also known as poly-houses, Poly-tubes or poly tunnels. But, in general, greenhouse is the commonly accepted and popular name.

Types of material used for making a structure of the greenhouse also vary. Two things are necessary for a greenhouse. i.e. the frame or structure and the cover. To support the cover and structures on the sides, pillars are needed. The structure of pillars or support may vary with type of frame and size of a greenhouse. Small and semicircular greenhouses, i.e. poly-tunnels may not need pillars or support in the centre but heavy or large greenhouses which may have heavy structure of iron or wood, do not need strong support to have the roof and sides intact.

Frames may be wooden or of iron or aluminum for a greenhouse of glass cover. Pieces of glasses are fixed tightly in the frames by putting some fixing material along the sides of the glass. In case of poly-house the sides and roof are covered with a polythene sheet, which is placed deep in the soil to hold it. In some cases the sheet is put in between the two tubes/frames.

Pillars of cement and concrete mixture are also built to support the roof of a greenhouse of glass or fiberglass. Number of pillars are needed will depend upon the size of a greenhouse but six pillars are necessary at the minimum level. The height of these pillars also varies but generally the highest pillar may be 3 meter tall. Pillars can be round, square or rectangular but in practice, the side pillars are usually square in shape while the central ones may be round to avoid unnecessary clash in greenhouse. When wooden pillars are used, they are usually square in shape of the width 8-10 cm. Iron or metallic pillars are also used but they are usually of round but hollow pipes of the diameter of 5-10 cm. Pillars of cement and concrete mix are usually white washed or painted with paint while the wooden and metallic structures are painted with paint. If money is available, these structures may be painted with a silver paint. Doors and windows may also be painted with white or silver paint. Some people paint the wooden frames with green paint which is also good but white or silver paint provides good sanitary look.

Depending upon the material to be kept, the specific period of keeping in the greenhouse and the type of material for construction and coverage, the size and shape of greenhouse varies. Heavy greenhouse with pillars of cement and concrete mix with a fiberglass top or cover may last for 2 decades or so while other may last for 5 years or so. On the other hand, plastic houses or poly-tunnels do last for 2-3 years only provided they are maintained in the best manner.

Shape of the greenhouse is generally, pentagonal from the front and back. For about 2 meters, they do not have the straight walls then it tapers toward the top to form a triangle which is needed for the easy run off of rainwater or snow. Wind may pass over it. Leaves etc., if they fall on the roof do not stay there for a long period they are blown off by the wind.

40.2.2 Equipments for Greenhouse

Common greenhouses are just shelters for the plants with the aim of protecting the plants or other plant material from extreme variations in the temperature like snow falls in hills, drought in plains and excessive rains. These greenhouses have only benches to keep the plant pots or trays in order. They do have a hard floor that may be a cemented one and there is good provision for the drainage of water. For control of temperature, the doors and windows are opened for free passage of air current.

Equipments and furnishings are needed in a greenhouse will depend upon the type of material to be raised or stored and under what conditions they are to be kept. Simple greenhouses are the structures which can house the plants under shade. They are not provided with sophisticated equipments. Generally, during a warm weather, plants are either watered frequently or pone of water is created artificially in the center to keep the humidity under check. Doors may be opened or closed to provide ventilation, air passage and to raise the temperature.

Misting, cooling, heating, balancing the air supply with its quantity and quality, artificial lighting, controlling temperature and humidity are some of the operations which are governed by different automatic equipments fixed in a green house.

An ideal greenhouse must have a good system for control of temperature, humidity, nutritional requirement and pathogens.