PROPAGATION STRUCTURE

 

Introduction

The structures which facilitate propagation of plants are called propagation structures. Propagation structures are required for propagating plants by seed, cuttings and grafting. Propagation structures are of two types; i) The first type a structure with temperature control and ample light, such as a green house, modified quonset house, or hot bed-where seeds can be germinated or cuttings are rooted or tissues culture micro-plants are rooted and acclimatized and the ii) The unit is structure into which the younger, tender plants can be moved for hardening, preparatory to transplanting out-of-doors. Cold frames, low polyethylene tunnels or sun tunnels covered by saran cloth and lath-houses are useful for this purpose.

Greenhouse

It is a structure constructed of cladding materials like plastic, polythene and fibre glass material having temperature control and ample light availability needed for propagating plants by seed, cutting and grafting. Experiments have shown that a greenhouse that runs from east-to-west is best for better light penetration in winter and early spring, and consequently preferable for raising seedlings at this time of the year. Moreover, it is important that the green houses should be well away from any kind of shade such as a tree of building, including other greenhouse. Some shelter, however, from north to northeast winds is desirable The glass covered greenhouses are expensive but for permanent long term installation, it is satisfactory than the plastic covered greenhouses. The construction utilizes wood or metal framework which are fastened with wood or metal sash bars to support sheet of glass embedded in putty. Mostly translucent type of glass, which gives a uniform diffused light, is used for the construction of greenhouse. Ventilation, which aid in controlling of temperature and humidity, is provided by arranging panels at the ridge or at the side of framework. In larger installation, forced air fan ventilation, controlled by thermostats, are used. As per requirements of season and types of plant to be grown. provision is made to heat the plants. Heating provision is sometimes made by blowing heated air into polythene tube of 30 to 60 cm size hung overhead and running throughout the length of the greenhouse. Small 5 to 7.5 cm holes spaced throughout the length of tubes allow the hot air to escape and, thus, maintain uniform heating throughout the house. These tubes can also be used for forced air ventilation during summer. Heat loss is a problem in greenhouse. It takes place through the roof. The loss of heat during winter is prevented by installing double layer polythene sheeting over the glass-roof. The two layers are kept separate by an air cushion from a low pressure blower. Another device to reduce heat loss is the use of movable thermal curtain which at night comes between the crop and the greenhouse roof and wall. Insulating the north wall reduces heat loss without appreciably lowering the available light. 

 

 GLASS HOUSE

Glass covered greenhouses are expensive but they have long life. However, for short –term benefits, plastic covered green houses can also be made. Two types of plastics are available for the construction, polyethylene and fiberglass. Both are lightweight and inexpensive as compared to glass. Plastic covered greenhouses are lighter than glass covered ones but there is very high increase in humidity is such houses, especially in winters, which results undesirable water drops on the plants. It can however be overcome by making adequate ventilation. Polyvinyl chloride (PVC) has also been used in construction of greenhouses but it tends to darken pre-maturely in sunlight.

Environmental control: During summer, the greenhouses can be cooled mechanically by the use of large evaporative cooling units. The 'pad and fan' system in which a wet pad of some material like aspen-wood excelsior is installed at one side of greenhouse and a large exhaust fan at the other side has been proved to be the best method of cooling large greenhouses. Thermostat control is recommended for greenhouse heating and evaporative cooling system. Depending upon the species, a night temperature of 13°C to 15°C is maintained under greenhouse.

Green house covering materials: The covering materials used for construction of green houses include glass, polyethylene, UV-stabilized polythene, acrylic, polycarbonate and fibreglass. The glass covered greenhouses are expensive but for a permanent long term installation under low light winter conditions because glass has superior light transmitting properties and less expensive relative humidity problems. Polyethylene materials are light weight and relatively less expensive compared to glass. Being light in weight, permits a less expensive supportive frame work than is required for glass. Polyethylene has relatively short life than glass. The UV-stabilized sheets can last for 3-4 years. Polyethylene having a thickness of 100-200 micron is generally used. The acrylic is highly weather resistant and does not yellow with age, has excellent light transmission properties and retains twice the heat of glass, but it is more costly and brittle.

Polycarbonate: It is probably, the most widely used structures sheet material today. This material is similar to acrylic in heat retention properties, with 90% light transmission of glass. It is light in weight 1/6th of glass and easy to install. It is resistant to impact. Polycarbonate textured surface diffuses light and reduces condensation drip. For providing rigid panels fiber glass is used widely for construction of greenhouse. It transmits 80-90% of light. New materials are continuously coming onto the market, for constructing better glass houses.

Lathhouse

It is a structure erected primarily to create shade to raise tender plants and to protect the plants from high temperature and light intensity. Agro-nets of different shading intensities are used for erecting lathhouses. Lath (lath - thin strip of wood) or shade houses provide outdoor shade and protect container-grown plants from high summer temperature and high light irradiance. They reduce moisture stress and reduce the moisture requirement of plants. Lath houses have many uses in propagation, particularly in conjunction with the hardening off and acclimatization of liner plants prior to transplanting and for maintenance of shade requiring plants. In mild climates, they are used for propagation, along with a mist facility and can be used as overwintering structures for liner plants. However, snow load can cause problems in higher latitude regions.

Lath house may be constructed with many kind of materials, although aluminum prefabricated lath house are available, but may be more costly than wooden structure. Most lath houses are covered with high density, woven plastic material such as saran, poly propylene fabric and UV-treated polyethylene shade cloth, which come in varying shade percentage and colors. These materials are available in different densities, thus allowing lower irradiance of light, such as 50 percent sunlight, to the plants. They are light weight and can be fastened to supporting posts. By controlling light intensities, the lathhouse reduces moisture stress and decreases the water requirement of plants. Generally, wood or iron poles are used to erect support. Through these poles, criss-cross arm of support is stretched. Over these supports, agronets are covered. Agro nets are light weight material and can be used on heavy wire fastended to supportive poles.

 Lath House

Hotbed (Hot frames)

A hotbed is a bed of soil enclosed in a glass or plastic frame. It is a large wood box or frame with a sloping, tight fitting lid made of window sash. The standard size is 0.9 x 1.8 m². Red wood or cedar wood which is resistant to decay, is used for preparing hotbeds. The seedlings start growth and cuttings root early in the season when hotbeds are used. In hotbeds heat is provided below the propagating medium by electric heating cables, hot water, steam pipes, hot air fumes or fermenting manure. Lead or plastic covered electric soils heating cables are quite satisfactory for providing bottom heat in hotbeds. Automatic temperature control is achieved using thermostat. For hotbed of 1.8 x 1.8 m about 18 metres of heating cable is required. The hotbed is filled with 10-15 cm of a rooting or seed germinating medium over the heating cables. Alternatively, flats containing the medium can be used. These are placed directly on a thin layer of sand covering wire netting. The layer of sand protect the cables from tools used. Hotbeds can be used throughout the year, except in areas with severe winter. In these areas its use may be restricted to spring summer and early winter (September).

Cold frame

Cold frame construction is similar to hotbed except that no provision is made for supplying bottom heat. The standard glass 0.9 x 1.8 m2 hotbeds sash is used as a covering for the frame. Although light weight less expensive frame can be constructed using polythene or fibreglass covering. The covered frame should fit tightly in order to retain heat and obtain high humidity. Cold frame should be placed in locations protected from winds. It is used primarily in hardening rooted cutting or young seedlings before planting. It can also be used for starting new plants in late spring, summer or early winter (September) when no external supply of heat is necessary. Cold frame utilizes only the heat of the sun retained by the transparent covering. While placing tender plants in cold frame, initially the covering is generally kept closed tightly to maintain a high humidity, The installation of mist line or irrigation provision in cold frame is essential to maintain humid conditions. Proper ventilation and shading is required to be maintained in order to prevent dryer condition. For this purpose, shading nets, reed mats or polythene are laid over the sash. In areas of extremely low winter, the plants can be over-wintered under cold frame. For such purpose, additional protective covering is required.

Cold Frame

 

Propagation frames

Sometimes in a greenhouse, the humidity is not enough to allow satisfactory rooting in the leaf cuttings. In such cases, enclosed frames covered with glass or plastic material may be used for rooting of cutting. These frames are useful only on grafted plants as these retain high humidity during the process of healing. Large inverted glass can also be kept over a container having cuttings. Though, high humidity is required is such frames but ventilation and shading is necessary after the rooting process has started in the cutting. Warm and humid conditions inside these structures provide excellent environment for growth of pathogenic organisms, which may infect the propagation material. It is therefore, necessary to maintain cleanliness and proper sanitation in such structures.

Net house

Net houses are widely used as propagation structures in tropical areas, where artificial heating is not required and artificial cooling is expensive. In these areas, net houses may be constructed with roofs covered with glass or plastic film and its sides are covered with wire net. It provides necessary ventilation and maintains an ideal temperature for germination of seeds and subsequent growth of the seedlings. The roof of net house may be covered with gunny cloth or even with live plant creeper to cut off the solar radiant energy and to keep the house cool. Net house can be constructed as per the need of the propagator and therefore its size varies with the requirements of the nurserymen.

Shade Net

Bottom heat box

It is a simple box for promoting rooting of cutting in difficult-to-root fruit plants like mango and guava. It consists of two chambers made from galvanized iron sheets. The outer chamber has a height of 70cm with 46 cm width and the inner chamber has a height of 68cm and width of 44cm. The space between the two chambers is filled with glass wool for heat insulation.

Another chamber is fitted inside the inner chamber with the height of 35cm fitted with two electric bulbs at the bottom for providing heat to the cutting. The innermost chamber is filled with soil mixture or any other propagation medium and the cuttings are inserted in it.

Two electric bulbs of 100 watt capacity are fitted at the bottom of the chamber to provide heat and light to the cuttings. Similarly, the temperature in the box is maintained and controlled automatically by a thermostat fitted at the bottom of the chamber. The most ideal temperature to be maintained in the box is 30± 20⁰C because at this temperature, cuttings of mango, walnut, olive and guava root easily and profusely. The initiation of rooting in cutting varies from species-to-species but in general, it takes 1-2 months for proper development of the roots.

Mist propagation unit

The rooting of softwood leafy cutting under spray or mist is a technique now widely used by nurserymen and other plant propagators throughout the world. The aim of misting is to maintain humidity by a continuous film of water on the leaves, thus reducing transpiration and keeping the cutting turgid until rooting take place. In this way, leafy cuttings can be fully exposed to light and air because humidity remains high and prevents damage even from bright sunshine. Mist also prevents disease infection in the cuttings by way of washing off fungus spores before they attack the tissues. While the leaves in this process must be kept continuously moist, it is important that only minimum water should be used. This is because excessive water leaches out nutrients from the compost, which may cause starvation. Moreover, a directly injurious effect on the cutting may occur from over watering. Hence, it is necessary to utilize nozzles capable of producing a very fine mist. A small mist propagating units are mostly used by small farmers, whereas, highly advanced impermanent units are used by the commercial nurserymen in advanced countries.

Mist propagation units are used for propagation of “difficult to root, cutting in most advanced countries. Mist beds are constructed within a greenhouse. A fine mist is sprayed intermittently over the cutting at regular intervals during day and night. The mist unit is controlled by a time clock, operating a magnetic solenoid valve and is set in a way to turn on the mist for 3-5 seconds to wet the leaves and turn off for some time and when the leaves are dry. The mist is again turned on.

In general, the mist has 5 control mechanisms. Timer, electronic leaf, thermostat and timer, screen balance and photoelectric cell. The two types of timers are used in a mist unit, one turn on in the morning and off at night and the second operate during day hours to produce an intermittent mist, usually 6 seconds “on” and 90 seconds “off”. In electronic leaf, a plastic with two terminals is placed under the mist along with cuttings, the alternate drying and wetting of the terminal breaks of the current, which in turn control the solenoid valve. A thermostat controls the temperature of the mist. In screen balance control mechanisms, stainless steel screen in attached to a lever with mercury switch. When mist is on, water is collected on the screen and when weight of water is more, it trips the mercury switch. The photoelectric controls are based on the relationship between light intensity and transpirations rate.

The mist unit can be set up in a glasshouse or in a polyethylene tunnel. Usually, it is set up on the propagation beds with 1.2 m width. The layout of the jets is very important. While installing mist propagation until, all the jets should be at equal height. For proper functioning of mist, there must be proper supply of water. The water should have good pressure and it must be free from salts. The optimum pH of water to be used in mist unit is 5.5 to 6.5. Hard water or alkaline water may be avoided as it may block the nozzles of the mist chamber and it may accumulate on the young leaves of the cutting and thus inhibit the growth and development of roots. Further, it is essential that a well-drained rooting media should be is used and there should be provision for removal of excess water. Similarly, development of blue and green algal growth is very common in mist propagation structure which is considered very harmful to the propagating material and thus every care should be taken to keep mist propagating unit free from any type of algae. After rooting in the mist, hardening of the rooted cutting is important for better success in the field. When cuttings are rooted, misting should not cease abruptly as this may help in drying out of the young plants followed by scorching, instead, a weaving off process should be adopted in which misting is continued but the number of sprays/days gradual reduce. The way is to shift the rooted cutting to a greenhouse, fog chamber, and frames, maintained at higher temperature and low relative humidity. After phase-wise hardening only, the rooted cuttings are planted at permanent location or in the nursery.

 Mist Unit