In the propagation of plants by soft wood herbaceous cuttings and leaf cuttings, one of the main problems is the loss by wilting due to desiccation before the cuttings have produced roots. In some other situations, when atmospheric temperature rises suddenly after rooting, the young roots are unable to absorb moisture from the soil at a rate, at which it is being lost by the transpiring surface of the young leaves. The result in such situations is again desiccation and death of the young rooted cuttings.
Even for the rooting of hardwood cuttings, they should also be maintained in fully turgid condition during the rooting period for a high rooting success. This can be accomplished by keeping the relative humidity of the air surrounding the cuttings at a high level (usually 90-95 per cent) by manual sprinkling of water on the foliage several times a day during the rooting period. Such procedures are difficult and costly and they usually fail to avoid heat build-up in the propagation chambers.
Mist Chamber:
To achieve this end water can be added to the air of propagation chamber by a number of mechanical means. Usually water is let into the chamber air through suitably designed nozzles under pressure (150-500 lbs per square inch, a high pressure system or 30-70 lbs per square inch a low pressure system) to form a fog or mist in the propagation chamber.
This type of mechanism creates and maintains a thin layer of water on the leaf surface. It lowers the transpiration rate as well respiration rate and thus prevents wilting, desiccation and death of soft leafy cuttings and helps in promoting rooting and subsequent vigorous growth of the roots.
The mist system consists of a regular water source, water strainers, electrically operated day-night timer, time interval cyclic timer, magnetic solenoid valve, mist nozzles and provision for bottom heat in the rooting beds. This type of intermittent mist operates automatically and maintains the chamber air at proper humidity and temperature to facilitate rooting.
The quality of irrigation water used in the mist system is very important. It should be very low in salts to avoid salt deposition on leaves of cuttings and also to prevent clogging of the nozzles and the lines. This is particularly important in arid regions when salts are usually high in the water.
Different kinds of nozzles are available in the market to regulate the delivery of water into the propagation chamber air. Oil burner nozzles operate best at pressure higher than 150 lb/m2, the others operate at pressure less than 100 lb/m2 and are usually employed for mist formation where excessive watering is required. High pressure nozzles are more costly than the low pressure nozzles.
The arrangement on nozzles in the chamber will depend upon objective, local conditions, and arrangement of benches and greenhouse structure. Above propagation benches, nozzles spaced every 3-4 feet will be satisfactory. The length to which a mist line can be extended shall depend upon the pressure of water source. But usually, the length of mist chamber is 100 feet or lesser.
Green Houses for Propagation:
To propagate plants around the year and to produce off season fruits the green houses are used. Their use is also essential for mass multiplication of plants through tissue culture. The optimum growth in green houses is promoted by maintaining high relative humidity and temperature control, adequate day length and light intensity. Good light conditions are essential for the sturdy growth of the seedlings. The green house runs from east to west are the best for better light penetration in winter and early spring.
Greenhouses can be constructed by using plastic polyethylene or fiberglass or other appropriate covering material. The greenhouse should be well equipped with elaborate structures and have precise control temperature, light intensity and humidity. The size and type of greenhouse primarily depends upon the need. Glass covered greenhouses are expensive but they have long life.
For quick and short term need plastic covered green houses can be constructed. Several types of sheets are available like UV stabilized polyethylene, polycarbonate and fiberglass which are light in weight and comparatively inexpensive. Plastic covered greenhouses are lighter than the glass covered ones. The proper ventilation should be made in plastic covered greenhouses to avoid undesirable water drops on the plants.
Polyvinyl chloride (PVC) has also been used in construction of green houses but it tends to darken pre-maturely in sunlight. In India, low cost green houses are constructed either of wood or metal framework and one covered with polyethylene sheet of 0.10 to 0.15 mm thickness which is resistant to ultra-violet rays. Such houses are equipped with thermostat, cooler or a conditioner or humidifier etc. for rigid control on temperature and humidity.
In green houses, two types of beds are used for growing seedlings. The beds may either by prepared on ground itself or raised beds or bench type beds are used.