Greenhouse Accessories: Heated Propagator
Propagation cases and mist propagation units
Vegetative propagation is the production of young plants by inducing the formation of roots on sections of vegetative tissue removed from the parent plant. The more quickly this can be achieved the better, and it is therefore important to maintain the tissue in good condition. Soft tissue is particularly prone to rapid wilting when removed from the parent plant, simply because the leaves lose their moisture by rapid transpiration, which cannot be entirely replaced by the usual uptake of moisture because the water-absorbing tissue has been severed. While a certain amount of moisture can be obtained through the exposed xylem, the condition of all the tissue quickly deteriorates, which inevitably holds up the complicated process of forming new root tissue. If the air around the vegetative tissue (leaves, cuttings or other sections of plant) can be maintained in a very humid state, transpiration is considerably reduced and the tissue cells remain turgid.
This can be achieved in several ways such as the use of a heated propagation case, by polythene drapes, or on a smaller scale in a porch or conservatory by the use of polythene bags. Alternatively it can also be achieved by maintaining a fine film of moisture over the young plant sections, as achieved by mist propagation. Seed germinates better too.
Propagating cases and polythene drapes both have the advantage of providing an environment which can be maintained independently of, and generally higher than, the greenhouse temperature, which offers considerable fuel savings. It is desirable also to have bottom heat which can either be provided by a pipe heating system beneath the bench on which the propagating case is placed or by the use ofwarming cables.
Heated propagator cases can be of any convenient size and can be purchased complete, the larger sizes with soil warming cables and the smaller sizes with an electric bulb providing the warmth. Any handyman can make a propagating case with a sheet of glass and some wood, or even glass, for the sides, a convenient depth being about 25-30cm (10-12in). A distinct disadvantage of a propagating case is that condensation builds up on the underside of the glass, causing drips, but a good slope will encourage these to run to one end. Adequate ventilation will help.
Plastic domes and germinating cabinets
Plastic dome cases for plastic seed trays have in many ways tended to replace propagating cases because of their cheapness and mobility, and if they are placed on a soil-warmed bench they achieve the same basic purpose as a propagating case.
Considerable heat build-up can, however, occur in propagating cases, polythene drapes and plastic domes unless ventilated, which can be detrimental to young plants; damage can usually be avoided because of the extremely high humidity but it is generally safer to shade the greenhouse with coloured polythene or other material.
Propagating cases and domes can of course be used for all other aspects of propagation, including seed germination. Germinating cabinets have, however, in recent years become popular for seed germination, and these are merely highly insulated, unlit, shelved compartments fitted with a humidifier and heater, or on a less sophisticated scale merely a heater, the floor being maintained in wet condition to give the necessary humidity.
This technique, invented in the UK but developed in the USA, aims at overcoming many of the disadvantages of closed propagating cases by maintaining a constant fine film of water on the propagating material, so keeping the tissue cool by restricting the elementary process of loss of heat by evaporation and simultaneously preventing rapid transpiration. To be effective it must be used in conjunction with high level soil warming 160w/m2 (15 watts per sq ft) to achieve 24°C (75°F), keeping the atmosphere moist by constant applications of water. A series of vertical mist nozzles 38-60cm (15-24in) high are spaced 90cm-1.2m (3-4ft) apart (or according to water pressure available). Overhead mist lines can also be used.
It is highly important that thein the bench is perfect to avoid saturation, which could have highly detrimental results. The film of moisture is maintained in relation to air temperature and solar radiation by using either a selenium cell light sensor, electrode-type ‘leaf’, or absorbent pad controller, all of which operate a solenoid valve to control water flow. Various types of ‘leaf’ are obtainable and it is necessary to obtain advice on which is the best for the particular type of water involved. A good pressure of water, say 2.7-4bar (40-60psi), is necessary to operate the mist nozzles effectively, and if this is not available from the mains, a pressure tank will be necessary. In most areas, however, water pressure is adequate provided the supply pipe is of sufficient diameter.
Weaning mist-rooted plants to become accustomed to growing naturally involves ideally a period of less frequent watering, and this can be achieved either by moving the ‘leaf’ nearer the nozzle, adjusting the particular type of leaf, or by a time control unit. Root temperatures should also be reduced gradually by altering the thermostat setting.