Several techniques are available that can improve the success rate of seed germination or of striking cuttings. They may also reduce the time taken to achieve germination or cutting strike. The most common techniques are heating and misting.
'Bottom heating' is a method used by most commercial propagating nurseries. This involves applying heat to the base of the propagating bed, usually by specially designed electrical cables or elements. The pots containing the cuttings or seeds are placed on the bed (usually sand or other inert material) and are heated by convection from below.
With cuttings, the aim of bottom heat is to maintain the base of the cuttings at a temperature of about 5-10 degrees C warmer than the top. This encourages root formation rather than foliage growth. With both cuttings and seedlings, the use of heating extends the period of active growth, enabling successful propagation to be carried out in the cooler parts of the year.
The main disadvantage of heating is its expense. Energy is not cheap and, unless the glasshouse or poly-house is well insulated, heat loss to the atmosphere can be considerable.
A second disadvantage, particularly for amateur growers, is the danger of the propagating mix drying out if watering is neglected.
Automatic misting is used to apply water to cuttings or seedlings on a controlled basis. It is often used in conjunction with a heating system and, once set up correctly, overcomes the problem of drying out of the propagating mix.
Misting involves the application of a fine spray of water to cuttings and seedlings at regular intervals based on a simple timer device or on a sensor which detects the drying out of leaf surfaces. A simple, commonly used sensor is a mechanical 'leaf' made of wire mesh which is supported on a pivot and connected to a mercury switch. Water falling on the mesh causes the leaf to swing down, breaking the electrical contact in the switch and causing an electric solenoid valve to close, shutting off the water. As the "leaf" dries, it swings up to re-establish the electrical contact and turning on the water supply again.
Once correctly set up, automatic misting systems can make the task of caring for seedlings and cuttings much simpler. For example, the plant propagation area can be left unattended for an extended period. A disadvantage, however, is that some plants react poorly to having the foliage continually wet and develop fungal diseases which can kill the seedlings and cuttings. This mainly effects those plants which have foliage with fine hairs (eg Flannel Flowers, Actinotus helianthi). These are best kept in a drier atmosphere.
For the home propagator who produces only a small quantity of plants, the simplest method is to invest in one of the commercial, self-contained systems that can be obtained from larger nurseries. Typically, these are around 40-50 cm square and include a heating element in the base. Some of these also include a misting system.
A larger heating/misting system can be constructed to suit a particular size requirement using materials purchased from a horticultural supply company. However, given the potential for fatal electrical shock when water and electricity are used in close proximity, the services of a qualified electrical contractor would be necessary to ensure safety.