Propagators are used to maintain a warm, humid environment for seeds and cuttings and can offer an excellent start to young plants. Unheated and heated versions are available; however unheated propagators should only really be used if ambient temperatures are high. This means that their use in the UK is usually restricted to rooting cuttings in summer.

How They Work

Propagators operate by the action of water vapour condensing on the inside of the lid and falling back onto the plants, creating a moist general atmosphere. This helps the plants to replace the water lost through their leaves. Rigid tops are more effective as they are more likely to retain the heat than thin plastic covers. You should also look for models with adjustable ventilators, as they allow some moisture to escape, preventing the atmosphere from becoming too humid.

1. Unheated Propagators

Whilst unheated propagators may be used effectively for root cutting during the summer, they do not usually provide enough heat to raise spring seeds successfully, unless they are heated in some other way. Other methods of heating may include soil warming cables, heated benches or trays, or simply a greenhouse that is fully heated over the winter and spring months.

Heated Benches and Trays

Heated benches and trays can be used with unheated propagators or ordinary seed trays, which are simply placed onto their surface. However, these systems do not raise the temperature as efficiently as propagators with an integral heating element. A plastic cover should be fitted over seed trays to maintain warmth and humidity.

Soil Warming Cables

Soil warming cables are primarily used to raise the temperature of compost in unheated propagators or mist units, but may also be used to heat the air in an enclosed space such as a cold frame.

For safety, choose a cable with a wired-in thermostat and make sure that it plugs into a socket with a residual current device (RCD) in case it is cut through accidentally during cultivation. Make sure that you carefully follow the instructions for installation and use.

2. Heated Propagators

Many biological processes are accelerated when temperatures are raised, and growing seeds is no exception. Increasing the temperature of propagation compost usually increases the rate at which seeds germinate and cuttings root.

If your seeds or cuttings need to germinate or strike root in warm conditions, a heated propagator can create a small, moisture-retentive, sanitary environment, and removes the need to heat the whole greenhouse. Heating elements should be capable of providing a minimum compost temperature of 15°C (59°F) for general plants, although tropical plants will need a higher temperature of at least 24°C (75°F).

Heated propagators can range from simple versions designed to sit on windowsills, to greenhouse models with adjustable thermostats to give greater flexibility of temperature.

Windowsill Propagators

Small domestic propagators are designed to sit on a windowsill, holding two or three seed trays. Although they are heated, they do not usually have an adjustable thermostat and are not powerful enough to raise temperatures sufficiently in a cold or cool greenhouse.

Fixed Temperature Propagators

Like the windowsill propagator, these models do not have an adjustable thermostat; they simply raise the soil temperature a certain number of degrees above the ambient temperature. For example, a propagator designed to give a lift of 12-14ºC (20-25ºF) above the ambient temperature will maintain the soil at a temperature of 17-19ºC (62-66ºF) in a greenhouse at 5ºC (41ºF). Greenhouse models tend to be more powerful than windowsill propagators (for example, 50W compared with 15W), which means they can provide more heat. They are also tend to be larger than the domestic models, holding far more seed trays.

Variable Temperature Propagators

Variable temperature propagators house a powerful heating element in the base and are fitted with an adjustable thermostat. The thermostat turns the heating element on and off to maintain the correct temperature for the type of seed or cuttings you are propagating.