There are two types of propagators – unheated and heated. Unheated propagators help keep humidity levels high, whereas heated ones also provide bottom heat and keep the compost and air temperatures higher and more suitable for good germination and rooting.
Some heated propagators are thermostatically controlled, so the temperature can be regulated to the optimum temperature. Thermostatically-controlled propagators offer the most versatility and are the best choice for those taking propagation seriously.
A propagation frame can also be made using soil-warming cables, which again ideally should be fitted with a thermostat.
To make a propagation frame, build a wooden box of the desired size, line the bottom 5cm (2in) with sand, lay the cable on the sand, so that it is in a wide S or serpentine shape, ensuring the cable is laid regularly over the sand and it never touches itself. Cover the cable with a further 5cm (2in) of sand.